Topic: honda 550 big bore pistons 61mm HC pistons in a stock bore 750?

[hondaman750]

How do you all feel about 4th over cb350 pistons (65mm) in a cb750?

[754]

You will, have to trim the skirts, and remachine the domes, and maybe the valve pockets... Then they work..

[hondaman750]

OK I found a set of 4 that have already been in a 750 I was just curious about what the estimated CR would be

[turboguzzi]

check the whole kawasaki 650-750 (and big bore kits) fours family: 15mm pins, bores in the range of 62mm to 69 (wiseco 810), gave a set of stock 66mm off a gpz750 to try to paulages, ask him about them

[HondaMan]

OK I found a set of 4 that have already been in a 750 I was just curious about what the estimated CR would be

This was one of the first 'big bore kits' out there in 1969 (using modified 350 twin pistons). The compression comes in at 175-185 PSI on a compression gage, so it's close to 11:1. If you can install a Megacycle cam like their 125-00 with the longer dwell, it helps to bring it down to about 10.6:1 so the engine isn't so finnicky, hot, and leaky.

The pistons are considerably heavier than "proper" 750 big-bore pistons, so don't expect it to be real smooth: it won't be. The crank weights are a bit light for the heavier pistons. You will also need some strong rods for this much compression.