BMW Airhead Boxers
About BMW Aircooled Twins
BMW motorcycles are the R-series - flat-twin aircooled; the K-series - 3 or 4 cylinder watercooled; and F-series - single cylinder.
We give a brief history, then this page carries on to talk about the Airhead R-series. Our BMW K-Series page is here.
A Brief Model History
BMW have made aircooled twins since the dawn of time. There have been model designation changes over the years, but the basic layout and style of the bike has not changed. Typical is the author's R80/7 shown - an 800cc twin produced from the late 70s to mid 80s.
These bikes are known as the BMW 'Boxer' from the engine configuration of the two pistons moving out and in as a boxer punches. They are also known (particularly in the US) as the 'Airhead', to distinguish them from the newer twin-cylinder 'Oilheads'.
In the early 80s, BMW introduced the highly advanced K-series. An example is the ex-Police K100RT shown (with a non-standard rear box). This was a completely new model, and a radical departure from the Boxers. 'K-Bikes' as they are known in most of the world, 'Bricks' (from the engine being shaped like a concrete block) as they are often known in the UK, are 3 or 4 cylinder water-cooled bikes with Eectronic Fuel Injection and Ignition. The engine is still laid 'flat', in that the pistons run left and right, with the head on the left and the crank inline, but these are a completely different style of bike to the twins.
BMW tried to phase out production of the twins, but customer demand was so strong that they could not ignore it, and 're-introduced' (actually they had never really gone away) the boxer range at the end of the 80s, expanding it from the early 90s, into what are now known as the 'Oilheads'. They follow BMW's trademark flat-twin cylinders-opposed configuration - 'Oilhead' distinguishing them from the older 'Airheads'. Perhaps the best known example would be the R1150GS models Ewan McGregor and Charlie Boorman rode round the world in the 'Long Way Round' series. These are much more than just the older Airheads re-made, they are thoroughly modern bikes, with electronic ignition, ABS, and so on.
In the early 90s, BMW brought out another completely different style of bike, officially called the F-series, known by all as the F650 from the only available engine size 650cc. This uses the Rotax water-cooled single-cylinder engine, modified slightly for BMW's use. Though not widely known, the Austrian Rotax motor has a long history as an engine used by specialist manufacturers.
Which model of BMW you want will depend on whether you want:
(Of course I am over-simplifying. You could easily ride an F650GS round the
world, or a K-Bike for the daily commute to work, and many happy owners do.)
The rest of this page discusses Airheads in more detail.
Assuming you are thinking of getting one, but never had one before....
Many years ago, I and a friend rode 100 miles to a party on my R80. Next morning, I was a little, actually quite a lot, hungover. Asked my friend to drive us home. He had never ridden a Boxer before, but he could see I was in no fit state... Stopped after 20 miles, and he begged me to take over. I couldn't. Stopped after 50 miles, and I asked if he was OK. He looked thoughful, said nothing, and carried on driving. We got home, and the next week, he had bought one. As far as I know, he still has it.
That is how people get into Boxers. You will not like your first impressions. It will seem rough, ugly, agricultural. The way it rolls to the right as you blip the throttle will be unsettling. The way the starter clunks into engagement. The slow, heavy gearbox. The ponderous handling. But bear with it. After a couple of days, you will begin to get used to its ways. After a couple of weeks, you will be looking at it with respect every time you get off. After a couple of thousand miles, you will wonder why anyone would want to own any other bike.
A good Boxer will be smooth and positive in feel, it will seem 'well-oiled' and 'fluid' to ride. It will be comfortable all day at speed on the Autobahn. It will have an air of solid dependability. It will take you round the world if you ask it. ...It just takes a little while for these qualities to show themselves to you.
When you go to look at a prospective purchase, you can see it is the colour and model you want. OK, now stop looking at the bike. Look at the Owner. This is true of buying any bike, but it is especially true of BMWs. Older ones are now cheap bikes, so many get bought as cheap hacks, often by young riders, who run them into the ground without proper maintenance. Walk away.
