Ladies, I thought I’d do anther ‘engine build’ type thread for those nerds that are interested in such stuff. I thought I’d do it separately to my previous one and title it as a 920 build to help those searching around for 920 build info in future.
Well, my story is that I bought a Maney crank to put in the ‘Blue Bomber’. Then after a lot of thought, I basically decided that the best place to put such a nice crank would be in some nice strong Maney cases. Then, further thought deduced that such a strong bottom end justified some improvements with regards to output. And so, another 920 project was born!
So I got a set of cases from Steve Maney, machined to accommodate a standard primary case, this weakens them somewhat, but I really wanted to keep the stock primary case set up. It will still be plenty strong enough for my needs, according to Mr Maney.
At my request, the cases have no holes drilled from crank chamber to timing chest, this saves me having to plug them for using a Comnoz / cNw reed valve breather. Maney cases come machined to take the Combat style breather at the rear of the case, so I’ll use the cNw style breather that bolts directly into this location.
As mentioned previously, I already had a Steve Maney crank on the shelf that I bought earlier. I sent this back to Steve for him to fit the main bearings and shim up accordingly to the new cases. I know I could have done this myself, but being in ‘this deep’ already, I thought I’d let a specialist do it right! The cases have been machined to accept a set of Steve’s 920 barrels, which have been finish bored to 81mm in readiness for the JS pistons.
I have had extensive conversations with Jim Schmidt (thanks Jim) regarding pistons. I have used his high compression 850 pistons very happily hitherto in the Blue Bomber. We arrived at an 81mm 920 design based on the spec of the 850 high comp pistons, with extra material in the crown to allow for a 1mm dish machined into the crown. According to my ‘fag packet’ calculations, this should allow a tight squish and keep the CR at a reasonable 10.5:1 (or there about). I’m aiming for a 030”-.035” squish clearance, which is what I am currently running with no sign of any contact anywhere. The pistons will be hard anodised with the ‘Diamondyze’ process as per my current 850 pistons.
The fabulous Comnoz ported RH10 head will be used as is, apart from opening up the squish area to suit the 81mm bore.
I really like the BSA cam followers that I am currently using with my JS1 cam kit, but the flat lined peak BHP graph that I had on the dyno (virtually flat from 6300 to 7500) led me to conclude that a hotter cam might help liberate some power above 6300, plus the increased displacement will have increased breathing requirements, so, after much deliberation, I have opted for a slightly hotter JS2 cam. The JS2 has the same duration as the JS1, but has more lift, and is very similar to the PW3. I will be using a Webcam hard welded JS2 ‘smoothramp’ version. Actually ‘Brooking850’ has a large part of the blame here as his glowing praise for the JS2 cam in his race bike certainly helped to persuade me in that direction!
Its not likely to be a very quick build, I am aiming to fiddle around collecting the necessary parts (and spread the cost) and hopefully will get the engine built and nailed in to the frame next winter (2017-2018).
I’ll post some pics of various bits n bobs soon.