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|Bike Profile - Posted 4th May 2011|
Converting a classic Airhead BMW from R80 to R100; just a matter of swapping pistons and barrels, or is there more to it than that? Martin Gelder prises open a Pandora's box of oily mystery...
"What if it's already a thousand?"
"Well your bike is making great power for a standard R80, let alone one with a lot of miles on it. So what if someone has already done the 1000cc conversion?"
Oh lor. I hadn't thought of that.
Last month, in the interests of science, I stuck my 100,000 mile BMW R80/7 on a rolling road dynamometer to see how much rear wheel horsepower it was making. Its exhaust valves have started to recess into the cylinder heads and as I've got a spare set of 1000cc barrels and pistons knocking about, I thought it would be interesting to do a before and after comparison of a standard R80/7 versus one converted to the full-fat one thousand (it'll actually be 980cc, but BMW called their bigger airhead boxers R100s so we'll follow their convention).
The general consensus of opinion was that the resulting 43.9bhp was pretty healthy for such a high mileage motor, particularly one with valves that were on the way out, tired piston rings and all the other afflictions of general wear and tear.
The possibility that it was already running R100 barrels hadn't entered my head. As far as I knew, it was completely standard and probably still on the original rings. The cycle parts certainly feel a bit baggy and you're never in any doubt that you're riding a bike that's been once round the clock. It weeps oil from every orifice and there's a bluish tinge to the exhaust at times, but by and large it's a reliable bike.1977 BMW R80/7: By and large it's a reliable bike....
It's only really the rapidly closing up exhaust valve clearances that have been giving me cause for concern... Until someone suggested that rather than a fairly fit eight hundred, I might have a flippin' feeble thousand.
Armed with a set of second hand barrels and pistons of unknown provenance, a set of 'good' R80/7 cylinder heads recently removed from The Vicar's Bike and a big box of new piston rings, circlips, pushrod tubes and gaskets from Motorworks there was nothing for it but to prise open Pandora's box and find out what horrors lurked within.
One of the beauties of seventies BMWs is that they come apart like Lego. Even after thirty five years of neglect. After a few squirts of releasing oil and a kettle full of hot water I managed to undo the exhaust nuts on *this* bike without breaking any special tools and from then on removing the heads didn't need much more than the tools I carry under the seat.85mm bore reveals that the engine is still an 800; 1000cc barrels have a 94mm bore.
The Vernier calliper confirmed that the bike was still an 800, and although the piston crown was bit oily, there was no sign of wear on the barrels. No sign of looseness in the big or small ends either, although there was a small amount of side play on the big ends. I could probably have got away with just fitting new rings, but where's the fun in that?Exhaust valve (left) is eating it's way into the head. Inlet valve (right), isn't.
With the old heads off and put next to the "new" ones, it was easy to see where the exhaust valve had been eating its way back into the valve seat. This photo doesn't really show how clear it is, but you can see that while the inlet valve (on the right) stands almost proud of the head, the exhaust valve (left) has sunk so that it is becoming recessed into the head. This is the main reason I'd opened the engine up; how much longer would the bike have continued running like this, while it slowly devoured itself?
The 1000cc pistons share the same little end size as the 800s so I used the gudgeon pins from the 800 in the bigger pistons, but with new circlips. The R100 pistons also got new rings and the top end gasket set provided o-rings for the upper cylinder studs and the barrel base. I fitted stainless steel pushrod tubes as these are a bit of a weak point; the stainless ones will probably out last me if not the bike. They're a snug fit in the barrels and despite having spent the night in the freezer they needed repeated firm application of Mighty Thor on the special fitting tool before they were fully seated.R80 piston on the left, R100 on the right.
And then I hit my first snag. There's very little taper on the barrels, and the new rings proved devilishly awkward to get home. I couldn't decide if my piston ring compressor was helping or hindering and I'd chosen to work outside on one of the hottest days of the year. After a bit of welcome advice from the rapidly responsive RC Message board, and after a shady break for lunch, everything finally slipped into alignment. Funny how an awkward job can be made easy by stepping away for a while and doing something else.
Heads back on and torqued down, tappets reset, valve covers on, exhausts on, carbs on, plugs connected, petrol on, choke on, ignition on, push the button and...
