Finding an 81+ R80 cylinder and headset used and in good nick would probably get you most of the way there, too.
Using R80 jugs was always the starting point - But they need to be "trimmed" to suit the short stroke of the R65. Measuring the amount to trim off and paying someone to do that work is non-trivial and would I suspect add up to a tidy sum very quickly.
At the moment the whole thing is a "flight of fancy" on my part, I'm really only interested in doing it if it is a bolt up job and if I am not buying myself a world of disappointment.
I have to say that my research in the early 80s turned up a few people who had by various means increased the capacity of the R65 engine, at that time none were deliriously happy with the results for reasons such as vibration and short life.
When in 1987 the original R65 engine in my wife's bike dropped a valve and punched a rod out of the block I could have obtained an r65 engine with damage to the opposite side nothing. At that time I again investigated "big bore" kits for the r65 and again could only turn up a few people who had done it. Also in keeping with my research of a few years prior the only peopel who professed happiness with their big-bore R65s strangely seemed to want to sell them to me rather than enjoy all that new found power.
Against all that my thinking is:-
The R65 bottom end is just as tough as any other Airhead - the only real difference being the cam followers and then they were only changed to keep the push-rod angles more or less the same as other airheads.
Seibenrock are a mature and well respected manufacturer who generally make well regarded products. The only negative I have seen regarding their R65 big-bore kit is that 2 independant tuning houses say that they have used entirely the wrong "squish" band for the pistons and that you need to modify the squish in either the heads or the pistons (one of the tuning houses showed detail of how they modify R80 pistons).
Now this may seem trivial to some people but the squish band in the combustion chamber is pretty much where trouble free performance lives and dies given the poor quality lead free fuels we must now use.
What I really want is someone who had fitted one to tell me dispassionately how it runs on 92 Octane (RON method) unleaded fuel and whether the whole experience is "pleasant" or not.
