Over size sleeve.

I was wondering if it was possible to take a Briggs flat head in the 3-4 hp range, bore out the block and press in a sleeve that would allow for a huge overbore. I haven’t had one of these engines in many years,so please forgive me if I don’t know the limits of what could be done. Has anyone considered doing this? The advantage, in my mind would be to have a small frame engine with a much larger displacement, that would easily fit a small frame mini bike. Jay


Well-Known Member
Sure.... it can be done but maybe not to the "huge overbore" you're thinking of {whatever than means}. I mean there's only so much room in the block before you interfere with the internals, the camshaft and/or the valve gallery. You'd be better off with a Briggs Raptor block or 5 HP I/C and going the high power/high speed route because there's just no aftermarket speed parts support for anything under the B&S 5 horsepower.

A 4 HP Briggs flathead is almost the same size as a 5. Why not go for a 1 or 2/2.5 HP Briggs?