Cylinder compression on 6 bolt 1g engine on 2g pistons

[Devon Headen]

I'm going to look at a 2g tsi awd this weekend and the guy is claiming that it has a 1g 6-bolt engine with 2g pistons. Also he says that the engine only has 200 miles on the rebuild. What kind of cylinder compression should I expect when I test it? There's no other reasonable way to prove that what he says is true is there?

Thanks,
Devon Headen

 

[Dream On]

Ask him if he did the work, and if not ask for receipts. Check to make sure it's a 6 bolt by checking the oil pan. The block would also probably be pretty clean if it just came back from the machine shop if it was hot tanked.

It's hard to say what the compression will be as it increases over time with the rings seating. I would say that it should have the stock compression of a 2g due to the pistons, which is 178 psi. Just make sure that they're consistent.

 

[JackM]

The dish is deeper in the 1G pistons over the 2G 7-bolt units. What I did to check to see if there were 2G pistons in an engine here was I measured the dish depth off the piston. One way to do this is to weld a 6mm stud to the end of a spark plug. Put the plug in the #1 hole (make sure the piston IS NOT at TDC!) and rotate the engine very slowly and carefully by hand until the piston hits the stud. Look at your crank pulley and see where it is sitting. Do the same test on a 1G piston car and see where it stops. If it's off by a little, then you have a different depth of dish, which means you probably have a different piston in there. If it is the same as the 1G, then you have a 1G piston. If you want to be really slick, you can figure out what the difference in depth is in MM by calculating the difference in the 88MM stroke in relation to the difference in degrees on the crank pulley. You can then verify the dish depth mathematically and figure out how much higher the 2G piston is really sitting in there to be sure you really have a 2G piston and not some NA piston or something, ! Once we made our little welded plug here, (for doing cam degreeing initially) we can tell very easily what pistons are in an engine now by just looking at where the crank pulley stops. The difference will be very small, so you need to mark and look at what you are doing very carefully. ...and remember to pull that plug out before you try to start the car!

Jack

 

[Calan]

I have a 1G 6-bolt that was rebuilt about 10k-15k miles before I bought the car (5 months ago), according to the guy I bought it from. Unfortunately, he didn't have the receipts and the machine shop that did it doesn't keep any specifics. They did say it was probably slightly over-bored though.

But... when I did a compression test, I read 173-175 across all cyls. So I'm guessing I have 2G pistons. (Could oversized 1G's give those compression numbers?)