The Owner you want to buy from is an older rider, has had the bike for 10 years, possibly toured on it, who can tell you what weight of oil goes in the engine, box, driveshaft, and final drive. They will be showing you a wad of paperwork for servicing and maintenance. They are selling because they have bought another BMW, or are getting too old to ride.
A well-maintained Boxer will easily do more than 100,000 miles before it needs significant work. A thrashed, poorly-maintained bike will be beyond economic repair long before that. You can tell one from the other in an instant by looking at the Owner.
#1 check. The critical area on any Boxer is the drivetrain. Almost anything else will be cheap and easy to fix, but any component gone from the gearbox to the final drive can get very expensive very quickly.
Get it on the centre stand and grasp the rear wheel. Rotate it back and forth, listening carefully all along the drive train. Rest your fingers on various casings, feeling for knocks. A light clunk in the gearbox is normal play in the gears. Anything else is worrying.
A clunk in the driveshaft itself will be £100 for a driveshaft and 4 hours to fit. Unless its a Paralever, which is a scary £250. (UK prices, and assuming an experienced mechanic).
A clunk in the rear bevel drive is either the crown wheel and pinion, or (on the pre-81 twinshock) the rear wheel drive splines. If the owner will let you, you could take the wheel off to see which. Whichever it is, it will cost you £300-£400 to fix, and need a specialist engineering shop. You can probably find a good used replacement for £100-150 and fit it yourself.
Serious clunks in the gearbox (more than the normal slack between gears) will require a specialist rebuild or replacement box. Either will be upwards of £300. It is possible to home-rebuild, but it needs specialist tools or advanced ingenuity to achieve. If the owner lets slip that it 'needs held in first', 'shudders when you accelerate hard in second' or similar - the box is dead.
If the drivetrain seems good, next check is the engine.
There must be a noticeable tappet noise. This is normal. If there is no tappet noise, you may have a serious problem. Before the mid-80s, these bikes expected to get Leaded petrol. With Unleaded petrol, valve-seat recession can cause the gaps to close to such an extent that the valves do not completely seal against their seat. It is then a short step to burnt valves. The good owner will have been using petrol additives and listening carefully for the tappets closing up - getting quieter - or will have had the heads converted (£250-300).
There will also be piston slap, a light 'ticking' from the barrels. Some do it more than others. As long as it is a light ticking, they did it from new and it is nothing to worry about. A heavy knocking from the barrels is very serious, either very loose pistons or the big-ends. Walk away.
There is an issue with rocker shaft end-play. A heavy knock from the tappets can sound awful and be confused with a more serious problem. If it is definately from the tappets, it can sometimes be adjusted out, sometimes you need parts. Either way, it is quite easy to fix.
And that covers the checks on the engine.
A quick aside on the clutch. These last very high mileages, but if it is slipping, the gearbox has to come out to change it, and that is a tricky and time-consuming job.
And 'They all do that, Sir'. If it smokes from the left side when it is started, don't panic. Even great billowing clouds are acceptable. If they are parked on the side-stand after a run, oil drains down overnight along the left bore and into the combustion chamber. This smoke is never terminal, it will burn the oil out as the engine warms, and it should clear in a minute or two. Cure: park it on the centre stand; replace the rings; or ignore it. Check for smoke again after a test run. If it continues to smoke, or smokes from both sides, most models can be helped with new rings and valve seals, but many models have nickasil bores which cannot be rebored, so you may be needing new barrels and pistons.
That covers all the critical checks. If it passes all those, you have a good one. If it doesn't, you have already walked away, or are expecting big money off the price.
A few notes, particularly on bikes which have lain unused for a while
- If the battery is dead, they are big and expensive - and a complete pain to get out and in. (Replace with a solid, 'gel' type, same power but smaller. About £80.)
- A common electrical gremlin is the original electro-mechanical charging regulator under the tank. They just fail sometimes. If the charging light does not go out, chances are it is this. A replacement solid-state device is £25 and half an hour. (The Rectifier is inside the front engine cover, more time-consuming to replace, but normally very reliable.)
