Your recommended piston clearance for a 250 aircooled.

[John Orchard]

I have to rebore one of my '79 KX250 barrels, can you guys give me your suggestions on piston to bore clearances please?

The piston is 70.25mm cast, I think they grow a little less than a forged piston.  I've been reading anything from .0025" to .005", my guess is .003" for a low revver or .004" for a high revver?

[SON]

John is your piston a 30 year old one?
Or a modern one?

[John Orchard]

The box looks 20 years old, has piston manufacture improved/changed over the years?

[SON]

John you have been around long enough to know the technology advances,
Oils, Ceramics, Digital ignitions,
No advances in Piston metallurgy
I would look to the Woosner range, I am a fan.

[Montynut]

Most aftermarket pistons have the correct bore clearance included with the piston. Wisco is more than Woosner and both are more than most cast OEM Japanese piston. If it is an OEN piston then the shop manual for the bike should have the clearance listed. I don't think there is standard clearance that applies across the board.

[John Orchard]

Thanks, I went to the manual, not thinking it would be in there but it was.  The piston kit is aftermarket and cast so I think adhering to the stock clearance may be close; the manual states .0020" to .0023" (.049~.059mm).  But that is with an Electrofusion bore; this has been resleeved; I think I'll run with .003" (.072mm).

[SON]

does the manual say to bore it tight, run the engine for 5 minutes, disassemble wet rub the high spots on the piston, reassemble a run it in, just like the old days

[mustanggrahame]

Don't run any piston other than OEM as tight as the original manual says. Cast high silicon pistons expand the least. Also once you have resleaved a cylinder it is not as capable of heat transfer to the cylinder fins. I don't know how much extra clearance than original you need John, but it must be a significant amount. The nikasil bore is the best. Once you have departed from that you are in unknown territory.
Cheers, Grahame

[Davey Crocket]

I have a Woosner piston in my resleeved A5 John....if you havent seen one yet then get one. I run them in all my bikes, they have a teflon coating on the sides and are very well made....I wouldnt use an 20 year old piston. They give you the bore size clearance on the box. Cheap investment.

[PCMAX]

quote author=Davey Crocket link=topic=27685.msg270365#msg270365 date=1360585304]
I have a Woosner piston in my resleeved A5 John....if you havent seen one yet then get one. I run them in all my bikes, they have a teflon coating on the sides and are very well made....I wouldnt use an 20 year old piston. They give you the bore size clearance on the box. Cheap investment.
[/quote]

I agree the Wossner pistons are nicely finished and generally cheaper than the Wiseco. I recently bought a Wossner for my CR125 from
  http://www.lionparts.com/
It was only $79 USD for the kit (piston, rings, circlips & gudgeon pin). I paid another $40 for priority shipping but still a bargain