Topic: Got the k3 engine powering my k7 cb750, but what pistons did I put in???

[M.Henry]

So I finally got 1 of my three blocks put together it is a k3 engine with a few k7 parts mixed in. It is primarily stock, except for pistons switched. They were ones I bought with one of my engines from Birmingham. I did all the engine building tests, clay in cylinder, checked valve gap from largest lift by pushing valve further in, and everything is good. I did all the effort to measure the compression ratio and it is about a 10.8:1 I also set the valve lash to the large end of the range at 0.012" and 0.024".
OK so the bike has run fine for 3 days now while commuting Its revved to at least 7.5k while fully up to temp in 60 - 70 ish deg weather.

But now I'm wondering more about these pistons I put in. They were Honda cast pistons. I don't remember the actual number but they were a 400 something (not 410 from what I can tell from Google images searching). When I stock put them in they left the piston flush with the deck surface with no recession down into the cylinder. They had a crazy dome coming up which scared me into doing all the engine building tests to check for potential valve to piston touching. The piston required the thinner piston rings form a 392 or 410 piston.

So what piston do y'all think this may be? could it be a dohc piston?
I will try to post some old pictures of the pistons from my phone If I can figure out how to post pictures.

[M.Henry]

Here are the only 3 pictures I can find of the piston, they are being held up to the piston I got for my 836 kit which is my next engine build. So disregard the bore difference there.

[PeWe]

It can be text or numbers inside?
I have old Henry Abe 900cc pistons with valve pockets for all cylinders. Wave shaped pocket for the 2 different valve angles. Those are marked inside HAINST...

[BPellerine]

they look like 77/78 f2/f3 pistons,the number would be 410 for those pistons,they are the only honda piston with that kind of dome.bill

[ekpent]

they look like 77/78 f2/f3 pistons,the number would be 410 for those pistons,they are the only honda piston with that kind of dome.bill

^^^^What he said ^^^^  https://www.ebay.ie/itm/Honda-CB750-CB-750-F2-1977-1978-SOHC-Genuine-4x-Pistons-Rings-61mm-/332328879077

[M.Henry]

Thanks guys. Those piston sure look the same. I guess I was confused with the other 410 piston I saw because my Google searches didn't quite show that dome. So far I'm glad to say it hasn't given me any trouble.

Well other then some decel popping. But I'm still reading up on the forums on that one. Gotta learn how to tune that out with the 77-78 pd carbs.

[M.Henry]

So my only question now is why did Honda let these pistons leave the factory with such a horribly rough finish on the dome surface? I get that it was a lathe operation going on, but wouldn't the visible ridges be nucleation points for secondary combustion (knock) within these already higher compression ratio pistons? I see the roughness both in my own pistons, and the various high dome 410 pistons I see online, such as the ones linked.

[M.Henry]

So is there much trouble to putting a set of 410 pistons in a k3 engine? Are there any specific known modifications that should be done?
The only major Thing I did was put a cork gasket with the regular base gasket to lower the compression ratio and give more room to avoid piston valve contact.

[BPellerine]

that is the only thing you have to worry about,ie valve to piston contact,and it seems like you checked that,the added compression may give it a little more pep.bill