- If it has lain unused for any length of time - clean and re-grease the drive splines. If you don't, wear will be rapid and expensive.
On a bike which has been unused for a while, keep checking the carbs aren't flooding and leaking all over your boots.
The gearbox will clunk into first. Many like a slight blip of the throttle to engage first, this is normal. You will probably miss a few changes until you get used to the box. They reward slow changes, and careful matching of revs and speed. Work out whether it is just you, or whether the box itself crunches or snatches badly as you let the clutch out. Early models had insufficient lubrication to the forward output shaft bearing, they break up and cause difficulty shifting and snatching of drive. Roll on and off the throttle in third or fourth at cruising speed, listening and feeling for this. Take some opportunity to pull away hard in first, if it jumps out of gear you will be rebuilding the box.
Both brakes should be firm and progressive. Original equipment drilled stainless disks are known to be useless. They should work well enough at modest speed, but they are not powerful. Replacement with cast disks is highly recommended, you may want to budget for the £150 each they cost in negotiating the price.
Tyre fitment has a big effect on handling. 'Castoring' can be bad at around 40mph hands off the bars. It may twitch over white lines. Both can be helped greatly by strict adherence to correct front tyre pressures, and different tyre choice. They do not, in themselves, indicate a problem with the bike.
The rest of the test ride is obvious: a good one will go vrooom smoothly through the gears, a bad one will be rough and snatchy.
What to pay ?
(I give UK prices - this is motorcycle.co.uk after all. Prices will vary in your locale.)
Old dogs with problems are worth a few hundred quid. If they need work, you can easily and quickly get into many hundreds for parts or specialist rebuilding work to make them good bikes. Bear in mind there are lots of the common models around, and it is often false economy to buy one as a fixer-upper. For rare, desirable models, you may be prepared to take one with problems - but do it with your eyes open. Basket-cases, or stripped-in-boxes, are up to how confident you feel that its all there.
Decent runners are rarely shiny-like-new, but will have been well-maintained by a sympathetic owner, capable of immediate reliable use, and worth anywhere between £700 and £1500.
Very nice bikes - garaged, cherished, maintained regardless of expense - come up quite often, but are not really worth much more than decent runners. Often people ask more, and if you want that particular model with that colour fairing, you may be prepared to pay it. But really, unless it is a concours example with very low miles, very few Airhead Boxers are worth over £2000.
Ex-Police bikes are common in the UK. Direct ex-police, they can be good, well-maintained bikes, and worth having. It depends who has had them since the Authorities. From a caring mature owner, they are worth only slightly less than the equivalent civilian model. From a less caring owner, I would be walking away.
Naked /6 /7 bikes and well-used RT (touring fairing) bikes are at the bottom end, £700-1200. The R100RS (make sure it is the 100, not a dressed up 80, and make sure it has the very necessary original fitment oil cooler) is a fantastic high-speed bike, so worth £1500-2000 for a decent one.
GS models are overpriced for what they are, but are very cool, stylish bikes if you want one, £1500 +. Late R100GSs seem to go for silly money, I have seen them sell at £3500-4000. Nice bike, but not £2000 nicer than the equivalent road models! If you want a GS at sensible price, you may have to go to Germany - try www.motoscout.de, better than eBay for German bikes, or Martin Kornhas classic BMW dealer SW Germany (find a page-translation tool at Google)
A good R65/80ST is a very sweet machine, fairly rare, and in good condition worth upwards of £1000. R45s seem to hold the same price as any normal unfaired Boxer. They are rather slow, though, and for the same money you could have something which goes properly.
Rare, charismatic models of any bike often have a cult following. The gorgeous BMW R90S, for example, may justifiably reach £3-4000 in shiny condition. In this rarified, concours world, price is whatever somebody who wants it is prepared to pay.
eBay is the most useful place to determine current prices. Search for, say, 'R80'. Then down on the left, click on 'Completed Items' and it will show you the prices for items over the last few months - green prices for bikes that sold, red for those that didn't reach what the owner wanted.
