Top end service - Piston to replace?

[Skippa]

Hi again,
I repeated the measures a tons of time at my home, where temperature is pretty standard conditions (circa 20?C). Result is always the same:
Cylinder 66,35
Piston 66,27
Delta = clearance = 0,08 mm
Piston an S3 Vertex, cast.
Cylinder as well piston appear in a good shape at my poor of experience eyes.
On the piston the black I was seeing is still the initial coating.
Rings put into the cylinder has a gap of 0,45 mm.
So everything seems ok except the clearance.
Right now? Should I change? What do you thing looking at the pics?
If so, my last doubt is? what measure? I?d go with a 66.35, so a C Wossner or a C S3 Vertex to remain on the same type. But I cannot understand the measures? my piston is a B .34; a C.35 should be only 0,01 mm more? so I?d have a clearance out (0.07)! This is really not clear for me. What?s missing to my mind??

Please help? !!

Here the pics:

https://ibb.co/NyDvXLQ
https://ibb.co/yRjs4wB
https://ibb.co/xzmHV8N
https://ibb.co/hWD5Bc3
https://ibb.co/hYCChqp

[JAD]

Any progress...?
I thought this would be a popular thread....


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[Skippa]

Ciao everybody,
at the end, I bought a Vertex replica 66.35. Compared to the old S3, it is more oval as on the bottom I found effectively 66.35 while under the pin hole is 66.25? old piston is definetively more round but about 66.27 ad most (bottom area).
I watched some videos where it was explained that is pretty normal for a brand new piston? After a good break in it will deform a bit getting a shape more round that will fit perfectly the cylinder.
Also this is not so celar for me as the piston does?t touch the cylinder and thermal deformation should be mostly elastic, not plastic! But it doesn?t matter? I mounted the new Vertex that has play (so my measures of the cylinder were not reliable for sure? otherwise piston should have seized immediately) despite of I don?t know how much it is? I assume circa 0.03-0.04, a little bit low? I will run an accurate break in and a quiet winter with specific thermal cycles in order to prepare the engine to summer period when I move my EC to supermoto version. Only at that time I will ask more to my engine but it should be well run in.

I completed my service few days ago and it was enough only a kick to the pedal to have my EC born again!! I run only 15 minutes and it was perfect? Let?s see if everything proceeds well!

[JAD]

Well done!
Glad it all went well and the bike is running.

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