Cylinder bore

[AndyT]

Gents quick question

I have my engine apart at the moment and have calibrated the barrels with the following results

Upper left.
Exh/inlet left/right
Top 56.73 56.73
Bottom 56.71 56.73

Upper right
Top 56.72 56.70
Bottom 56.71 56.71

Front left
Top 56.70 56.71
Bottom 56.70 56.70

Front right
Top 56.72 56.71
Bottom 56.72 56.72

The pistons are 1st oversize at 0.25mm. My question is there enough meat left to throw in a new set of 0.25 pistons or do i need to rebore and fit 0.5mm pistons

Cheers Andy

[phildu31]

What did you use to make measurements and how ?
This would make 12/100e mm of free play with new pistons, way too much.
What is the mileage of the motor since last rebore ?

[AndyT]

Used cylinder bore gauge and also double checked with telescopic gauge and outside micrometer.

Dont know when the last rebore was done

[silverstrom]

Looks like you'll be going to 56.90 if using Yamaha pistons. That should be enough to clean up any ovality in the cylinders.

[MK]

As most measurements deviate s bit, what's the current piston clearance? I.e. use a micrometer gauge at the piston, then some inside calipers to adjust to zero.
If you then use them in your bore they show exactly the clearance.
On the 350 the wear limit is 0.1mm, not 100% sure if that was the same for the 500, but 12/100mm is anyway too much.

As you don't exactly know what dimension the new piston has, it'd be your own risk to order new ones and find they're too small.
I'd go to 0.5mm oversize (especially when you consider that we're all not immortal and don't use our twostrokes for 30.000 mls a year).

[phildu31]

Pistons from Yamaha generally come with clearance ie original size ( 56,40 ) piston comes at 56,35.
At least it was the size of the last I bought ( and 35 mark on top ).
I guess first oversize would be 56,60 mm max.
That may vary with other brands.
That is for a brand new piston that would give 12/100 mm clearance with the measurements done.
But with the old pistons that clearance can only be way bigger ...

[Luther]

It looks like you may non stock pistons?? 500 is 56.4 stock so 56.65 then 56.9 oversize. Others come in 56., 56.25, 56.5, 56.75 If your pistons are 56.75 then it looks like you have a good fit