98 GTX 717 Piston Size

[mikelhuff]

Hi all,
Bought a project ski and had to take the engine completely apart so I’m wanting to replace the pistons, rings, pins and needle bearings.
Looking at the top of the piston I don’t see a size or STD mark. The bottom of the skirt is 82.9mm
Anyone know if this is stock and if not, how much over stock?

I do see a mark on the top that reads J86H2

 

[mikidymac]

You would be much better off having the cylinders bored and new oversized pistons fitted.
New pistons in worn out cylinders will do nothing for you.

 

[Mekanix]

81.89 is typical std. And goes up in 0.25 increments for first bore. First over is 82.25mm. Best to bore since wear and a manufacturing differences can place you out of tolerance. New pistons in that bore may be more than the 4 thousands clearance required. It will either not seal properly or slap itself to the breaking point.

 

[etemplet]

Don't do the first over bore of .25mm. Go to .5mm. That said.... I hate millimeters. I am an old school machinist and still use thousandths and... always will.

 

[mikidymac]

Have your machine shop determine the size you need to bore, don't just guess and waste an overbore due to guessing. If there are no marks on the cylinder from mechanical damage, just low compression then .25 over is typically fine.

 

[etemplet]

As a machinist by trade, I can say with conviction. .5mm is what you need to go to to insure the bore will clean up properly. I won't even buy .25mm and I do my own stuff. The reason....

.25mm is roughly .010" That gives you .005" to handle out of round, scouring, cylinder wear, taper, and a factory bore that may not have been perfectly true. If there is a mark you can get your fingernail into... you've got a problem. Just touching the tool off you need .002" and you need to leave a bit of metal for a hone. You could easily end up with only .008" or .007" total to work with. Wayyy too close. I like easy peasy.

For comparison's sake a female pubic hair measures .003" and they are a bit out of round. Yep, I brought my 1" mics into the bedroom when I was an apprentice. My wife says I"m weird for bringing a flashlight but I need to be able to see under the covers.

 

[etemplet]

Don't let me be a thread killa... half hearted joking there. YMMV.

 

[Mekanix]

For comparison's sake a female pubic hair measures .003" and they are a bit out of round. Yep, I brought my 1" mics into the bedroom when I was an apprentice. My wife says I"m weird for bringing a flashlight but I need to be able to see under the covers.

Lmao!!

Well that explains the unit of measurements of the C#*t hair.

 

[etemplet]

I am a cornucopia of insignificant information.