measuring pistons

ahistand · Post by **ahistand** » Wed May 02, 2007 9:58 pm

OK, where exactly should you measure the piston diameter so that I can compare it to my cylinder bores to see if I need to get a bore job and bigger pistons. I see the markings on the tops of the pistons, for instance, mine say 76.48 and then below that it says Sp.03..... So does this mean the piston measurement I use is 76.48mm and the cylinder to piston clearance that they are looking for is .03mm (about .001")? If i'm correct, then it looks like I need to go oversize because using that rationale my piston to cylinder wall clearance is like .004". I saw on a chart somewhere that the wear limit is like .0025".

So, then if I need a overbore, when I get the new pistons in, then I take the block to the machine shop and tell them I want the bores .001" larger than the piston diameter that's stamped in the top of the pistons? I've never done an overbore before, so please excuse my ignorance.
Thanks guys.

Post by **Fatmobile** » Wed May 02, 2007 10:46 pm

That sounds right to me.
My machinist said there is so little material to remove that he can't bore it.
He has to hone it to the right size.
The cutter would just skip across the surface when removing such a thin layer.

dexev · Post by **dexev** » Fri May 04, 2007 11:23 am

Is it really ok to just trust the measurement stamped onto the piston? The bentley says to measure them -- but I guess they're not going to wear?

ahistand · Post by **ahistand** » Fri May 04, 2007 9:43 pm

I got new pistons, and the measurement stamped in the top corresponds with a measurement taken about a half inch up from the bottom of the piston skirt 90 deg to the pin. So the new factory clearance is .001" so I'll just add that to the stamped piston size and thats what i'll have them bore it to. I've been doing a little more research and measuring since I posted this first.

rwest1 · Post by **rwest1** » Fri May 04, 2007 10:17 pm

Use a well known, reputable machine shop that has worked on VW diesels if at all possible, else take your Bentley to show machinist proper fit-up dimensions. Take block & new pistons,machinist should measure then bore & hone cylinders to proper dimension. They s/b the experts as to how much to cut & how much to hone to achieve final specified dimension and proper crosshatch pattern needed for ring seating. I've got into trouble using bargain machine shops in the past and found your usually making a fools bargain in dealing with them!!! This one area where it pays to find the best, and pay the price! Regards

CoolAirVw · Post by **CoolAirVw** » Wed May 09, 2007 10:44 pm

Here's some specs from Jacks page.