Double-torquing Head Studs?
 

As I was torquing up my head studs per the directions in Wayne's book and other advice here (7-15-24lbft) I found that after the final torque step when I went back to the center nuts the torque had dropped down to about 17lbft and they needed about another eighth turn to get back to 24. As I got further out from center the needed turn dropped back down to nothing. Is this normal and expected or should they have gotten to spec on the first full torque turn and stayed tight?

The studs, washers and nuts were all lubricated with copper anti seize.

Further, my camshafts are out for regrind so I can't check the heads for straightness just yet.

pelicanparts.com/porsche-911-technical-forum/1117123-head-stud-tq.html

Thanks, I guess I'm curious about how this works out with what I saw. Going from 15lbft to 24 ended up being just shy of ninety degrees like they were calling out, but when I got back to the first nut in sequence after going around it had more turning left to get back up to 24. So it seems the torque plus ninety method would give uneven torque values, or I've overstretched my center studs a certain amount. Either way had I not gone back through twice at 24lbft those studs would have shown low when I retorque in a thousand miles.

Scratching my head some, other engines with joined cylinders haven't behaved like this but it's my first time with the separate cylinder and head style like on the 911.

Just engine oil, no anti- seize. Where did that suggestion come from?

The Bentley and Dempsey's rebuilding books both, if I'm not mistaken, but I'm away from desk this afternoon. I think the recommended was optimoly something or other.

So, torque isn't the true measure of bolt tightness—it's tension. But lubrication on the bolt threads—and a whole host of other factors—can cause fluctuation in torque readings. So once you add a lubricant of any kind to the threads, all bets are off as to whether the same torque setting will truly secure the bolt.

So, torque isn't the true measure of bolt tightness—it's tension. But lubrication on the bolt threads—and a whole host of other factors—can cause fluctuation in torque readings. So once you add a lubricant of any kind to the threads, all bets are off as to whether the same torque setting will truly secure the bolt.

Yes, indeed.

However, the factory manuals don't mention anything that was commonly-accepted workshop practice for a trained technician - the expected audience. So they don't say "threads not full of grit" or "don't drink the contents of the battery" because they assume you don't have to be told that.

Whereas period manuals, aimed at owners, for other marques might be more explicit. And most of those say to lubricate.

If the factory manual simply gives a torque rating, I read that as a spec for a clean & lubricated (eg a light oil) thread - and if they wanted it dry or with a specific technique or lubricant, they'd say so.

The question for me is the impact of lubrication and sequence of torqueing. I followed the Bentley manual use of coating everything with high temp anti-seize, and then torqued in steps from 7 to 15 to 24 lbft as I found here. The only weird part is that going back over the nuts after the 100% torque the center nuts needed a scooch more to get back up to 24. Had I just used the torque + 90 degree method my actual torques would have been all over the place and I don't know why.

I guess I'll find out when the cams come in and I see whether or not they spin freely in the journals.

Edit: Just to be clear I torqued the heads and cam housings to the cylinders as a unit, I didn't go heads then cam housing.

Also, I'll keep not drinking the battery water in mind for the future. :)

As far as I am aware, the use of anti-siege (Optimolly HT) on the threads and nut surface originally applied to 84-89 3.2 engines but was later updated to earlier engines when the updated nut was used along with using the updated torque angle method. The earlier nuts were specified to still be used dry, with the original two-step torque method (7/23ft/lbs) .

Todd

http://forums.pelicanparts.com/uploa...1654526514.png

Yeah according to that procedure my center head nuts would have been about 16 lbft with my outside nuts at 24. I'm confused but I'll be patient on the cams.

Thank you for that information!

there are two factors at play , torque and clamping force.

Clamping force is much greater at equal torque if threads are lubricated.
so stands to reason that different clamping forces can be obtained with equal torque dependent on lubrication used. This is why Porsche spec'ed what to use.

This is true, but I believe what the OP is really asking (correct me if I'm wrong) is the ultimate goal to have uniform clamping force across the heads? Based upon his experience, following Porsche's procedure, he did not have uniform torque and hence, very likely, non-uniform clamping forces.

Yeah this. I got the impression that I was bending the cam-housing as I first got to the appropriate torque, and as I torqued the outer bolts it straightened out and took the force off the middle, reducing the stress on those studs.

Either that or I somehow misaligned the heads and cylinders and completely scrunched the machined surfaces, but I don't think that's it.

I would assume (and yes I know this can be dangerous) that the goal is to have uniform clamping force across the head. To that end, I would further expect that going through the torque procedure and then verifying final torque (even reapplying additional torque as necessary) would be prudent. It seems unlikely that the mating surfaces are not coplanar (by enough to cause this issue), but it is not unreasonable to imagine the studs either ever so slightly relaxing (at the crystalline lattice structure level), threads of the studs bedding into the block/nut slightly (seems plausible but not by much) or the cylinder base gaskets were not completely seated/bedded (seems a bit more plausible). Either way, I would say that the goal is to get uniform clamping force and achieving uniform torque values is likely the best way to assure yourself that you have achieved that.

At the best everything is fine, at the worst I'll let everyone know when my engine grenades. :)

I wouldn't wish that upon anyone....let's hope your diligence produces many, many miles of enjoyment and not the opposite!

[QUOTE=tholyoak;11710605]As far as I am aware, the use of anti-siege (Optimolly HT) on the threads and nut surface originally applied to 84-89 3.2 engines but was later updated to earlier engines when the updated nut was used along with using the updated torque angle method. The earlier nuts were specified to still be used dry, with the original two-step torque method (7/23ft/lbs) .

Todd



Earlier nuts and dry is new to me... I slathered the bloody things in optHT and 32Nm as per some repair manuals, only one nut on the right bank was still at 32Nm 8000 miles later.

Torque is good method since it's straight forward to check that the nuts are holding down the line. Don't skimp on the full Nm when checking at 1000Miles so forth, I checked on a couple of nuts at lesser torque, they were fine so stopped, should have checked every one of them at full torque, I was and am 100% sure things were done correctly, so thought how can they come loose, and I don't want to disturb the masterpiece, well 8000 Miles later leak, checked again and right side all were loose as heck, left side all tight. It's holding now that it's re-tightened, but I still see a small weeping at the cyl/head interface.

How would you make sure it's holding using the 90deg method? The arse answer would be, if you've done it right it will hold, or use these nuts, etc..well, you will end up checking anyway if you mean business. It's so easy to check the torque, takes 1/2 hour per side if everything is clean, no oil change necessary. Check again at 3000 Miles, then every valve adj. Why the heck not, it's simple to re-tighten if you find something. (of course if it needs tightening every year then that's another story) Make sure the torque wrench is good, obviously. My two cents on this.

Phil

Notes in the above.

Was revisiting another old thread of mine and thought I'd add that my engine has seen near 7000 rpm at load on track a few times now and has had no issues after using this torque procedure. Haven't had a chance to re-check the torque on the studs but might give it a shot this winter, time (and baby) allowing.