Head re-torque debate

[SIVAllan]

Air-pine has been in the shop for a few things. Should be on the streets tomorrow!

Shop guy is an engine rebuilder. For years all he did was rebuild engines.

Now he works on anything that rolls into his shop and prefers the odd and unusual. Runs his ads to bring in every odd ball car in need of a repair.

We have been 'debating' retorqueing the head, since Air-pine has a new head gasket.

He maintains that the type gasket used today (black type from SS) does not need retorqueing, that it can even be counter-productive, especially should an old thread boss give way when retorquieng the head bolts.

In his view, the "book" was written during an era when a different style head gasket was used and that back in the day, retorqueing was necesary, but not with today's gaskets.

So...thought I'd post on the need to retorque with modern head gaskets...?

Allan

 

[Bill Blue]

Allan, you know my automotive background, not technical. But I do not buy it. The bit about a boss giving away while re torquing. If it does, don't think it was doing much good anyway. As for the rest of his argument, isn't the new gasket from SS giving trouble? if they were worth a damn, they would not be blowing, would they? So I don't buy that argument, either.

Bill

 

[RootesRacer]

The manual says to retorque, plus the "new" gaskets are still 60s technology and materials.

It absolutely could not hurt to retorque unless something else is fundamentally wrong.

 

[SIVAllan]

Thanks guys,

A followup (rookie) question...

"Retorque" means merely to retorque to make sure all head bolts (and the two stud nuts) are still at 48 pounds...NOT to loosen and start over.. ??

Allan

 

[Green67Alpine]

Allen, There's been a tech update (back in '68 IIRC) that lowered the value to 45 lbs. I'm sure others will chime in.
congrats ! ! on getting the Air-Pine on the road , what ever you do don't add up the receipts..............

Tom j

I think retorque is to back-off then bring back to torque value, not loosen and start over.

 

[jumpinjan]

I read many, many years ago that the composite gaskets do not need re-torqueing. The sheetmetal head gaskets used on the small block Chevy V8, do require retorqueing.
I have tried to check the torque on the head bolts of Alpine engines that I rebuild and I found they didn't need any retorqueing.
Jan

 

[SIVAllan]

Criminey! 45 pounds..?

The forum tech page lists 48 pounds...

What is the impact of 45 vs 48 .. ??

Allan

 

[Nickodell]

Thanks guys,

A followup (rookie) question...

"Retorque" means merely to retorque to make sure all head bolts (and the two stud nuts) are still at 48 pounds...NOT to loosen and start over.. ??

Allan

Correct. Simply ensure that the wrench is at the correct setting, put it on and twist until it clicks (or reaches the proper setting if a dial type). Usually after a few hours following the initial torquing the wrench will turn a fraction before the click.

By the way, torque wrenches, especially cheap ones, can be inaccurate, and in any case even a good one can change its characteristics with time and use. It's a good idea to check it against a known accurate one, or if you aren't certain that even the other one is accurate, against two others (to average out any error).

 

[Green67Alpine]

Allan, check this. http://teae.org/forum/viewtopic.php?t=773

Tom j

 

[sd_pace]

I would re-torque ... but i dont know what the impact would be to re-torque from 48 to 45lb and also curious as to why the change

 

[lemansvk]

Interesting...

If the head has been initially torqued to 48, then I suspect that putting the (same) wrench on set to 45 will achieve very little - will probably just click immediately without moving anything. My experience is that when retorquing even at the same setting, very little happens anyway.

Also, I know someone who has worked on these engines for many years and says that the best way to retorque is to back the bolts off (one at a time, working in the torquing sequence) and then bring them back to the specified torque. He claims this reduces the incidence of old bolts stripping or snapping. I have no knowledge of whether this is correct or not (so don't blame me).

Cheers, Vic

 

[sd_pace]

the proper way to re-torque is to back off the tightness of a bolt and then bring it back to its specified amount of torque... as the properties of a new gasket seats when first heated up .... the question was by doing this to a lesser amount what will the impact be on the gasket... neutral, negeative, or positive

 

[Jim E]

Never blew an Alpine HG, always retorqued at least three times at 300 to 500 mile intervals with motor stone cold, never backed the bolts off before running them, bolts were always a bit loose on the second torque.

 

[Alpineracer8]

Hey, guys:

I don't mean to add more confusion to this discussion, but I thought it interesting that even the pros aren't definitive when it comes to the torque/retorque subject. The copper head gasket that I just installed on my race car came with a detailed set of instructions. Under the "Torquing" section, it states very clearly that the jury is still out on whether or not you would need to retorque the head after the installation of the copper gasket. The manufacturers personal experience was that he saw no reason for it and never did it on his cars or any others he worked on. I should point out, however, that he races a TR4 which has a cast iron head...

