RM250 Piston-bore clearance

[Baracus]

Hey Guys - I was hoping somebody would be able to shed some light.
I'm doing the top-end on an '02 RM250.

I measured the bore and it is still within spec according to the manual (66.4mm)

I measured the old piston (PRO-X) and it measures 66.05mm.
I then measured the new piston (Wiseco pro-lite - pn: 756M06640) and it also measures arount 66.05mm. This seems to be the only piston in their catalogue.

The manual says the piston to bore clearance should be 0.05-0.06mm with a service limit of 0.12mm.

But if I use the above measurements - the clearance is 0.35.
This seems like too much?

Would a forged piston require a greater clearance? I realise the manual will refer to standard cast pistons.

Seems odd that the bore size is within spec - but both the old and the new pistons seem to be undersize. I checked on the PRO-X website and they do oversize pistons starting from 66.34mm. This would be closer to the factory specs?

Also to further my confusion - I measured the ring-end gap in the bore for the new wiseco piston and the are within spec according to the manual.

Am I missing something?

Thanks!

[Motoxparts]

Give us a call we have more than 25 year experience machining engines we own a engine reconditioning shop and check piston to bore clearances on engines all day long car bike etc we also have a website www.motoxparts.co.nz or 06 8749495 Craig

[xwhatsit]

Am I missing something?

Thanks!
I'm not sure about two-strokes, but normal pistons don't have straight up-and-down sides. They taper a little, they're eccentric (the top of the piston expands more than the sides and bottom -- would probably be even more with a forged piston?). Where are you taking the measurement? Definitely take it on the skirt somewhere, but check at a few different heights (like right at the bottom, too) to see if you get a bigger measurement.

[IIIRII]

Pistons are always measured 90 degrees to the pin , on the skirt
Forged pistons dont always run higher clearances when cold (they used to) because of the more modern higher silicon aluminium used.
Basically the high silicon forged pistons can run less clearance than a cast piston, because they grow less than a cast one and are more dimentionally stable.
You need to find out what wiseco recommend.

[LittleJohn]

Also remember that there as specific places to measure both the piston and the barrel, not sure on the RMZ or suzuki's but my kawasaki manual has specific places (down 15mm and at certain position etc) which then gives you the factory specification clearances.....

[Robert Taylor]

Wiseco forged pistons do indeed require more generous clearancing, but more so in highly stressed 125s than 250s. They are strong pistons.

[Baracus]

Thanks for all the responses!

I went back and measured it 25mm up from the bottom of the skirt and it measures correctly. Didn't read the fine print at the bottom of the spec table in the manual.

Pretty interesting that it tapers quite a bit up to the top.

Thanks again.