1970 911T Compression Gasket Problem

[thilmes]

Ok all you early 911 experts, shed some light on this one for me.

I purchased a 1970 911T about a year ago. The motor was locked. Upon dis-assembly, it was apparent that one of the pistons was seized. I went all through the motor, but did not split the case. I installed new pistons and cylinders (84mm), sand blasted the heads, relapped the valves, cleaned up and inspected everything. Zenith Carburetors have been refurbished. Engine appears original except for pistons and cylinders I installed. All is good.

Motor started and ran well. After about 4 hours on the motor, while flushing out some tuning issues, a significant miss started to develop. got worse. Compression test showed cylinder 1 was about 30 psi. Cylinder 3 was about 100. The rest were about 150psi. removed carbs and looked in cylinder with bore scope. no issues.

Used compressed air to diagnose. Air leaking between head and cylinder. I checked head bolts. they were loose. Weird. not like me to not torque the heads... Tightened up a smidge and compression went up to ~90 psi

So i pulled the motor and the heads on the drivers side. The compression rings that fits into the groove on the cylinders was in place. Evidence of blow by on 1 and 3, some minor on 2. Weirdness was that the "spring" in the compression ring on 1 and 3 seemed to just crumble. 2 did not seem to do this. Is this "crumbling appearance" normal. I do not think it was over heating but... maybe cause is blow-by heating up gasket?

So, now what? Why do I have this compression ring(s) failure?

Porsche factory manual did not mention it but, should I "lap" in the cylinders into the heads like is done with VW's???

I do not think studs are pulling. Not sure how to test definitively...

When cleaning up the motor I sandblasted the heads. I am thinking that I should have heads resurfaced (sealing surface is not perfect, but not bad)? Is this the smoking gun?

If heads are resurfaced, I suppose I will have to add additional spacers between case and cylinder, correct?

Looking forward to your comments and thoughts!

[Jon B]

Compression test showed cylinder 1 was about 30 psi. Cylinder 3 was about 100. The rest were about 150psi ...So i pulled the motor and the heads on the drivers side. The compression rings that fits into the groove on the cylinders was in place. Evidence of blow by on 1 and 3, some minor on 2.

It sounds like the center cylinder or base shim may be slightly larger than the others.

Jon B.
Vista, CA

[thilmes]

Come on guys. I would really like to hear some feedback

Thoughts on resurfacing?
"Lapping" in cylinders into heads?
Anyone ever see head gaskets that the spring appeared brittle?

[Richy]

Yes , the cylinder heads should be resurfaced . The machinest will tell you how much is removed , probably not enough to need to add shims . #2 usually runs the hottest and that cylinder deck is usually the lowest . The head studs being loose is not good . Can they be torqued to specs ?

[edmayo]

The blow by you're getting, probably due to the loose studs you mentioned, will cause that kind of damage to the head gaskets. Easy enough to check the studs, put a head and cylinder back on and see if you can reach torque. When you assemble the engine the heads and cam housing should be torqued alternately with a cam in the housing to test for rotation. If the cams starts to bind during torqueing then something is not level.

[thilmes]

I checked the studs. they were solid, not pulling out of the case...I believe the factory manual states torque head nuts to around 26 ft-lbs. I tested studs to about 40+ ft-lbs

Has anyone ever had reason to assemble motor. warm it up, let it cool down and then re-torque head nuts?

Another note: I placed a straight edge across the surface of the individual cylinders that mates up to the head. The outer lip is about .002" lower than the inner lip. I checked the old factory cylinders that I still have. Completely level. thoughts?

[Mr9146]

The blow by you're getting, probably due to the loose studs you mentioned, will cause that kind of damage to the head gaskets. Easy enough to check the studs, put a head and cylinder back on and see if you can reach torque. When you assemble the engine the heads and cam housing should be torqued alternately with a cam in the housing to test for rotation. If the cams starts to bind during torqueing then something is not level.

I have seen this happen. Cam wouldn't turn until cylinder head torque was loosened. I wonder how much horsepower that was stealing...

[edmayo]

Thilmes, you may have a cylinder problem, this what I have done that has worked well for me. I use a magnetic base dial indicator that I set up on my drill press table, set the cylinder upside down on the table, and read the the height off the cylinder gasket surface, then lap the top of the cylinder on a piece of 180 grit paper on a thick piece of plate glass in the parts cleaner with the cleaner flowing over the sandpaper. Lap the cylinder in a rotating motion until you get a shiney appearance across the both sides of the gasket groove. Do this to each cylinder, but the important part is to check all three cylinders with the dial indicator, lapping all three until they are equal in height to no more than a .001" difference. Now you should have three equal height cylinders that are perfectly flat across the gasket groove. Torque heads and cam housing as I mentioned in the above post.

[thilmes]

I had already assembled the motor when I read edmayo's comment above. I had thought about doing something similar. I am wishing that I had now...

Anyway, the motor is assembled with the new "fire-rings" and cylinders 1-3 lapped into their respective head. I did take off the valve covers on the right side. interestingly all head nuts were a little loose on that side as well. I am going to guess about 5 ft-lbs low. Weird.

So, here is my current theory. the new cylinders were not completely seated when I assembled them and while the motor ran and went through heating and cooling, the cylinders seated more into the case, causing loosening of the head nuts. either that or i can blame it on my torque wrench; it is in in-lbs and i forgot how to multiply by 12...

update to follow after fire-up

[thilmes]

Ok. Successful motor start up. Three or four hours on it. after thirty minutes I checked the head bolts. all were still at torque. I checked compress. all good. crossing my fingers!