Torque specs for Raceware and ARP Head Studs

[Rob 930]

I have been chatting with another 930 owner and I understand he has installed ARP head studs, and has torqued his heads to their recommended spec of 36 ft-lb. I have a 930 with Raceware head studs. They have a torque spec of 24 ft-lb. I talked with the man at Raceware and he said a slight overtorque is okay, but not above 29 ft-lb. (All these numbers assume using the lubricants recommended by each manufacturer.)

If my memory serves me, the factory spec, for factory hardware, is 24 ft-lb.

I'm confused as to this rather large (50%) torque differential for what it essentially the same task. Can anyone comment on why the recommended torque numbers are so different? I have a hard time believing that the differences in material or manufacture between the two studs would be responsible for such a large range.

[Rob 930]

Okay guys. 43 reads. No responses. Doesn't anyone have any insight on this issue? I'm not sure if the answer is so obvious that no one has bothered to respond, or if it's so perplexing that no one knows the answer. (I suppose there's another possibility -- no one cares!) I'd say this is a big deal for anyone torquing heads with aftermarket studs -- which is just about everyone who's building a higher performance 911 or 930. I think it's good to understand what you're torquing and why. I'm stumped on this one. Got ideas anyone? Maybe I have to call ARP and Raceware (again) and sort out their responses if I want an answer. But I thought I'd start here...

[ChrisBennet]

Rob,
Just go by the manufacture's recommendation. Raceware head studs have some sort of coating that makes them slippery which might account for the lower torque spec.
Torque is just a crude way of getting a certain fastener stretch. The amount of friction between the nut and washer+threads makes a huge difference.
-Chris

[JeremyD]

raceware, ARP and Supertech all have different specs. Just follow the mfg specs. Did he get instructions or just decide, I guess I'll torque to X?

[Rob 930]

Thanks guys. I'm good at following directions, and that's just what I did with my Raceware studs. In fact, I went beyond that and talked to Raceware, for further clarification on how much overtorque is acceptable. But I don't want to blindly follow directions -- especially when there's such disparity -- without some understanding of what underlies the reasoning. I'd be surprised if a coating on the Raceware studs would account for the 50% difference from ARP -- but I'll try to find out. I have a hunch that the clamp load with the ARPs is higher, and if that's the case, I want to know why. As I see it, what matters here is not torque or even bolt stretch -- it's clamp load. And if each manufacturer is recommending a different effective clamp load, I'm concerned. I'm not sure I trust either stud manufacturer to be the expert on what the correct clamp load is for a particular Porsche engine.

This issue is particularly of concern for a turbo engine, where overboost is possible and head liftoff is sometimes an issue.

[JeremyD]

My directions from Supertech


SUPERTEC HEAD STUDS


INSTALLATION INSTRUCTION
1) USE BLUE LOCTITE ON CASE END (10 x 1.50)
2) USE LIGHT WIEGHT MOTOR OIL OR ANTI SEIZE ON NUT END
3) CLEAN CASE THREADS WITH 10 X 1.50 BOTTOMING TAP
4) STALL ONE NUT UPSIDE DOWN AND ONE RIGHT SIDE UP, LEAVE ONE WRENCH ON STUD AND INSTALL IN CASE. THE INSIDE WRENCH MAKES IT EASY TO LOOSEN INSIDE NUT AND STILL LEAVE THE STUD INSTALLED PROPERLY.
5) INSTALL STUD UNTIL IT BOTTOMS AND TURN OUT ½ TURN
6) INSTALLATION LENGTH IS NO LONGER IMPORTANT


TORQUE SPECIFICATION + or – 2lbs STANDARD TORQUE PATERN IS MAINTAINED
a) 2.0 aluminum case 32 lbs
b) 2.0, 2.2, 2.4, 2.7 ( all magnesium cases ) 26 lbs
c) 3.0, 3.2, 3.6 aluminum case 32 lbs
d) 3.0, 3.3, 3.6 aluminum Turbo 34-36 lbs
THERE IS NO RETORQUING NECESSARY

Supertec Performance guarantees these head stud against BREAKAGE forever. If for any reason [short of intentional destruction] this stud breaks, Supertec will replace it. We only warranty the part. No incidental damage or labor is covered.

I assume the 4 lb difference from regular to Turbo is for head lift/blow off.

[ChrisBennet]

Rob,
Who knows, maybe they have a different coefficient of expansion? If say, the Raceware expanded more when it heated up, it wouldn't necessarily need to have the same initial clamping load.
Maybe it's because (as Raceware states in their directions) their studs are stronger so the don't need the higher torque.
-Chris

[Tom F2]

All of the studs have the same coefficient of thermal expansion as Porsche steel studs. I called Raceware to ask about this a few years ago. I believe that only Porsche Dilavar have an engineered coefficient of thermal expansion for an aluminum cylinder.