Where to buy
In the UK, most Airheads will be sold privately. eBay is the obvious place, but prices (particularly for rough bikes) are high. Classic Bike magazine always has a few, this is the best place to find a low mileage, garaged example from an enthusiast, but they are priced to reflect this. An occasional cheaper buy can be found in there. Cheaper bikes and bargains will most often be found in your local evening paper or 'Free Ads' weekly.
Your local Dealers are not really interested in old Boxers, but you can often find something languishing at the back of their second-hand stock. Given the low value of the bike to them, you are unlikely to get 5-star service and a long dealer warranty. But they may well be happy to shift what they took as a trade-in 'sold as seen' to you for a reasonable price.
There are very few specialists. If you are reassured buying from them, they are worth travelling to find. Others are generally local back-street workshops whose owner happens to like these bikes - and you will only know of them if you live there, or ask around (stop a local BMW rider and ask, we are a friendly bunch :).
In R 80 RT - R = Boxer engine; 80 = 800cc; RT = Touring Fairing
|Boxer engine||dawn of time - about 1994||
|K||3 or 4 cylinder watercooled||1983 - 90s|
|F||single cylinder, chain drive||1993 - current|
Newer K-Bikes and Oilheads (early 90s on) are known by their engine size, e.g. R1150GS, K1200LT, etc.
Of the R-series bikes, these are the main 'Type 247 Engine' model variants (Type 247 is the Airhead Boxer engine)
Some had single front disks, some twin disks.
Pre-80 - points ignition
/5 introduced 1969
From 1980, no /series designation, just known as R80, R100, etc until end-of-line models R80R, R100R etc in early 1990s.
Barn-door tall fairing. Much loved by long-distance tourers.
R80RT and R100RT variants
Bike advertised as '80RT' may be same-fairing 80RT TIC - where TIC means 'Police Spec'.
Excellent fairing for high-speed touring. A well-sorted R100RS will show you 135mph, and hold over 100 all day.
There was no official R80RS, though you often see a naked R80 which has been converted. The 800 can't really take the sustained speed the RS fairing allows.
Proper RS-faired models were fitted with an oil-cooler as standard. It needs it.
Disk rear brake on RS/RT from about 1979.
|LS||Rare R65LS is only known variant. Angular wedge-shaped styling. R65LS FAQ here|
R90S is the best-known S variant. Often bright orange. Fast, highly collectible.
R100CS looks similar, but the 90S is the desirable one.
Limited numbers in the UK. R65ST (and rare R80ST) had GS frame and R45 forks.
(R65s use the Type 248/1 engine)
Different frame from road models. Early 80s G/S is competent offroad. Often had kicker box.
Later GS are too heavy to be good as trail bikes. Riding Round The World, though, a monolever GS should be top of your list.
Paris Dakar variant, built to captalise on Dakar rally success. Mostly a cosmetic exercise.
Many had 'exo-skeleton' fairing frame, plastic sump guard, too much weight, lairy paintjobs, etc...
Which one you want is up to you. I give some pointers:
- 650s and 800s are sweeter bikes to ride than the 1000, which can be lumpy and vibrates more. The 800 ridden solo is almost as fast as the 1000 in real life. But if you regularly carry a pillion, the 1000's extra grunt will be worthwhile.
- RS fairings are excellent, but the bars are narrow and some people find them cramped. The rider with unusually long legs may have trouble fitting behind the RS or RT fairing. The footpegs are adjustable, but not by much.
- GSs (particularly later models) have high seats.
Most 80/100 road models should come with panniers. An RT advertised without panniers and frame is probably ex-police. Giveaway is colour, white is ex-police, RT civilian models usually had very nice smoked paintjobs.
TwinShock / MonoLever / ParaLever / TeleLever
Early bikes - up to 1984/5 - were twin-shocks of traditional design, with the driveshaft running in oil in the right leg of the swingarm.
