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BMW airhead conversion


Mike Meakin

2CV bellhousing
2CV gearbox

Of the many choices available to the Pembleton builder, one option is to use a BMW Boxer engine – a 2-valves-per cylinder, aircooled flat twin. The "R" series use a common crankcase with capacities from 598 to 980ccs, slightly wider than the standard 2CV, but shorter from front to back. The slightly heavier BMW unit contains an integral alternator and starter motor, units which with the 2CV have to be added externally. Typical BMW outputs range from 45 bhp (R65) to 70 +bhp (R100) as standard, before any tuning. The R80 engine (50 to 55 bhp) will propel both trikes and Brooklands comfortably – there is always the option of upgrading an R80 to R100 capacity.

Used Boxer engines are plentiful and properly maintained are reliable and long-lasting. Buying a functioning BMW bike gives the opportunity to test the engine, before dismantling and ensures you have all the necessary ancillaries for a complete conversion. There is a number of specialist motorcycle breakers who will sell a complete engine, sometimes with limited warranty.

PMC flywheel

Physically, the BMW engine requires some modification to enable coupling to a 2CV gearbox. The pre-1980 solid steel flywheel is not used, instead a bespoke flywheel from Pembleton Motor Company comes with the substantial boss on the back, to fit to the BMW crankshaft and oil seal. The ring gear from the later BMW fabricated flywheel has to be sourced, the rivets drilled out so that M6 bolts can fix the ring gear to the bespoke flywheel, enabling the use of the BMW starter. It is VITAL to tighten these bolts with Loctite or similar – with flywheel installed, these bolts are not accessible. The other side of the flywheel is drilled to accept a standard 2CV clutch. Socket head cap bolts (single use) attach the flywheel to the BMW crankshaft. It is essential to accurately measure the internal diameter of the recess on the back of the flywheel to compare with the external diameter of the crankshaft boss. It has been that the flywheel recess has been machined slightly smaller than required, preventing the flywheel from fitting fully to the crankshaft boss. Signs of this are when the ring gear teeth touch the inside of the BMW housing. The cure is to remove the flywheel and have the recess machined to match the crankshaft boss – an ultra precision process.

On the motorbike engine there are 2 lug mounts cast in to the crankcase top behind the engine. These have to be carefully removed, taking care not to break through in to the sump.

Modified clutch forks

The sump on a Boxer engine extends beyond the underneath of the flywheel in to the 2CV bellhousing, which PMC cuts back (allowing the gearbox input shaft to reach the clutch centre and flywheel), with a register plate of heavy aluminium being welded to the shortened bell housing. A shortened clutch fork, necessary because the bellhousing has been cropped and the register plate, prevents the standard arm from coming forward, is part of the conversion kit from PMC. The modified fork MUST be checked to ensure that the "fingers" allow full retraction of the release bearing. Initially, my shortened clutch fork caused pre-loading of the clutch, preventing full engagement.

Modified clutch forks

The length of the 2CV gearbox input shaft beyond the splines has to be reduced to prevent it from bottoming out in the centre of the flywheel. Grinding off the recessed end of the shaft using an angle grinder, whilst someone turns one driveshaft plate (with the other fixed) the opposite way to the grinder allows an even finish with rounded shoulder. This helps when inserting the input shaft in to the oilite bush, which supports the plain end of the shaft in the flywheel centre.

It is wise to fit the loose clutch cable to the fork at this stage. With the extended sump in the way, it is extremely difficult to hook up a clutch cable . Connecting engine to gearbox is the usual process, aligning clutch driven plate splines with the input shaft. An engine which goes "clunk" when the bellhousing and crankcase meet, is good. A mating where the 2 surfaces do not quite meet would suggest the clutch fork problem may be to blame.

The engine/gearbox assembly hangs from a fabricated bracket with a tubular top, fixed by 2 outer bolts through the register plate and the long starter fixing bolt. This long bolt screws in to a thread at the front of the starter and is notoriously difficult to "start". Fix this bolt loosely before the others. The lower 2 crankcase/bellhousing bolts are almost impossible to access once the car is built up – the front discs are in the way.

