Piston to bore clearance

[TS1-200]

Just rebuilding a standard iron barrel 200 after a major seize, honed the barrel alittle to clean up, got new piston same size, have checked clearance and it is 6 thou. I assume this is not tight but is it too slack ? what would be the ideal piston to bore clearance for an iron barrelled 200.

[dapper]

Everyone seems to recommend 3 thou clearance, maybe 3.5 thou on an iron barrel. Your 6 thou is far too big.

[goldeneye]

it"ll rattle a bit, but will probly run ok. try it 1st.

[drunkmunkey6969]

Apparently Harry Barlow runs 5 thou clearance on Vespas as standard. Reckons most P2B clearances for cast iron are too tight and account for a lot of seizures? Give it a go, and see how you get on.

[TS1-200]

I will give it a go, the barrel has done a few miles which would account for the extra few thou. clearance I expect. Don`t really want to go for rebore and new piston cost at the minute rather plan a decent replacement next winter.

[lamblast]

i would run it ,my brother was in the same situation with a tuned 175 barrel tried a few pistons at a shop .picked the tightest at i think 5 or 6 thou it runs fantastic never seizes .i think vespas run 5 to 5.5 thou from the factory which i suppose is one reason they dont seize

[tha83]

As far as clearence in general, the type of piston comes into consideration as well. High quality thin ring forged pistons with high silicone content and anti friction/heat treatments tend to work well at closer tolerances as compared to generic cast pistons.

[Muppet]

As far as clearence in general, the type of piston comes into consideration as well. High quality thin ring forged pistons with high silicone content and anti friction/heat treatments tend to work well at closer tolerances as compared to generic cast pistons.

do they?

[tha83]

As far as clearence in general, the type of piston comes into consideration as well. High quality thin ring forged pistons with high silicone content and anti friction/heat treatments tend to work well at closer tolerances as compared to generic cast pistons.

do they?

Sure. Less expansion=less chance of seizing. I've built hot rod Ford small blocks. Naturally aspirated (as opposed to supercharged, nitro or turbo which call for more slop in the bore)high output engines suffer longer abuse when high silicon content forged pistons are used. Two stroke dirt bikes raced in the Baja 1000 don't used cast pistons. Pro street hot rods don't use cast pistons. I only use cast pistons in vespas.

[lamblast]

ive noticed iron barells with rt forged pistons dont tend to seize while my cast asso was always seizing