Bigger Piston sizes

[Hunsta]

OK this one is just a question Ive been wanting to ask. Being reletively new to Airheads, I have heard stories of people putting larger capacity cylinders and pistons on smaller capacity bikes like 100`s on an 80. So is there a larger capacity pot and piston that fits the R65?

[Yikes]

From what I''ve read, the high reving R65 engine isn't happy with the bigger jugs and pistons.  Best to get a bigger bike.  I've no real experience though.

[Bob_Roller]

Luftmeister, a BMW aftermarket parts supplier in the Los Angles area (now out of business), had I believe an 850 cc upgrade kit for the R65 .

From the accounts of owners that installed the kit, the engine would develop bottom end problems within 10,000 miles or so .

Now if you want to install a larger engine into the R65 frame, that seems to be the way to go for more power .

If you want a faster bike than an R65, it would probably be easier and more cost effective in the long run, buy a faster bike .

[Barry]

I've always heard big bore conversions on the R65 engine are a bad idea so the conventional wisdom is fit a bigger engine.

Hard to understand why that should be though as the crankshaft and bearings are just as substantial as bigger airheads. I've even heard it suggested that the short and long stroke crankshafts are the same basic forging machined at different centres as there is only 9.1 mm difference in the stroke. Might just be a myth that grew in the telling.  The place to ask would be the Airheads list where the long time shop owners like Tom Cutter may have done some of these conversions.

I've just checked the Airheads list and someone has recently started a thread on R75 pistons in an R65 block. Only 2 posts so far.

[Hunsta]

I've always heard big bore conversions on the R65 engine are a bad idea so the conventional wisdom is fit a bigger engine.

Hard to understand why that should be though as the crankshaft and bearings are just as substantial as bigger airheads. I've even heard it suggested that the short and long stroke crankshafts are the same basic forging machined at different centres as there is only 9.1 mm difference in the stroke. Might just be a myth that grew in the telling.  The place to ask would be the Airheads list where the long time shop owners like Tom Cutter may have done some of these conversions.

I've just checked the Airheads list and someone has recently started a thread on R75 pistons in an R65 block. Only 2 posts so far.

I do understand the thinking of most of the replies. Yes If I wanted a bigger bike it would be easier to just get a bigger bike. i just wanted to know if it was possible, like the larger airheads can do.

I`ll check out the airheads list
 Cheers
Craig

[nhmaf]

I think it would be in most cases, more economical to just get an R80 engine from an otherwise broken bike and transplant it.   The crankshafts are milled and balanced based on the piston weights, so to keep the engine running smoothly you'd be replacing much of the bottom end even if the existing crank and con-rods are "strong enough".

With the r90 - they bored the R75 cylinder out and used the same stroke, but to keep the engine vibrations in check they needed more crankshaft offset mass.   They couldn't make the crank sufficiently massive enough to fit in the same gallery space in the cases, so they drilled out parts of the crankshaft and inserted tungsten plugs to achieve the greater mass.   It could become a "diminishing returns" kind of exercise, plus with the Nikasil cylinder bores, one can't simply bore out and re-hone the jugs like one can with the old cast iron cylinder liners, so it becomes a wholesale swap of cylinders&pistons&heads, etc.