POP Y CB450F Help... what are they?

[ColCB400 4]

So, I finally decided to trip up a 400 four engine. they are selling 460cc pistons online for about £150, cant afford yoshimura
Bought some spare barrels with 0.5 oversized pistons.
When it arrived the pistons wont slip in the barrels??? I pondered and saw the pistons have the following markings:
POP Y
CB450F
on the top it has 0.5 and a E for the exhaust valve.
The recesses for the valves are bigger than standard and the top looks "fuller"

These are the Yoshimura ones right? has anyone seen these... in the flesh as it were?
I understand Yoshimura did 54mm and 54.5mm bore and presume the ones I have are 54.5mm because of the 0.5mm?
Can anyone confirm this?
They measure about 54mm but its hard to tell with a ruler.
I have 4 pistons with intact ring sets.... can I just get a rebore and chuck em in ... using a different head gasket
will I need different size con rods with different sized little ends?

[Bryanj]

You are going to have to purchase a decent set of vernier gauges and do some measuring, 400f gudgeon pins are a smaller diameter than most so I would start there

[stevush]

I think I have a pair of these pistons with the POP Y markings on them, STD size

Steve

[Dave_Canada]

Non-Japan market CB400F bores are 51mm, so 0.5 overbore would be 51.5mm.
The Yoshimura kits were 458cc or 466cc - 54 and 54.5mm I think. The 466 was sort-of intended as an overbore for the 458 kit, but most hotrodders just installed the 466 kit.
It's no surprise that 54/54.5mm pistons don't slide into 51.5mm bores.
A decent engine shop will be able to measure the pistons you have and rebore the barrels to fit. These become rather this but have been proven durable. The forcing cone on the bore bottoms get mostly removed but inserting rings is pretty easy without compressors.
Proper 400F overbore pistons use the same gudgeon pins. You will need a head gasket to suit the larger bore, a kit should include this. MC-Again in Japan used to sell correct (Yoshimura branded, although I doubt the providence) gaskets, haven't bought one in a decade or so though.