Engine rebuild first valve job. Do I need to re-torque head studs
 

I just rebuilt my first 930 engine. One of the BESt things that I have ever done. I highly recommend it!

I just broke in the engine and need to do the first valve adjustment. (500 miles)

They say to re-torque the head stud bolts at this time. Is it enough to just check them or should I loosen them to take the load off them first, then re-torque them down?

I used ARP head stud bolts, torqued to 38 ft-lb.

Thanks

Just torque, to check the spec. Dont release, you just want to know youre holding.
Bruce

ok I agree. I called ARP to check the torque requirement and they told me to release the load on the bolt and re-torque.

I was reluctant becuase I don't want to create any un-intended issues, just confirm the torque.

Thanks

ARP should know what it is doing, but I'd not follow this advice (of course, I'd not have asked for it). What Bruce says is what guys do, and all they do. Makes sure that if somehow you didn't get enough torque on something, or something compressed after running, you still have proper torque. Not likely that you overtorqued anything initially, is it?

And retorquing was most important when dealing with magnesium cases and pulling head studs. If it lost torque, you could flag it. And if it wouldn't retorque, you knew it was time to pull things apart.

If you torqued with lubricant the first time, would not you want to reapply it for a release/retorque? As the lubricant might have left or altered its lubricity after the heat of running?

Sounds like an engineering project:
Retorque and carefully note the angle of the wrench. Release and retorque and see if the angle stays the same?

Torque specs are accurate only when the fastener is rotating while tightening. Thus, if the torque spec is 38 ft.lbs, the breakaway torque is typically higher (torque necessary to overcome initial friction). I would set the torque wrench to 38 ft.lbs. If you reach 38, stop. The torque should be fine if you tightened it properly during assembly.

Too tight? The only way to confirm is to back off all the fasteners, then retighten in 3-4 gradual steps to the final spec. in the recommended tightening pattern.

MHO,
Sherwood

are 911 heads really torqued to a fixed value? i've never rebuilt a 911 engine before, but all the other german cars i've worked on required a multi-stage torquing process, where the final stage was based on angle.

for example, on a diesel mercedes where i changed the head gasket, it has 24 head bolts. they are to be all torqued in a specific pattern to x ft/lbs. then you let it sit for an hour. then you torque all 24 bolts again in the specific order to y ft/lbs. then you lit it sit an hour. then you tighten all 24 bolts in the specific order by 90 degrees. it's quite a procedure, but that's what the factory called for in their workshop manuals.

Carrera

The torque to angle method is a fixed value system also. The goal of torquing is stretching the fastener (bolt, stud) to a suitably high tension that the forces upon it never exceed the force that tension creates. Under those circumstances, the parts clamped together never separate. As a bonus, the friction between the nut (or bolt head) and the parts, and on the screw threads, is sufficient to keep the fastener from backing off (and thus losing that tension, etc).

Torque to angle means that you are stretching the fastener at least the amount determined by the thread pitch and the amount of rotation. I'm sure the factory engineers worked out how all those steps meant that the final use of thread pitch achieved closely equal results, and at the needed values. Just a bit complicated. I think Porsche specifies this nowadays for some engine components, just not quite as many steps and settling periods.

Torque to a torque wrench value is thought to be a reasonable approximation of the needed stretch, but much depends on various friction characteristics, much discussion of what lubricant to use, the effect of metal plating materials, and so on. .

Which is why the proper way of dealing with rod bolts is to measure the stretch. But that is kind of difficult for studs.

An engineer once explained to shade tree folks like me how they torqued huge bolts on some industrial equipment: they heated the bolts up to some temperature, installed them, spun the nuts on, and let it cool. Shrinkage tensioned the shanks just fine. No wrench would have been large enough, heat could be measured accurately, thermal expansion rates well understood and uniform, etc. Basically the principal behind the old hot rivets in bridges and skyscrapers and such.. Not practical for our 911s, though.

When I worked for LA DW&P they were rebuilding a 180 MW turbine. They placed the case nuts for the lid on cold studs and tightened with air wrenches that looked like manual air chisels guys use to tear up streets. Then they measured stretch


and...
Walt at Competition Engineering said to check stud nuts after 500 miles by only tightening with certs installed on my 2.7. I don't know if this procedure applies to an SC engine? [although it can't hurt ] His rebuild info might be on his web site? Although I must say that after 80,000 miles since a good full rebuild and machine work in 1999 oil psi is perfect and rings are still tighter than a crabs ass. I'm also big on good break in technique

I used Raceware for heads and rods