...It started. Just like that. Blimey. Nice and steady on half choke, the engine noticeably quieter than before so perhaps there'd been some piston slap prior to the rebuild. Blip the throttle and the engine's still quiet. Blip the throttle again and check the exhaust; no smoke. Blip the throttle again and...
Oil! Oil everywhere! A pool the size of an LP record has formed under each cylinder and is spreading rapidly. Oil is oozing steadily, relentlessly, from the joins between the crankcase and the cylinder barrels. On both sides. And all the way round the base of each barrel.
It's quite an impressive, fascinating sight and I watch it for a few seconds, the way you can watch your finger bleed profusely before it starts to hurt. Then I realise that pretty soon all the oil will have pumped its way out of my freshly rebuilt bike.
I switch it off and it sits ticking and dripping, mocking me.
Something not quite right, obviously. There's no base gasket on an R80 or R100; they're sealed with one big o-ring round the base of the barrel and two small o-rings round the upper cylinder studs. Simple.
So confident were BMW in the barrel to crankcase fit of their bikes that some models even dispensed with the large o-ring, a point which is discussed in my most excellent Clymer manual. "If the bike has a groove in the cylinder base for an o-ring, fit an o-ring" it says. "If it doesn't have a groove in the cylinder base for an o-ring, don't fit one" it continues. What they don't say is what to do if the cylinder has a groove for an o-ring, but the crankcase is expecting a cylinder without a groove or an o-ring.
The following day brought a strange combination of Christmas morning anticipation and exam results dread. If the bike leaked (puked, more accurately) oil with the o-ring in place, instinct said that it couldn't possibly be any more oil tight with it removed.
I start the bike, and there's no oil leak. I ride round the block; no oil leak. Ride to petrol station, no oil leak. Take out mortgage, fill up with petrol, ride home, no oil leak.
That was too easy. For the next fifty miles I'm constantly checking the cylinder bases for signs of oil, but none appears. I change the oil, just to check there is enough in there. Still none of it leaks out. I ride round some more, feeling the engine free up with the passing miles. The sun shines, there is a wedding, there are street parties, the roads are quiet, all is well.
In an ideal world, I'd be writing this report having fully run the bike in and after dyno testing it, so that I could document the improvement. Well, I'm afraid that'll have to wait.
What I can give you are some initial impressions on my R80 with R100 cylinders and pistons.
It's smooth. I was worried that the balance factor might be different, or that just having bigger pistons would make the bike vibrate more. In fact, it's at least as smooth as before up to 4000rpm, and I haven't ventured much beyond that yet. Round town it's as pleasant as before, chuffing along quietly and politely.
The engine itself is quieter than before the strip down, although the racket from the gearbox and clutch is now more obvious.
The blue haze that would sometimes accompany a blipped throttle is gone, and the new pushrod seals have stemmed the flow of oil from below the cylinders to the point where the bike no longer leaves its calling card couple of spots on the floor when parked overnight.
It's crisper. This could just be better compression due to the new rings and better seated exhaust valves, but it's now got 22.5% more capacity and I'm sure that's added more grunt.
Throttle response definitely feels brisker at low revs, but I haven't given it its head at higher revs yet. And because I know it's a thousand now, I'll expect it to feel faster even if it's not, hence the need for the dyno test. I want to put some numbers to the changes, but I want the bike to be properly run in first.
And I think it might need a new clutch before that. One of the signs of the increased gruntiness is the occasional squeak from the dry clutch plate when accelerating briskly, a sure sign that it's ready for replacement.
Finally, if the bike is making more power, the Stop will need improving before I can use that extra Go.R80 piston on the left, R100 on the right.
One of the reasons I wanted to try this conversion was that there is so much conflicting 'advice' about this on the internet. Based on some of my research, I was in danger of ruining my perfectly adequate R80/7, turning it into a vibratory horror which made less power than standard, and to see any real improvement I'd need to spend several times the value of the bike on aftermarket parts and machining.
So far, I think I've proved that wrong. The results with different combinations of heads, barrels and pistons may be different, and it may be that any power gains I'm feeling only exist at part throttle and at low revs, but to ride - and really, that's what counts - the bike is better than before.
And I call that a result.
Words and Photos: Martin Gelder
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