Torquing a Head

[Guest]

I sat my newly shaved head in last night and now may need a hernia operation. Kidding aside, I bought a set of ARP chrome-moly head bolts. Their products are truly works of art. The TR manual says to torque from 60 to 80 lbs. ARP says to torque their bolts to 70 lbs. so that is a happy medium. My question is, is it really necessary to retorque a head after some run time? I don't relish the thought of taking the valve train off again to do this. I torqued the ARP studs using the only Snap-On tool I own, a very accurate torque wrench. I could almost feel those studs stretch! Thinking of ordering all my manifold studs from them. I like those 12-point nuts.

Bill

 

[Mickey Richaud]

Hey, Bill -

When I rebuilt the MGB, I read the same thing about re-torqueing the head, and wondered about that. When I did re-torque it, sure enough, it took more. Don't know about using chrome-moly, but the head is still the same, and I suppose things could loosen up. Guess the old saying applies: "An ounce of prevention..."

Mickey

 

[Bugeye58]

Bill, I've been using nothing but ARP hardware for years, and yes, you should re-torque the head. I know its a pain in the nether regions to pull the valve train to do it again, but not nearly as painful as replacing a head gasket.
The head gasket will compress a tad more during the running in process, and everything else will seek its own level.
Bite the bullet and do it right.
Jeff

 

[martx-5]

This link from the ARP fasteners site says to re-torque the bolts and studs after the engine has run. It mentions it in the "stud vs. bolt" box on page one. /ubbthreads/images/graemlins/cheers.gif

 

[waltesefalcon]

Always Always Always retorque your heads after awhile. They will always take a little more torque.

 

[Geo Hahn]

Just so I'm clear on the proper procedure... is it only necessary to retorque from where the tighten nut now sits? i.e. you do not back the nut off and then torque it to spec?

 

[jimbar]

from where it sits.

 

[Simon TR4a]

Bill, I would agree with the others on the need to retorque the head, but not sure about the process. I have read it is worth loosening the nut just a flat or two before retorquing in order to overcome initial "stiction" which might give a false reading.If you really loosened off the head you might have to retorque it again, though possibly not as the gasket would have been compressed and through several heat cyles.
Also puzzled by the wide range, 60-80 ft.lbs., that you quoted. My TR4 has a much smaller range, I think 100-110ft.lbs. only 10 % difference compared with 33%?
Simon.

 

[JamesWilson]

https://www.triumphstag.net/start/phorum/read.php?f=1&i=793&t=753

Has a fairly extensive discussion of the issue. The pertinent part is:

". the ONLY way to re-torque is to slacken off the nut/bolt first and auto engineering convention says this should be by one 'flat' only, immediately re-torquing to the correct torque. The reason for backing-off is that stiction between the surfaces including threads will probably not allow any movement of bolts/nuts if the correct final torque is applied, but if backed off slightly the torque can then be smoothly and proggressively applied right up to the recommended torque."

 

[piman]

Hello all,

in contrast to the above, I never slacken the nuts but usually find that I need a few part turns to get them back to the specified torque. I'm quite happy with the way I have done it in the past and don't suffer from head gasket problems.

Alec

 

[Keoke]

HI Geo. yes back off the nut before retorquing.But no body has said any thing about torquing the studs before the head is set in place. That is a crucial step in this process.It is also interesting to note the information contained in the reference Martx- 5 provided above.---Keoke

 

[Camping57]

I design head bolts (and other bolts) for a living so maybe I can help.
The reason the you have to retorque the head bolts is because the head gasket compresses under the thermal load during driving. This is an initial set type of thing (OK, it does continue after the first 10 heat cycles, but it is such a small amount that you can ignore it). The head gasket compresses and that reduces some of the stretch that is in the head bolts. All bolts stretch; they act like very stiff tension springs, and that is what keeps the head sealed from the combustion loads. (Torque to yield bolts are different animal and we'll save that discussion for a future date...entirely different set of rules).

So you have to regain that initial amount of stretch. There are ver precise ways to do this if your building a Cosworth engine (read; expensive), but most Triumphs use simple torque as the measurment method. The contraversy here was about whether to loosen prior to retorqueing and the answer is that it is not neccesary. There is certain amount of static friction that has to be overcome, but on a newly assembled engine it will be overcome by the drop in torque resulting from the clamp load loss due to the gasket set. If you want to loosen the bolts/ nuts and then retorque them it doesn't hurt, it just doesn't make things any better.

Two things to remember:
1. Always move the torque wrench in a smooth arc, don't ever jerk it or the readings will be worthless.
2. Read exactly what the directions say about lubricating the fasteners and follow them to the letter. The difference between a dry bolt and one with oil can be a doubling of the clamp load at the same torque, and too much clamp load is almost as bad as too little.

Dick

 

[Keoke]

Camping 57, My point is that the studs must be torgued prior to setting the the head inplace.---Keoke /ubbthreads/images/graemlins/yesnod.gif

 

[piman]

Hello Keoke,

the information in the ARP site refered to seems contradictory.
They say to install studs hand tight only, not tighten at all. Then it says in a later paragraph that it is necessary to re-torque the studs and bolts. I think that it means the nuts on the studs or bolts depending on what is fitted. I cannot see any way of re-torquing studs without removing the nuts completely, which seems pointless.

Alec

 

[Camping57]

Keoke,
Interesting point about torqueing the studs into the block. There are a lot of different ideas here too. One thing to be careful of is to make sure that if the studs go into the water jacket that you use a thread sealing compound on those thread prior to installation. Forgatting that will give you a chance to take everything apart a second time when coolant leaks into the crank case.

The real reson for torquing the studs into the block prior to putting the heads on is to prevent the studs from backing out during future disassembly. From a clamp load perspective it doesn't make much difference whether the stud threads are rotating in the block or the nut threads are rotating on the stud as the final torque is applied. The majority of the applied torque is lost as friction in the threads and on the bearing surface and since the nut is rotating against the bearing surface in both cases that is not a variable. The difference will be in the friction characteristics of the threads at the top of the stud from the ones at the bottom. Becasue the diameters and pitches are very similar the friction will be similar (the tapped threads in the block may be dirty, but if they sieze up the stud wil stop rotating and we'll be back to the nut friction...). Actually, the variation caused by using a hand torque wrench is likely to be more than the difference caused by nut vs. the stud rotating.

Sorry to be so long winded.

Dick

 

[martx-5]

I think what the ARP site refered to when mentioning the studs, is that the studs are put in hand tight. Then everything is assembled and the nut/stud combination is torqued down. Upon re-torqueing, I don't think they meant you have to re-torque the studs, after all, they were only installed initially hand tight.

 

[Guest]

Great info! That is what this site is all about. Now, torquing for dummies, retorque after it has been run, right?

Bill

 

[Keoke]

Yep Bill, and in spite of all the GOBLY GOOK check that the studs are tight before setting the head a reasonable figure is 12LB/FT.---Keoke /ubbthreads/images/graemlins/hammer.gif