Additional frame

The whole assembly hangs between the frame top mounts on a substantial pin with threaded ends. It can be very reluctant to push through the tubular mount, so burnishing may be advisable. Having got it through without damaging threads, it can then be difficult to screw up nuts on each end. Cutting a spare nut in half enables a locknut arrangement. There is a built-in hole through the crankcase from side to side on the bike, for footrest attachment. Using M12 threaded rod through this fixing provides a very substantial mount for a tubular frame, with fixing hooks which hook over the engine mount spindle. This frame can act as a "100mm test sphere" obstacle, sump guard, number plate and oil-cooler supports. "Airhead" engines can run hot: sustained engine speeds above 6,000 rpm will significantly heat the oil, especially if touring abroad in summer. The fitting of a welded-gallery, oil cooler is recommended. Flow and return ports are on a replacement oil filter cover – a clever system where the oil does not travel round the cooler until hot, after viscosity drops. Allowing the engine to get excessively hot (oil temperature 140 degrees) runs the risk of differential expansion pulling the head studs out of the crankcase threads.

Something to check on a potential donor engine is the condition of the exhaust port threads. The exhaust headers are held in place by finned collars, which run on threads cut in to the outside of exhaust post castings. Sealing of the headers is by a compression type spring steel, split ring, bearing against a solid "V" ring. The collars are done up with a "spider wrench" and heavy hammer. The integrity of the threads on the cast exhaust port is vital.

Bing 40mm carburettors

Carburation – BMW Boxers come with a variety of carburettors. Those which attach to the inlet stub via a rubber sleeve, would normally also be firmly connected to the bike’s air filter chamber. Since the Pembleton’s 2CV gearbox is in that position, the carburettors need physical support to keep them at the correct attitude (they are sensitive to angle) and to prevent them "popping off" their rubber sleeves. The 40mm Bing carbs normally on R100 engines are physically large. The forward frame downtubes restrict where the carbs may sit and the inner driveshaft gaiter (especially on the offside) is uncomfortably close to the underside of the float chamber. On the bike, BMW carbs are gravity fed, from a tank above. Float jets and needles need to be in pristine condition. Using a low-pressure fuel pump, a line pressure of 1.3 psi in 8mm bore pipes from pump to carbs, has been found to be the optimum, with low pressure but adequate volume. Some have fitted an in-line, adjustable fuel regulator as an alternative to an excess fuel return back to the tank. An alternative for a 980cc engine is to fit inlet stubs from an R90S – these take Dellorto, clamp-on 38mm carbs – only a very small amount of top end power is lost. The 32mm carbs, standard on the R800 are physically much more compact and can be accommodated easily. Correctly jetted, they will satisfactorily fuel the 980cc engine (the R800 and the R100 have the same size inlet valves).

Ignition: standard on the BMW is points, condenser and advance/retard bobweights in a "bean can" housing under the alternator. The system is sound but requires maintenance; condenser leads have been known to abrade and earth out. A digital, electronic ignition system (particularly if the engine has been twin-plugged) requires no maintenance, once set up and should be considered.

Electrics; the on-board alternator is not "self-exciting" and needs a feed to the earthed brush to begin generation. Reliance on interconnected components to provide an earth path is not advised. Direct earth leads from the battery earth to the diode board mount, the starter body and a dashboard earth point is advisable. The use of a solid state regulator commonly used by Police and the armed forces allows a full charge from the alternator at earlier stages than normal.

A properly fettled BMW 980cc engine, twin-plugged and with digital ignition will produce some 75bhp and weighing less than 400 kgs provides for around 180 bhp per tonne. Subjectively, it makes a wonderful noise between 4,000 and 6,000 rpm but burbles along nicely at just under 4,000 rpm, normally returning between 40 and 50 mpg, driven sensibly.

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