Piston and Piston Ring choices

[hauser775]

In my 94 4.0 there is very low compression in the #6 cylinder. It was at a mechanic and he told me that it needs a piston and piston ring in that cylinder. Now I trust this mechanic because I have been using him for a while now. But When looking on rockauto.com I came across the parts that are needed but they come in different options. They have the option of standard, .50 MM oversize, .75 MM oversize, or 1 MM oversized; and the piston rings come with the same options. So I am wondering if I were to order the parts which would I order? The engine is stock except for an intake and throttle body spacer.

 

[Big Jim M]

How low is the compression in that cylinder? Have you taken off the oil cap and listened and looked for blowby from that cylinder?
Engines needing only ONE new piston are rare. Usually all piston rings wear at about the same rate. Far more common would be a valve causing the low compression in only one cylinder.
Big JIm

 

[Beef52751]

yeah, its really odd that you would need just one new piston. if for some reason that were the case, you want a Standard Size piston, unless your rebuilding the motor and boring it, then you would need 6 oversized pistons.