Forum

[Felix]

Just went to install new US-made Robbins piston and fresh-bored cylinder.
The only crown mark is 040 near one valve recess. Does it matter which way round?
No mention in the instruction label.
Thanks.

[Felix]

PS
The piston is symmetrical as far as I can tell.

[Alan R]

Hi Felix-------- If there are no specific instructions and / or markings then it doesn't matter.....But if it makes you feel any the better then place the 0.040 maked recess under the exhaust valve... put the head on and nip the securing nuts..........Turn the engine over SLOWLY by hand for two revolutions ( ie one working cycle)....If ok then continue with the re-build.....Just to be on the safe side overall why not consider a thin compression packing under the barrel......This will raise the head in relation to the piston just a small amount and give a bit more clearance with the loss of a small amount of Compression Ratio...

[Alan R]

Just a thought but did you give the piston to the person who re-bored your barrel ??--

[Felix]

Thanks, Alan
There are no marks on the rings either, so it's probably no issue.
It's nice to finally be assembling this bike. I was away on holiday while my machinist fitted the piston. I'm using a 2mm plate to soften the compression since this looks like a real difference in crown height. I'll be checking for clearance as I go. Ring gaps are filed to spec.