Wire wound piston clearances

[Shelby-Right]

The new bore size is the standard size plus the oversize . Depending on usage some owners add .0005" to .001" , the wire wound part of of these pistons serve the same purpose as a Invar strut in some car pistons ."thermal expansion "we had a Berco AC750 , and a power hone and like the reconditioners over there you measure pistons all day long, bore and honing barrels and blocks . And resleeving .

[g5wqian]

i think youll find that the wire wound pistons used .001" total running clearance due to fact they dont expand much if anything , but that would have been for a run of the mill everyday motorcycle , if it were a more sporty or racing type then it would use bigger clearances

modern alloy pistons like GPM need .0015" per inch of bore but it seems modern alloy pistons that are available for our machines do require quite a fair amount of clearance , typically .004" to .007"total , most people doing rebores will go for around .0045" clearance when they are dealing with GPM or JP pistons on engines with around 3" bores .. .

the manual for the bike shows clearances for rebores somewhere i think and it shows .001" total clearance for the wirewound pistons used in a G11/G12 matchless .

one thing i will say is that i bought my ajs model 30 some years ago and the previous owner had it rebored and i found it had partially seized on the nearside cylinder at the rear , it had GPM [gandini] pistons in it and i found that the running clearance in the bore was .001" and no more , which was obviously "old information taken from the manual " but too little for those particular modern pistons .
i then honed it out to give me .006" clearance and all was fine after , although perhaps that was a little on the slack side for some and i do get some piston slap when cold .
it has not seized up at all and will rev flat out if i want it to .

seems that with original wirewound pistons , you could make a tightly bored engine and it will be fine .

ian

[paulvk]

Thank you Ian for that info .
Regards...Paul

[GSAX1]

F.W Neill says in his book that M20 / G9 wire-wound pistons, when new, were fitted with a clearance of 0.0009" only.

I had my M20 barrels nicasil plated and the original std size wire wound pistons that came with the bike when new were installed with a clearance of 0.03 mm / 0.0012" some 7-8 years ago. I was afraid they would sieze immediately, but they have run for a long time now without problems. Last year I had to repair one of the spark plug hole threads, and took of one of the heads. The cylinder bore looked like new, and no signs of tight running on the piston.