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?
Ahh ok, so you're talking about the bore of the cylinder sleeve external diameter relative to the bore of the Crankcase cylinder clearance bore internal diameter, gotcha. How much meat do you have as a safety margin to the studs by boring out +2.25D? What's the intended application? Race?
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.
