787 cylinder sizes

[rkkoeb]

So I picked up a 96 GTX that I am that I am going to fix up and sell but the motor has low compression. 130 on one cylinder and 110 on the other cylinder. Bottom end seems good. I did a quick measurement of the cylinder and looks like they are about 82.25. I am hopping to be able to clean up the cylinder but at what point do I need to go to 82.50? I guess after cleaning them up what is the max the cylinder size can be to work with the 82.25 pistons?

 

[etemplet]

I always go (.5mm) oversize from a standard bore. Easier to clean up the bore completely. (.25mm) means centering the boring tool and cleaning up all the bore damage in (.010") and that adds up to (.005") on the wall surface which is not much especially accounting for uneven cylinder wear or marks. Being a machinist by trade and boring my own cylinders every now and again... that extra (.010") makes life a whole lot easier. 1mm is the max I've done but people go bigger. Don't look for a performance increase at least I've never noticed any. Ha ha. You should be 150psi on compression on a good gauge. Good Luck !

 

[rkkoeb]

So I measured my cylinder tonight using my cheap harbor freight digital calipers and I am getting about 82.3. What does this mean?

 

[etemplet]

If the bore isn't scoured up .75mm over would be a safe size to order. You want to bore the clean up. It doesn't matter what the final bore size is your goal is the get a good clean bore.

 

[rkkoeb]

the cylinders look in good condition, can't i just clean them up and hone them and run the 82.25 pistons again? This is a flip so don't want to spend a ton of money on it if not needed

 

[etemplet]

You got 110 PSI compression, the ski needs a new top end.

 

[rkkoeb]

Correct I would be getting a new top in kit/pistons

 

[mikidymac]

No, you need to have the cylinders bored to fit the next oversized pistons.
Just a hone and new pistons and rings of the same size does not work on these.