Torque at different temperatures

[curtis cutter]

As I was tightening something at about 25 degrees F this morning, it occurred to me that there may possibly be a change of a torque value if say a 25 degree head bolt 6" long goes into a 25 degree engine block as opposed to both pieces being at say 90 degrees.

Is there a particular standard temperature that is used when establishing torque values?

[John Hasler]

If both materials have approximately the same temperature coefficient of expansion it doesn't matter.

[Steggy]

As I was tightening something at about 25 degrees F this morning, it occurred to me that there may possibly be a change of a torque value if say a 25 degree head bolt 6" long goes into a 25 degree engine block as opposed to both pieces being at say 90 degrees.

Is there a particular standard temperature that is used when establishing torque values?

"Officially," torque specifications are at room temperature. However, as John noted, as long as all parts involved have similar coefficients of expansion you can rest easy.

Now, if it were -25 F while you were doing this I'd say put down the torque wrench and go for coffee.

[spro]

These are things we wonder about. A very cold iron block outside, and an aluminum head with new bolts. The head,gasket and bolts are at least 50 degree warmer than the block. Sure there is a raise of temperature around the block with lighting and bodies holding and breathing. Under a hood with a tarp covering. Ideally everything would be the same temperature but as long as the torque pattern is correct and retorqued in sequence, it seems to work. There may be a reason to this, in that the head gets hotter than the block anyway.

[John Hasler]

These are things we wonder about. A very cold iron block outside, and an aluminum head with new bolts. The head,gasket and bolts are at least 50 degree warmer than the block. Sure there is a raise of temperature around the block with lighting and bodies holding and breathing. Under a hood with a tarp covering. Ideally everything would be the same temperature but as long as the torque pattern is correct and retorqued in sequence, it seems to work. There may be a reason to this, in that the head gets hotter than the block anyway.

It's the temperature of the aluminum head and the steel bolts that matters. The bolt shanks stretch when you torque them. Aluminum expands more with temperature than steel does so if the head had been very cold the bolts might have been in danger of breaking when the engine reached operating temperature due to the extra 50 degrees worth of expansion overstretching them. If both were at "room temperature", which is probably what the torque spec assumed, you should be ok.

[curtis cutter]

I appreciate these responses.
Certainly the different expansion rates of the various metals plays a part in this. I assume that in the case of cylinder heads on blocks, the head gasket itself can absorb some of this expansion (or contraction. Could this be why you may want to let an engine idle or warm slowly before you really heat up a cylinder head while the block and bolts are still cold?
I have always been one to look up torque specs on just about everything I do that I consider critical such as transmission band adjustments etc. Just never see it noted that they are to be made "At room temperature" or some specific condition.

[GlennW]

A five inch long head bolt would only change about a thousandth and a half in length over a fifty degree ambient temperature swing.

That would be far within the acceptable stretch range.

The rod bolts I use call for a .007"-.010" stretch and they are only two to three inches long. (Yes, you use a stretch gauge, not a torque wrench)

A torque value on a fastener is only an approximation of stretch to begin with as there are a number of factors that can affect it.

[curtis cutter]

Thanks Glenn. I had not considered the application of torque in regard to the stretch of the object being torqued.

If I am adjusting a bolt on the end of an arm, I am not only applying pressure to the bolt but an amount of stretch to the arm. Albeit most likely minimal stretch, but still there never the less.

Gregg

[NP317]

Shall we assume the torque specs are for dry un-lubricated threads?
~RN

[warmstrong1955]

Most all torque charts are for lubed or plated threads.

Bill

[John Hasler]

A five inch long head bolt would only change about a thousandth and a half in length over a fifty degree ambient temperature swing.

That would be far within the acceptable stretch range.

If the head is cast iron it will expand almost exactly as much as the steel bolt so there will be no significant change in the stretch as long as both bolt and head were at the same temperature when torqued. I agree that even if they were fifty degrees apart when torqued the excess stretch would be negligible. However if the head was aluminum it would expand about twice as much as the steel. If the bolt was at room temperature and the head 50 degrees colder at the time the bolt was torqued you'd have three thou excess stretch when the head came up to room temperature.

The engine in my Dodge truck has a broken manifold bolt. This problem is common for these engines and it is clearly due to an error in accounting for differential thermal expansion.

[Rich_Carlstedt]

Shall we assume the torque specs are for dry un-lubricated threads?
~RN

Torque Specs require lube on the threads as well as the load bearing surfaces ( ie bottom of the head)
Dry torque is 15 % higher to account for the lack of lube

Rich