Heads leak after new rebuild.

[dbryant61]

Dear Wayne,

I recently rebuilt the engine in my 1980 911SC. I did a top-end rebuild down to new rings and piston base gaskets. Nothing connecting rod or below was touched. I completed the build two weeks ago and first got the car running on Monday, 3/12. Everything sounded and seemed fine until I heard louder then normal exhaust-type noise on full throttle acceleration. This began after about 300 miles of breaking in and was heard on the track at VIR. Perhaps I should have waited longer before going to full throttle, but I was still short-shifting at about 5000 rpm.

I have thoroughly checked the exhaust (factory headers and Monty 2-in 2-out) and find no leaks. However I do find dark, gooey liquid around the head to cylinder interface. It appears to me that I do not have a good seal between the heads and cylinders and that combusion air/fuel is escaping on the power stroke. #1 is the worst. #3, #4, and #6 also appear to be leaking. The heads were machined .010" and their surfaces are perfect. I did nothing to the cylinders other than clean them and scrape the carbon out of the groove for the head gasket. I used new head gaskets (C-E Rings?).

If any of you have experienced this, have you tried re-torqueing your head studs? How much can you torque them? This is an SC aluminum (930) case and has all new steel head studs (old style. Same as the original ones that were on the intake side.). I re-used the washers and nuts. I torqued them the night I installed the heads and re-torqued them the next day.

I would really like to try re-torqueing the heads if there is any chance I can re-establish the seal. Not looking forward to pulling the engine and tearing everything apart again. I would also like you thoughts on what I may have done wrong. I tried to follow your book religiously.

Thanks,

Don Bryant
'80 911SC

[anol]

Hello Don,

Crap, I know how It feels, I had the same problem after my rebuild, the cylinder/head leaked on cylinder 4-6 and some on 1 & 3. I tore down the engine again and after measuring the cylinders I discovered that the cylinder tops weren't flat, the inner surface was higher on some and the outer surface was higher on the others. It was about 0,06mm difference. I had all cylinders machined flat an also the CE-groove cut equal amount. Now I hope it will be good, I am about to reassemble the engine again.

Read more:
Resurfacing heads and cylinders

/ Anders - Sweden

[john walker's workshop]

the head to cylinder mating surface may not be absolutely parallel to the head to cam tower surface. happens sometimes when the heads are flycut and they're not set up on the mill or lathe real well.

[dbryant61]

I am pretty sure I have found the problem. It is two-fold: I did not properly lubricate the washers, nuts and threads, and my torque wrench is reading about 20-25% higher than it is actually putting out. I am going to re-torque tomorrow night and see what happens.