Piston clearances

I had a few seizures on my return some of these are we believe down to carburation, but we are now thinking it may also be down to insuficient pistion clearance or the wrong ovaility.

so there seem to be many different types of barrels- any comments about what your running would be helpful, I have been talking to a few on this off the site and thought it may be a good topic to do in the open.

the barrels currently in use from my reconning are as follows.
1) iron barrel d1/d3 air cooled (125)
2) iron barrel with alum top fin's shrunk on d1/d3 (125)
3) BTW aluminuim barrel with iron liner (125 and 175)
4) BTW aluminium barrel with alloy liner (125 /175)
5) water cooled barrel with iron liner d5/d10/d14 (125)
6) water cooled iron barrel d1/d3 without liner (125)

Talking my engine is an iron barrel D3 With aluminium top fins shrunkl on.

I need to know what the piston clearences should be, ? any help here would be greatly appreaciated.

this is what I set them to as I started last season off with.
above the ring 0.009" thou
below the rings 0.007/8"
on g/pin cetre line axis 0.005"
below the g/pin bore 0.002"
btm skirt 0.002"
ovality 0.003" extra

I seem to be geting slight smearing but only either side of the exhaust port, don't as one would expect them 180 degree's apart, I run 5% oil, any comments ?

Best regards Derek

Hi Derek,
You should consider the possibilities of air leaks and liner/barrel distortion as possible problems. The air leaks you can check for with a good pressure test system. The distortion will only be checked by the marks on the piston.

Your markings around the exhaust port area on the piston are symptomatic of the ports being close to being too big. It could well be the inlet port is too large, this then lets the piston "rock" forwards in the bore and you get scouring on the exhaust port side. It helps to keep the piston as long as possible inan attempt to keep it mechanically upright in the bore , maybe even create full length legs on the piston either side of the inlet port to give support as much as possible.
The quoted piston clearances sound ok to me for an Iron bore and modern pistons, others may say different!
I run the 25:1 oil content so less than you.
Cheers,
Alan

Since 1985 I have gone from 2 to 1.5 thou clearance with Wiseco RM125 pistons, 20:1 synthetic (Pete Ts recommendation) i my iron D14 barrelled short stroke 125 water cooled motor. Never seized. Holed a piston once when Devcon came loose on cases, now welded.

Long stroke iron 125s with Omega then Suzuki RG250 ran 2.5 thou, only seized once when contact breaker cam came loose.

Long stroke iron 185 ran Wiseco, seized once because of wrong carb setting change. Now use forged Wosner piston with2.5thou clearance very reliable

Hi

John thanks for those comments -

Alan i had not considered the piston rocking forwards, as actually my piston is a little short on the skirt, if you look back at my return to bantam racing posts , I had trouble locating a piston running a Go kart piston, the only one I can find that avoids the transfers and is pegged right at the top so avoids the top of the inlet, inlet is 33/5mm width tapering, but tall, and yes on one of the seizures ,identified an air leek between reed and piston face. ?

any other comments would be helpful.

regards Derek