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If I send the cylinder off to be replated, I send the piston with it. If I bought a complete kit, cylinder included I normally just check the ring gap.
I always measure. Mistakes are made even with new top end kits. Uncommon, but it does happen
If I don’t have an inside mic. , wouldn’t feeler gauges between piston and cylinder be close enough to check for clearance ? I’ve done .001 , .0015 , .002 , etc. until piston doesn’t move freely. The thin ones bend to the shape of the bore.
If you're inserting the guage between the piston skirt and cylinder you can probably get away with it. Have to be careful not to scratch the skirt though. If you try doing it between the piston crown and the cylinder it won't be accurate (the crown is undersized at room temp to allow for thermal expansion, so the clearance will measure too large if measured at the crown)
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so this might be helpful, but the manual suggests you measure the piston width (at least on my 2017 ktm 350 sxf) at a point 5 mm up from the bottom of the skirt. Apparently the top of the piston is slightly smaller and designed to expand with heat. https://youtu.be/oBmrGro0MhM?si=zCCZFV6B85gZVqDs&t=250 so maybe measure from the bottom side, if you are using feeler gauges.
Also the manual suggests you measure the cylinder in multiple spots on an X and Y axis, then compare the measurements to find the largest and smallest piston to cylinder clearance gaps. Again, if the cylinder is new, you might assume its all the same, but as harescrambled mentioned, mistakes can be made even with new parts.
I can't find another video I wanted to reference as well that compared measuring the cylinder width with a telescoping gauge and then calipers instead of using an actual ID micrometer or bore gauge. The measurements were different enough that the telescoping gauge method showed it was within spec, but the bore gauge said it was too far out. There might be too large of a margin of error using the wrong tool when your dealing with hundredths or thousandths of a millimeter.
I know a guy at a machine shop I am taking both my new and old piston and cylinder to to be measured next week with proper tools. Maybe call a shop near you and see if they can help measure as well.
I should have read this short response first, ha ha. Already said what I did, 10 hours before me!
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