Looking to see if this has been done before. I assume it has, as the fat fin cylinder isnt that old of a thing. Looks to me like you could just fill the area next to the cam chain tunnel with devcon or something and forge ahead.
This was quite common in sidecar cross during the 1980s, combined with 87mm XT/SR500 pistons.
Stock stroke results in 880cc, long stroke (84mm) crank very close to 1000cc.
Depending on core shift of the cylinder casting, the 91mm bore would, or would not, break through into the center air channel.
If it did, some oil would sweat out of the channel to the rear of the cylinder. Which could be remedied by closing off the air channel with some kind of epoxy goo, or welding it shut.. blocking the cooling air through the channel at the same time.
Support for the cylinder sleeve would be nil at the break through, resulting in (even more) distortion of the bore when hot.
Not pretty, but it worked.
The method described by Smedspeed looks much more refined and reliable, also w.r.t. support for the cylinder liner and the gasket area.
However, if you just want to have fun on a limited budget, using the original casting should work.
Another variant is to use 85mm pistons, like from an SP370 (840cc with stock stroke).
They're rare as hen's teeth now, but maybe there is a usable alternative somewhere.
That would leave some more meat in the cylinders and/or sleeves to keep things in shape.
Pursuing a big bore engine like this is (potentially, depending on your ambitions) more than putting in bigger pistons and sleeves.
At least, pistons need modifications for compression and valve reliefs, and you need to check all kinds of details along your build.
Choose the correct length conrods for your particular piston.
Consider welding the crank pins for some reliability.
Once it runs, you'll find it has loads of torque down below, but runs out of breath at higher RPMs.
To compensate, you'll want head modifications, different carbs, different gearing.. and you'll get sucked right down the rabbit hole.
Is it fun, even in its simplest form? You bet!
Top to bottom:
- 87.5mm cylinder with modified XT piston
- 85mm cylinder (XS1, note the bridge in the cam chain tunnel)
- 84mm cylinder with Porsche piston
- stock 75mm cylinder with XT piston on top for comparison
