Who Made These 812cc Pistons?

[KMan]

Howdy All,

Looking for some help determining who made the 812cc pistons in the attached pictures. I recently acquired a project '71 CB750K1 that's been sitting in a garage for at least a decade. The previous owner took it on as an unfinished project from another dude but for various reasons never completed the bike either. From what I've been told, the guy who had it before me had an 812 big bore installed by a now defunct local shop (DC / Baltimore area) way back when. The previous owner also told me when he first fired it up he heard a knocking, shut it down and never got back to the bike. Because of the knocking comment and the fact that the once nice black paint job on the head & cylinders went to heck sitting all these years, I decided to pull the top end rather than try and see if I could get the bike running.

I've looked around on the web but can't seem to find any info on pistons with these markings. One side of each piston has the number "812"; the other sides have some Japanese / Kanji style characters. Inside each piston it has what looks like "KSK" inside a small circle with "KGY-9002" next to it. The domes all have the number "50" stamped in the top center.

The rods all have the part number S25 on them. I don't know if those are stock or were part of the 812 kit.

Thanks in advance!! This is my first ever CB750 so I'm sure I'll be reaching out to all for your wisdom and experience. I'm to the challenge of the rebuild / restoration.

 

[dirtdigger]

Going to be hard to pinpoint them. Everybody made 812 kits for the 750 back in the 70's. Could be Action Fours, but they could be modified cb350 pistons. They are cast pistons so I wouldn't go drag racing with them. I will see if I can find some info on them. Are you going to run them? You will have to have rings made for them. Be best to send a piston off to Total Seal so they can measure the grooves for rings.

 

[KMan]

No plans to drag race or make a bike show winner at all, just trying to build a solid, good looking, regular rider. Why new rings? Is this a failsafe kind of thing? The cylinder walls don't look scored at all so it looks like the current rings were proper for the 812 upgrade kit. Other than being a little gummed up here and there from the oil sitting so long, the bores look really good. Actually, I wouldn't mind pulling the 812 pistons and cylinders altogether and swapping a stock sized CB750 set back in their place. Given the choice between some exotic antique hardware and something easier to obtain / maintain, I'd rather go the easier & more dependable route and pass along the 812s to somebody looking to build a dedicated '70s hot rod bike.

 

[dirtdigger]

Anytime pistons are removed from the cylinders you need to at least hone the cylinders and replaced the rings. They get worn together and when you remove them you disturb that and risk compression loss as well as oil consumption. This is with any engine. You may be able to trade the pistons and cylinder for a stock one to someone who wants a little bigger motor or possibly sell them, but they are not worth much.