A stab in the dark here. I've got some 748R crankcases that I need to bore out to receive some 916SP cylinders (which I've had bored to ultimately make a 955 lump). The question is what is the preferred clearance? I've measured the original cylinder to crankcase clearance and it's about 0.25mm which seems a bit sloppy to me. Considering everything starts getting a little thin in this scenario, I wonder if the clearance in the corse bikes was a little tighter?
Nope. The cylinder extends into to crankcase below the base gasket. On 748/916 machines the external diameter of the cylinder at this point is 100mm. 916SP machines were 102mm so the bore could be increased to 96mm and thus 955cc for racing. The recess in the crankcase was larger to take the oversize cylinders. Subsequently the crankcases were redesigned with a larger stud spacing to allow bigger bores and hence the 996. On the 748R I have on the bench the OD of the cylinders is 100mm, the ID of the crankcase is 100.25mm, it is this clearance to which I refer. The 916SP cylinders I have are a gnats pube shy of 102mm OD. Should I bore the crankcases to 102.25mm to maintain 0.25mm clearance or go a little tighter?
When I built my 955 851 I turned down the outside of the 916SP barrels to 101mm and then machined the c/cases to a sliding fit. This increases the strength of the C/cases, it was a mod' used on endurance race motors when they found the 102mm bored cases failing. I also used NOS late model ST4 cases as they incorporated the lessons learnt from the 851/888 and 926 racing programme, so they're a lot stronger then standard 888/926 Corsa and road items.
Thanks for this, thats pretty well much what you told me over on the 851/888 forum years ago when I started looking for the 916SP cylinders. As an aside, yesterday I was putting the 748R crankcases together ready for machining and I noticed the bolts didn't match what was stated in the 748R service manual, odd I thought. Checking various part lists and service manuals has lead me to conclude this 748R engine was built with ST4 cases. I don't know much about the history of this lump, I bought it from a dodgy garage in Peterborough 10 years ago, they were breaking the whole bike. I guess it must be a late one if the cases are ST4?
Yep my bad, serves me right for only half researching and jumping to conclusions, the 02 Service manual and the 02 parts manual show different joint arrangements. The late 748R does have different crankcases and the jointing bolts appear to match that of the ST4, which in turn is very similar to the 996 with the exception of the two in clutch side which are M6 on the 748R.