MonoLever came in from 1984 on - and is simply a single-shock single-sided swing-arm. Shaft running in oil.
ParaLever, from about 1988 on, is single-sided, with an additional Torque Arm below the right side of the swingarm. It was designed to reduce the rising and falling of the rear of the bike 'jacking up' the rear under acceleration/deceleration. A bit of a non-existent problem really. All bikes will rise at the rear as you accelerate, and squat as you decelerate. Anyway, BMW invented the ParaLever, and claimed it did away with the 'problem'. ParaLever driveshafts run dry, and need replacement sooner than older types running in oil. And there are many more bushes and pivot points to keep an eye on.
TeleLever came in around 1994, as a refinement of ParaLever.
Exact dates of introduction will vary by a year or two, depending on the model you are looking at. (Early G/S models, possibly also ST, were all monolever from the start of the model run '81. Police-spec models seem to have stayed twinshock for longer.)
A good description of how ParaLever works is here www.largiader.com/paralever
For detailed Technical Specs by model - weights, wheelbase, power, etc:
- verrill.com - Chronological listing with pictures and detailed specifications.
- A big text page with all the 1970-85 engine 'engineering' specs you could want, from compression ratios and power outputs to valve spring free length and gudgeon pin diameter and torque wrench settings and......
A selection of real world photos are here, to help you choose which model you like. And look at other pages there for lists of Paint Codes, engine and chassis number lists, numbers of each model produced, and loads and loads of other detailed reference stuff.
If you have the bike's VIN / Serial Number, this lookup tool tells you exactly which model it is.
Airhead Boxers are very well-engineered and well-built. This makes them nice to work on. There is a lot of information available on the web and via maillists, and there are specialists who will supply any component you need, or refurbish what you can't do yourself. Do not be scared of mechanical work on a Boxer, there are only a few jobs that are beyond the home mechanic.
All electrics are very reliable. Problems will be down to broken wires or corroded connectors.
Anything around the heads / barrels / carbs is easy. Simple engineering, readily accessible.
Frame and Cycle Parts are long lasting. Bearings last forever unless the grease has dried out, or 5-yearly adjustment has not been done.
The clutch is buried deep, the swingarm and box have to come out. (I have done it by unbolting the box and levering it back in the frame, then wriggling my fingers in the gap, but I will not do it that way again.)
Any work on the drivetrain is tricky. The shaft itself is the only 'Amateur-replaceable' component. Any Gearbox or Bevel Drive work needs a specialist. Or at least a very well-equiped workshop and considerable experience. For example, the gearbox output flange is tightened to 200 ft-lbs. That's more than just about anything else you will ever work on. And there is nothing to lever against unless you make/buy/borrow the specialist tool. If you replace gearbox bearings, you should re-shim all shafts. The cover must be heated to remove and replace. ....If you feel brave, go for it. Or buy an exchange rebuilt box from a specialist for £300-£350, fit and forget for another 100,000 miles.
Same with the Bevel Box. It is possible, with extreme care, to replace the output shaft oil seal - but if your twinshock drive splines are gone, you will not replace them unless you are a specialist engineering firm. But again, £300 or so to a specialist solves the problem for another 100,000 miles.
BTW. If you remove the rear shock(s) - never ever allow the swingarm to drop. Support it with block of wood or something. Letting it dangle knackers the driveshaft.
That sums up Working on a Boxer. Everything is a pleasure to work on and rewarding to do. But you will get on a lot better if you have some mechanical experience and a decent workshop. And sometimes you have to take a hit for specialised work. Oh, about every 25,000 miles or so on an old bike. In other words, in the time it takes your mates to have gone through 2 or 3 new Sportsbikes, you may have had to replace a tricky oil seal.
With regular preventative maintenance, quality replacement parts, and occasional specialist work - you could run a Boxer for 20 years. On that timescale even expensive parts become cheap.
Yep, here come the boring bit where he tells us to spin our nipple-nuts every week......
