I want to use some 88mm pistons and cylinders in a 650cc engine and, as I expected, I was told by a local machinist how difficult it would be to bore out the spigots in the case to the larger 750cc size, which is about 96mm; mainly, the oddness of the case makes it hard to set up. An alternative would be to turn the 750cc/88mm cylinders down to the 93mm size of the 650 case, but that would leave only a 2.5mm-thick cylinder wall at the bottom. Is that acceptable? I do see that the thickness of the cylinder walls in the 90mm kits is 3mm, and they do OK, but even-more thin makes it more risky that the cylinders will warp. Any help with this is appreciated. Does anyone is the US have recent experience with someone boring a case for them? Thanks, MikeV

Mike,
That's really easy to do. Any place that works on two strokes can bore that.
Otherwise I have a guy here in Minneapolis that can do it. A little slow, but great work.

I wouldn't turn the spigots down that thin. I saw an 888 motor that had the 94mm cylinders bored to 96mm. The spigot OD was 100mm, so 2mm thick wall. The spigots broke off as a ring, and rattled around the con rods. Wrecked everything.

Reed

With steel liners you could probably get away with 2mm walls,but it would be better to bore the cases.I've seen bevel cases taken out to where you could almost see the stud threads in the spigot hole.I'm pretty sure the 650 cases have enough meat to bore,remember it's only about another.060" closer to the studs.
The problem is finding someone with a milling machine tall enough to accept the cases standing on end or finding a horizontal milling machine.

The spigots can be made as big as the F1/Sport/Paso/Sport, 96.3mm, it is just finding someone near me who can deal with the odd setup that is the problem. It does seem that laying the case on one side would be much easier, so if there is a machinist around with a horizontal mill that would be the good setup. MikeV

Mike,

We've got a small Cincinnati horizontal here in the shop in Springfield. If you're not in a hurry, I could do it.


Geoff

Thanks Jeff. Maybe we can. Do you know how many Springfields (home of the Simpsons) there are in the US? We have on about 18 miles from here. You are in the US? MikeV

Interesting topic Mike.
It is a modification that I have thought in my project...
but for a future.
I have the engine(650) and cylinder/Pistons(750).

Hey Geoff, I'm in Springfield too! Well, North Springfield....
Bill

The one 18 miles from you. You sold me a fuel tank a little while back

And the home of the Simpsons