Identifying standard / oversize pistons

[priderj1]

Hi. We are doing up a 1990 60 HP FETOL 2 stroke. Only problem is the crankcase shows signs of being opened previously. We are therefore unsure whether the pistons in it are original, first oversize or even second oversize.
Are there any marks we should be able to find on the pistons that will tell us?
Thanks.

[rodbolt17]

if its a yamaha piston they are typically stamped either 25 or 50 if oversized.
best way to tell is measure the BORE.
that will tell you if its been bored oversize before and is the bore still within service limits.
kills two birdies with one brick.
if after ALL machine work on the bore its still round and straight then measure the piston to see if it will still properly fit the bore.
if the bore is round and straight and inside service limits AFTER any honing or boring, then you can check the piston or replace as nessasary.
piston to wall clearence is fairly tight on that engine and critical.

[priderj1]

Identifying standard / oversize pistons.

Thanks Rodbolt17. Our measurements said they are standard pistons, we are just looking for confirmation. There are no marks on the pistons apart from a 'W' so that adds to the evidence they are standard.

[99yam40]

does a W stand for something? Maybe an after market piston?

[rodbolt17]

I really really dont recall having typed anything about measuring a piston to see if its oversize.
key is, the BORE, then the piston.

[priderj1]

Ok, Mr Specific. Our measurements of the BORE indicated that the pistons are standard. On the 'W', 99yam40, the only thing I can think of is maybe Wiseco made the pistons for Yamaha. That's just a guess though and I don't need an answer to that. Cheers.

[rodbolt17]

your the one with a blown motor not me.
I rebuild 20-30 2 strokes every year.
its easy side money.
just because the pistons are standard doesnt mean you need standard replacement parts.
thats why you CAREFULLY measure the bore as per the service manual and then make a determination what parts to use.
wont be the first rebuilt wore out junker fresh from a "rebuild" I ever REDID.
bore takes 6 measurements to CORRECTLY ID what you need to do.
6 minimum.
3 in the thrust 3 in the axial directions.
its actually in your service manual with pictures.
taper LIMIT is .003" or .08mm.
out of round LIMIT is .002" or .05mm.
anything more after honing means a wore out fresh rebuild.
piston to wall clearence on that engine is .002"-.0022" or .050-.055mm.
kinda tight when the average human hair is .003".

so am I mr specific? yep. I also give a 1 year warrenty on the mechanical parts of my rebuilds.
the W doesnt mean wiesco either.