As soon as you get the bike, and every year or so, touch every nut and bolt you can see with a drop of oil. You could easily have a 30 year old bike that has never been apart. In 5 years time, if you do have to remove something like the rear subframe, you will be glad you put those drops of oil on. The anally retentive among you will see if the long bottom engine mounting bolts are free. If they are, get them out, clean them up, and put them back in with copper grease. If not, leave it, and worry about getting the hacksaw out if you ever do have to remove the motor.
Be religious about oil changes. Every year, drop all oils and refill. Oils should come out clean, and not look like they need replacing. They probably don't, but one of the things you are doing is looking for metal filings or swarf in what comes out - an early indication of failure. Do not forget the driveshaft tube, 100cc of fresh oil in there every year is cheaper than a new shaft. Inspect the rubber boot at the front of the driveshaft - replace if hardened or perished. DO NOT OVERTIGTEN oil drain plugs, they are easy to strip.
If you have the rear wheel off (for a tyre change, perhaps), clean out the oil drainage drilling which runs from next to the bevel box oil seal to the bottom of the casing behind the drain plug. It is always blocked with crud on a high-mileage bike. This drilling allows anything seeping past that seal to drain away before it gets on the brake shoes, which is good. But more importantly, you will then see any oil dripping out giving early warning of seal failure (later models and disk brake models don't have this drain.)
While the wheel is off - and every year - wash clean and lightly re-grease the drive splines with plain grease. Grit and brake dust accumulate in there, and make excellent grinding paste on the soft splines! Inspect them, splines should be clearly 'square'. If they are pointy and sharp, you are on borrowed time.
Learn the bike. If anything changes, gets 'sticky', feels 'funny', just 'something aint right' - investigate. On a Boxer, catching problems early is always beneficial.
Use quality parts. They cost more, but are always worth it. I recently had an HT lead fail. On a 28 year old 80,000 mile bike, one of the original leads failed internally. What would you do? Buy £5 worth of cheap lead and a plastic cap from your local accessory shop? Or replace both leads with original spec at £20 each. ...And not expect to have to do it again for another 28 years.
If you are facing some significant work and need to run the bike on a budget, remember that most parts are interchangeable across engine sizes and models. If your R80 bevel box goes, and you can find a good used R100 unit, just bolt it on. There were many minor changes around 1980/81, but most major components and sub-assemblies can be swapped between models of the same year/series. Ring a specialist parts supplier like MotorWorks or MotoBins and ask. (MotorWorks are themselves well known suppliers of good used parts, they break bikes for this purpose.) (R65s used the Type 248/1 engine, not the R80/100 Type 247. Individual engine/box parts are not so easily interchangeable, though major assemblies are, for example the entire 80 motor can be dropped into a 65 frame.)
The Haynes Manual is worth having for the wiring diagram and the oil capacities and such, but it stops well short of what I seem to want to know. Far too often it is 'remove the assembly and take it to your local dealer'. The Clymer manual may go further, I don't know. Original Factory BMW manuals sometimes come up on eBay, or can be had from MotorWorks or MotoBins for £30-40. The official manuals don't explain things in any detail, they have more photos and diagrams than Haynes or Clymer, but they assume you are a skilled mechanic.
Nowadays, if I want the last word on something, I go straight to Snowbum's site. If you want to do anything seriously mechanical to your bike, follow his advice to the letter.
If I can't find the answer there, I ask the Airheads mailing list.
If you are replacing fork springs or shocks anyway, go for 'Progressive Springs'. They will not turn it into a sports bike, and are not worth fitting for the performance improvement alone, but if you are replacing soggy springs anyway, it is a useful upgrade. I also like a fork brace, but this is of marginal benefit.
Twin plugging the heads is much discussed. It may help the bike burn cleaner, particularly with modern, less-volatile fuel. But it is unlikely to reward the expense of having it done. This is not a sportsbike, an extra half horse-power isn't going to change your life.
