Super Jet Piston clearance

[be_lain]

Just honed my 61x cylinder and bore is measuring 81.5mm at top of the cylinder. Am I good to run an 81.5mm piston kit? Or do I need more clearance? I’ve read that you need a clearance of 0.0045” or 0.115mm (bore minus piston dia). Previous pistons measured 81.38mm at base of the skirt perpendicular to wrist pin so not sure what dia these were ordered as.... 81.5mm?

Am I measuring these properly? Just want to know what size pistons to order

 

[augustaf]

You seem to have a 81.5 mm bore which requires 81.5 mm pistons (pistons measure smaller than the intended bore. Clearance of 0.0045" is good if using stock or ProX pistons.

 

[mikidymac]

What are you using to measure? You need a bore gauge, not a pair of calipers, just making sure.

 

[be_lain]

@augustaf thanks, thats exactly what I was trying to figure out.

I am using a digital caliper. I dont have access to a bore gauge unfortunately. But I have double triple checked the measurement so I am fairly confident in it.

I also just read that there is a stamp on top of the old piston that will say the amount of oversize. So going to see if I can find those markings later today to make sure I measured accurately.

 

[Big Kahuna]

Would you stake your life on those measurement?

The only way to do it correctly is with a bore guage and your digital micrometer.

 

[mikidymac]

You will not get an accurate measurement of the bore with a pair of calipers if you are trying to check clearance.

 

[Quinc]

If you stick with the same brand pistons you will be fine. Keep in mind different brand pistons have different measurements and requirements.

 

[JetManiac]

Just honed my 61x cylinder and bore is measuring 81.5mm at top of the cylinder. Am I good to run an 81.5mm piston kit? Or do I need more clearance? I’ve read that you need a clearance of 0.0045” or 0.115mm (bore minus piston dia). Previous pistons measured 81.38mm at base of the skirt perpendicular to wrist pin so not sure what dia these were ordered as.... 81.5mm?

Am I measuring these properly? Just want to know what size pistons to order

Take your cylinder to a machine shop and have it measured properly with a bore guage by a machinist. You cannot measure a cylinder accurately enough with a caliper.

If you stick with the same brand pistons you will be fine. Keep in mind different brand pistons have different measurements and requirements.

Cylinder may have been bored correctly for brand X pistons originally, but bore will wear with use, then it was honed an unknown amount. Bores may be out of round or too large now even for the same size and brand of piston.