My R90S Project - Airhead Archeology

So far my engine fitted in the bike appears to be:

R100RS cases from 10/80
R90S Barrels & Standard bore pistons
R90S Heads
and I have assumed that everything below the pistons is R100RS from 10/80 to match the cases. I've checked under the front cover and it has the 10/80 bean tin points.

I want to build a motor around the new '76 90S cases, but avoid buying any (correct) components that I've already got, so I've been comparing the brilliant exploded diags and part numbers from each engine (on Max BMW) to understand interchangeability.

It shows that the crank and rods are the same, although the cam chain sprocket and other components differ. So I hope to use my current crank and rods.

It shows the cam as different, which I assume is cable tacho in 76 vs electronic in 80? (my bike currently has a 75 speedo and later e-tacho, so I'll need a mechanical tacho with white dial to make it proper). If I have to buy a new cam is there any good reason not to get a 308deg standard one?

It shows the alternators as almost the same, so I'm hoping the '80 set up, and rest of the charging system, will transfer.

The cam followers and pushrods all look the same, so plan is that they will transfer.

I'm planning on all new bearings, seals etc, so the big question remaining is on flywheel options…


I'm hoping to replace the (recon 4k miles ago) later gearbox with a correct looking early one, but don't want the (very good for a BMW) gear change to get worse or the engine to feel soggy due to an over-heavy flywheel. Should I (can I?) use hybrid internals to have the lighter flywheel in the older cases to get a good gear change and (relatively) faster pickup?
 
Re your gearbox.

I love the heavy flywheel engines but the light flywheel engines rev quicker and do give a cleaner gear change. You could fit a light flywheel and an earlier gearbox case with later internals, if you want to go this way I can help with the smooth sided case, if your current gearbox is an 80 then there's virtually no difference internally to the last of the heavy flywheel boxes. Ideally you want an 83/84 onwards internals if you want the best parts and then make up a box to give you the look you want.
 
The later alternator will be fine on your earlier engine. The 90s cam obviously has provision for points and a tachometer drive. You will need the sprockets and timing chain to convert to duplex chain. The mechanical tach drives off the crank sprocket and you will need the correct inner case and an outer with the cut out for the cable.
 
Rob Farmer said:
Re your gearbox.

I love the heavy flywheel engines but the light flywheel engines rev quicker and do give a cleaner gear change. You could fit a light flywheel and an earlier gearbox case with later internals, if you want to go this way I can help with the smooth sided case
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Yes please! Would you add it to my pile of bits, with the R90S intake stubs too.:thumb2

The gear change on the blue 'un is only slightly worse than that on the Kalahari, and I put that down to the (un-shimmed) excessive side play on the "rear set" lever mounted to the sure foot side stand bracket and my fat feet catching the (far too close) engine bars a couple of times.

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Have you got a thick swinging arm or one with a waist on it?

for all extents and purposes with your light flywheel engine it should be exactly the same as your kalahari gear change.
 
Rob Farmer said:
Have you got a thick swinging arm or one with a waist on it?

for all extents and purposes with your light flywheel engine it should be exactly the same as your kalahari gear change.
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The shaft side has a flare out to become wider near the (Mustn't call it a diff) di… Bevel box.

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This Krauser crash bar is no good for size 11s and was catching my foot, and the lever was wobbling too. I've fixed the crash bar by removing it, but haven't put a shim/washer on the lever yet. It certainly didn't feel bad, although I've never ridden any BMW with a really good (Modern Japanese 4 cylinder or 70s triumph good) gear change, so my expectations were not too high:D.

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That's possibly the original swinging arm from your 90s they beefed up the swinging arm for the 81 models onwards and fitted a shock absorber,it gave a better gear change and improved the handling.
 
Rob Farmer said:
That's possibly the original swinging arm from your 90s they beefed up the swinging arm for the 81 models onwards and fitted a shock absorber,it gave a better gear change and improved the handling.
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It has a bracket on the nearside underneath, but I dunno what for?
 
It's not the original then. That's the bracket for the anchor arm for the disk calliper carrier.
 
It will still have the solid driveshaft. The Cush drive unit only fits in the straight swinging arm.
 
With the three blanks removed (hairdryer and screwdriver) I got loads of blast media out of the oil ways, so thanks all for the advice.:rob

None were actually blocked but I'm sure all the sludgy bits wouldn't have been a very nice start for the new bearings and oil pump she'll be getting.

I've now found a proper R90S tank, taps and filler cap (albeit without a key), so I can make plans for the lovely ginger paint:)

Does anyone have recommendations for a brilliant spray painter of R90S colours, preferably really cheap and in my village?:P
 
markjackson said:
I've now found a proper R90S tank, taps and filler cap (albeit without a key), so I can make plans for the lovely ginger paint:)

Does anyone have recommendations for a brilliant spray painter of R90S colours, preferably really cheap and in my village?:P
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Apart from not being cheap and not living in your village, I can recommend THIS person. Some of the finest work I've seen. I've had work done done by him and he painted my friend's R90/S and it's stunning.
 
Humbug said:
Apart from not being cheap and not living in your village, I can recommend THIS person. Some of the finest work I've seen. I've had work done done by him and he painted my friend's R90/S and it's stunning.
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Yeovil! It's not even the next-but -one village from me!:D

I've already spent most of this year's motorbike money in the deep South West, with both GS and XR having come from the land of Creamed Rice, so The North's balance of trade figures will start to suffer if I shop below Bristol any more this year.:P
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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