Head gaskets destroyed but why? 74 2.7 911.

[moneymanager]

I'm told that torque techniques have changed over the years. In the old days you torqued heads to some level, following a particular sequence, then took them to another final level. I'm told that current Porsche guidance, I think for the 993, is different. Now the advice is to torque them to X pounds, and then turn the wrench another 90° or some such. Anyone know if we should be adapting this approach to the earlier cars? Maybe this technique is somehow related to the new wisdom Snowman comments on above?

[snowman]

There are several types of bolts in use today. They are not interchangeable and the way they get tightened is unique to each type.

Newer cars use a torque to yield bolts. These bolts can only be tightened ONE time, then MUST be discarded. This technique allows the maximum stretch for the type of bolt and uniform clamping force. The two or three step process is to first torque to some small value, then turn the bolt some specified number of degrees, maybe more than once. An excellent, but expensive (for us, not the mfg) method of clamping things together. Porsche has used them on connecting rods for a very long time.

Stretch can be used on any kind of bolt, its the only accurate method (not necessary for torque to yield though) to tighten a bolt. The torque to yield values were determined by using stretch to produce a tightening sequence that lands the final bolt in a safe area between the yield and breaking points. Torque to yield bolts are permanently stretched, consequently cannot be safely tightened again.

"Regular" bolts, usually only used in non critical applications are tightened using only torque. To safely use just torque the bolt must not be tightened anywhere its maximum capability. The result, vast differences in clamping force. This is due to large differences in threads, the amount of burnishing of the threads, the lubricant used, dirt and other things that cannot be tightly controlled.

Old Porsche's have a big problem. Dissimilar metals, extreme length studs, exposed to the weather. I doubt the problem will ever be completely solved for this vintage.

[SXSMAN]

I'm not sure if it was a good idea to read this thread .

My 2.7 is sitting on my toolbox at work having a top-end refresh done .

My heart sank when it was mentioned that heads were torqued with a new Snap-on digital torque wrench and later had problems .

I borrowed one of my co-workers new S/O digital torque wrench after I got back my heads from B&D machine . I haven't installed my cam boxes yet so now might be the time to do some rechecking .

I've been waiting for an email from Henkel/Loctite Adhesives about the differences between 518 and 574 loctite and it just came .

I've got some thinking to do on mine , keep us posted on yours .

Best , SxS

[dtw]

SxS, were your head stud bores fitted with case-savers? If not, I strongly recommend getting that done...

[moneymanager]

Ditto about the case savers. And make sure they are the 14mm ones, not the smaller 10mm units which I believe are time certs, not case savers.
Only advice I can give (I started this thread) at this point is to torque carefully with a torque wrench that has been recently calibrated or a beam type that can't be very far off. I have no idea whether it will be useful or not, but I plan to have mine redone after 100 miles or so. After two engine top end tear downs, I'm inclined to be conservative. Jim