Cast disks. I'm sorry, but BMW got this one wrong - the Original Equipment stainless drilled disks on twinshock and monolever bikes are complete crap. With high-quality pads, the stainless disks may work reasonably well, but will wear quickly and are prone to cracking. #1 Boxer performance mod is to get front brakes that work. Fit cast disks, good pads, and stand the bike on its nose with 2 fingers. (If you are replacing the front disks anyway, they are often cheaper than OEM, making this a no-brainer.)
They got it right on the later Paralever bikes, a different design to earlier bikes, and first class stoppers.
BTW, some say that a handlebar-mounted master cylinder conversion is the way to improving master-cylinder-under-tank brakes. For up to £200, I am sure it works fine, and here is an article. But I reckon the reason it improves things is because the remote master cylinder under the tank often gets ignored in regular maintenance. Pull the tank off now and again, grease the pivots and check the cable adjustment. Spend your money on cast disks instead. I never found much benefit in braided lines either.
Lecky ignition. Post-81 bikes are electronic ignition from new. Older Airheads are traditional points, located behind the front engine casing. These points go out of adjustment easily and should be checked every 6 months or so to ensure smooth running. Replace with any aftermarket electronic setup, time it very carefully, and forget it forever.
/5 /6 Gear Linkage The /5 and /6 series bikes have a gear lever which some people find short and awkward to use. The later /7 series 'kinematic linkage' is easier to use, and an easy upgrade.
Anything else is personal preference. Heated grips are available - and often fitted to RTs from new. Radios can be fitted to RT fairings, some had them from new. GS models have a huge range of ally boxes and topboxes and handguards and sumpguards and crashbars and....
A matter of personal choice, but why not end on a contentious note ;)
Many Boxers will be running Metzeler tyres, in particular the 'Laser Pattern' front. This has a fairly smooth tread, with the blocks in a 'V'-pattern. Fine in the dry, rubbish in the wet, and I never got good mileage out of these. They were fitted by most Police Forces when the Boxer was a common bike for them - and who am I to disagree with a Police Class 1 rider, but......
The better choice is Continental, they used to do a tyre known as the 'Big Twin' or 'Super Twin' - TK22. A traditional ribbed pattern, it gripped well wet or dry and gave good mileage. I never had cause to worry about different rear tyres on a Boxer, but usually fitted Continental's TK44 to match the front. For late-model tubeless wheels, the designation was TKV. You will be lucky to find these now, Continental have changed all their ranges, and what they now call it is RB2 and K112. They look the same as the TK22/TK44, and come in sizes for older boxers. For more modern (late 80s/90s) bikes, you need bigger sizes, the last time I changed a rear all I could find in a 140/80x17 was ContiGO, which looks a bit sporty for my taste, but seems to work fine.
For significant mileage on rough roads - say RTW on a GS - no road tyre will be good enough. I used to use Avon Speedmasters (showing my age :) but nowadays you probably want 'Metzeler Saharas' or similar.
References and interesting reading
Some nice cut-away Boxer Engine Pictures.
An article on serious tuning of R100 engines. How much to skim off the flywheel, which rocker shims are lighter, etc, etc. I can understand 'making it run sweeter', and if you need to have your valves seats refaced anyway, have them cut as he suggests.
A huge list of maillists is here, dig down for, BMW-GS, BMW-Tech, BMWCYCLES (in Dutch), ....
I'll add more links here as they occur to me.
If you are looking for other detailed stuff yourself in Search Engines, the good keywords are: Airhead, Type 247.
We have concentrated here on Airhead Boxers made from the 70s on, the /5 /6 and /7 models, the ones we are likely to find in the classifieds and buy and ride regularly. Older BMWs are rarer, come available less frequently, and are only really of interest if you want a 'proper classic'. If you are looking for older, pre-70s BMW information, start at vintagebmw.org
There are other resources out there, see in the text above or your favourite Search Engine. This has been a Guide to Airhead Boxers for the UK biker. If you know of more useful Airhead Boxer resources - relevant to the UK biker - that I should add to this page, let me know.