I've always been told that the proper way to retorque a head was to back the nut or bolt (whichever you have) off enough to get it off it's original torque value, then run it back down to the proper torque value. It seems to me that the whole reason for doing this is because you are dealing with a crush factor on a factory head gasket. The pure copper gaskets do not crush at all, therefore the need to retorque the head should be virtually eliminated.

Personally, unless I start having any weeping of water between the block and head, or any other tell-tale problems, I do not intend to retorque my head on the race car. However, if I were running a stock gasket, you bet I'd retorque it...

Take care,

 

[Nickodell]

IMHO, the only possible rationale for backing off bolts or nuts before retorquing would be to reset at a lower value. The action of torquing down head bolts is to compress the gasket, and the purpose of retorquing a few hours of use later is to compensate for the gasket "squishing" slightly. I don't buy the twaddle of less chance of stripping threads, etc.

Also, and again IMHO, it is pointless and will possibly lead to a blown gasket to back off from an existing setting of 48 lb/ft to 45. If you set it to 48, leave the bloody thing alone.

I would like to see a definitive technical ruling (as opposed to a nontechnical opinion) on this, however.

 

[MikeH]

Must you use new head bolts when replacing the head or head gasket. I had a Datsun pick-up that had an aluminum head and the bolts were not reusable. They were called torque-stretch or stretch to torque. Found out the hard way when the gasket blew and the head warped after 4000 miles from replacing it the first time.

 

[RootesRacer]

Not sure if this qualifies as a technical explanation, but it seems that each time I put a new gasket on the alpine, the lash seems to magically open up (get noisier) until the gasket is retorqued some period after the initial break in.
I then reset the lash and it stays put for a long time.

It makes sense that a squishy gasket would "set" and need additional tightening to insure proper sealing long term.

New gaskets tend to have less springy materials, often not even having a composite layer. As a result these newer materials dont often require a retorque.

 

[sunbby]

I always thought you must loosen a bolt before retorqueing because you're supposed to measure torque while the bolt is rotating. Don't most wrench instructions say something like use a smooth stroke and do not stop and start again? (that came out funny didn't it )

Since static friction is greater than dynamic it will take more torque to start rotating a bolt than to keep it rotating. So if your bolt tension is reduced due to gasket compression or whatever, and the resulting "torque" was reduced to say 40 ft-lbs, it may take 48 ft-lbs to get that bolt turning again. Unless you back it off and start over your click type wrench will just click right away.

 

[Nickodell]

This whole subject was debated some two years ago, including the contrasting opinions of whether to lube the threads or not.

Let's look at the basic engineering as I remember it. What we call a "torque" setting is really nothing of the sort. Threaded fasteners apply a force to the parts to be joined by the movement of the movable item (bolt or nut) down the helical path of the threads. To reach the appropriate force, and ensure that they don't loosen, a designed amount of stretch is applied to the bolt or stud, and a designed amount of friction is produced between the male and female threads (crude description, isn't it? Only a man could have thought that one up). We use a torque setting to arrive at these, but what we are actually measuring is friction.

It is true that dynamic friction is always lower than static friction (or "stiction"). Stiction is produced between two solid objects pressing together without sliding, and requires a certain amount of force (parallel to the plane of contact) to overcome static cohesion before the surfaces can move against each other. Stiction is a threshold, not a continuous force. However, the act of moving the fasteners in order to reach the desired torque setting by definition is measuring dynamic friction and stretch, so I see no reason to back off and retorque to overcome stiction.

To open up an old controversy; dynamic/static friction with dry threads is always considerably higher than those for lubricted ones, so I was taught that unless the specs require lubricated threads you should always assemble with clean, dry threads. If dry threads are specified, or assumed, tightening with a torque wrench to the same reading with lubricated ones can result in a much higher stretch of the fastener than specified, and potential failure, so the torque reading should be reduced as much as 10% with oil and 20% with antiseize compound or molybdenum disulfide. I imagine the lubricating effect of Loctite etc. would have the same effect.

If you don't believe it, try a little experiment that I devised some years ago. Get a bunch of 3 1/2" X 1/4" bolts and nuts, and a piece of brass 3" thick, in which you drill a 9/32" hole. Put a bolt through the hole and put a flat washer and nut on the other end. Then use your torque wrench and apply steady force until the bold fractures. Note the reading. Then repeat three times to average the failure force out. Now repeat the experiment with lubricant on the bolt threads, first oil and next MoS2.

 

[wframe]

Nick,
Is it some of that "stiction" that holds your "twinpine" together? Or is it more "photoshop-iction"?

Your whole text seems to fits with my dear, departed mother's favorite saying: "I know you believe you understand what you think I said, but, I'm not sure you realize that what you heard is not really what I meant!"