Blowby issues

[nicknorth11]

I bought my car with a rebuilt motor. It had .20 over pistons. I stroked it and ordered .40 over pistons from wiseco, but they sent me .20 pistons. My machinist said they would be fine. I broke the car in exactly like I was told to do by my machinist and the motor now has about 20k miles on it. The car has excessive blowby, especially at higher boost levels. I did a compression test and one of the cylinders is about 10-15 psi lower than the rest, and the other three fluctuate between 5-10 psi.
Do you guys think I should just get new rings, or do you think my machinist made a mistake using .20 over pistons again?

 

[dnhieu]

im wondering if your machinest used a larger then normal ring gap to accomidate for nitrious.

 

[nicknorth11]

If that were the case, would I need to go with bigger pistons (.40 over) to fix the blowby? If I boost 21 psi, I get tons of oil shooting out of my dipstick. I have a catch-can with a breather coming off of the valve cover and still can't keep the dipstick from spewing oil.

 

[bastarddsm]

was the motor bored .040 over?

 

[nicknorth11]

No, it was not bored to .40 over.

 

[bastarddsm]

do a leak down test and see where your at.

 

[nicknorth11]

do a leak down test and see where your at.

I tried that once, it didn't work. I guess I probably did it wrong, but I was hoping to get some thoughts in case I can't get it to work again.