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Old 06-14-2010, 02:53 PM   #1
Ron/IA
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Default Head Bolt Torque-Hot or Cold?

I have been reading with interest the posts of Larry Brumfield and others on cylinder head subjects such as: what compression you run, calculating compression ratio, head and block surface smoothness, etc.

While installing a club member's rebuilt engine a couple of weeks ago; a discussion occurred whether a head's bolts should be torqued hot or cold. There were four of us that day, and our consensus was the engine should be cold; at least that is what we had heard. But, we had to conceed that we had heard the bolts should be torqued with the engine hot too (just not as many times as cold). In other words, none of us really knew for sure which would be best, or why you would pick one way over the other.

Today I read an article written by Murray Fahnestock (1961) stating: "Standard shop practice is to adjust the tension of the cylinder head bolts when the engine is warm." But no where in the article did it say it was the right way. For that matter, Murray didn't actually state what temperature was correct.

If I torqued the head bolts with the engine hot; after shutting it off, getting the torque wrench in hand, and going through the torque sequence could take say - five minutes. The engine will be cooling during that time. Would the head bolt torque vary from the beginning to the end of the sequence? If I torque the head "cold" (usually meaning room temperature), is that meant on a 72-degree day here in Iowa or a 92-degree day in Texas? Would that 20-degree seperation in temperatures make a difference? (Sorry, this is just the way my mind thinks.) Or am I being too critical?

I would be interested in answers to the following question:

1. If you torque your head bolts cold; do you let the engine cool for so many minutes, over-night, or somewhere in between? Why?

2. If you torque your head bolts hot (assume engine operating temperature); do you think as the engine cools the torque is being affected? Why or why not?

3. If you torque your head bolts warm; what do you consider warm? Why do you torque them with engine warm?

In case you were wondering; the above mentioned engine had the head bolts torqued cold when the head was installed, and a re-torque was done warm after the initial start up. The car was then driven 50-miles/day, 8-times, over a 2-week period. The roads used were a combination of long and short flat stretches, hills (mild and steep), and towns. After that period, the head was re-torqued cold. It is now into the second day traveling to French Lick, IN, and as far as I know; all is well.
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Old 06-14-2010, 07:27 PM   #2
kevinmac/toledo
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Default Re: Head Bolt Torque-Hot or Cold?

hi; we used to rebuild lots of engines at the shop i worked at... never started them up... always torqued them cold... engine out of car... never had a comeback or complaint... i still do it that way... good luck... oh by the way , follow the sequence of torqueing in steps gradually - like 35lbs, then go up to 55lbs, then go up to 65lbs.. yo get the idea... always coated my head gaskets with silver paint,and let dry first.. never a problem -yet... lots of engines brought to life to my credit... and stll going....kevin
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Old 06-14-2010, 08:21 PM   #3
Larry Brumfield
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Default Re: Head Bolt Torque-Hot or Cold?

Except for the first time (explained below), if you want accurate torque, you better torque and re-torque COLD. By cold I don't mean pack the engine in dry ice, just cooled off. The time for it to cool off would vary depending on the outside temperature and how hot the engine was when you turned off the ignition. Naturally the outside temperature will dictate how cool it will ultimately get. Now having said this I wouldn't want to re-torque the engine with 100 degree Texas sun beaming down on it. Just use common sense.

Now as referenced above, I tell people to re-torque the first time when the engine is warmed up and all others only when the engine is cooled off. The reason is because I have seen the copper-clad gaskets compress so much and the nuts get so loose after the first run that if one waited for the engine to cool off, water might leak or seep into the engine. Since one re-torque is not enough there's no need to worry about the loss of accuracy the first time.

Now back in the Model A days, some had a different theory. Since pistons, when heated, often run with only a single thousandths of an inch clearance, the theory was that the block might get distorted from being RE-torqued while the engine is cold and this might put the squeeze on the pistons (binding with the cylinder walls) when the engine warmed up to operating temperature and everything expanded. Consequently, the re-torque of the cylinder head was often done when the engine was warm or at full operating temperature.

I've seen no evidence of this. In my opinion if they get in a bind, the pistons were too tight to start with.

Always Re-torque when cooled off.


Larry B.

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Old 06-14-2010, 10:21 PM   #4
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Default Re: Head Bolt Torque-Hot or Cold?

A reprint of an old HOt Rod Magazine article from the 50's said to retorque cast iron head cold and aluminum heads hot. Did not explain why the difference.
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Old 06-15-2010, 12:41 AM   #5
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Default Re: Head Bolt Torque-Hot or Cold?

The Ford service bulletins say to do it warm.
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Old 06-15-2010, 08:03 AM   #6
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Default Re: Head Bolt Torque-Hot or Cold?

Thank you gentlemen; so far we have an interesting group of answers, ranging from cold to maybe they used to torque the bolts hot.

I was aware of Larry's comment: "I have seen the copper-clad gaskets compress so much and the nuts get so loose after the first run that if one waited for the engine to cool off, water might leak or seep into the engine." My own experience has shown me that. Also, Larry's "binding" comment that torque done when cold could put the "pinch" on the pistons is one I have not heard before.

As for Mike's comment: "The Ford service bulletins say to do it warm." But is there a reason the bulletins would state this?

PC/SR's comment that cast be done cold and aluminum hot; I now remember that statement too. Is the reason because of the expansion/contraction properties of the two metals? (I don't know what they are.)

The consensus so far is leaning towards torqueing head bolts when the engine is cold, or had enough time to cool off to the touch.

What say you, other Fordbarners?
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Last edited by Ron/IA; 06-16-2010 at 07:22 AM.
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Old 06-15-2010, 12:57 PM   #7
Larry Brumfield
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Default Re: Head Bolt Torque-Hot or Cold?

ALWAYS re-torque aluminum heads when cooled off! Just ask Edelbrock.

The Ford Service Bulletin says "warm" for the reason that I cited in my post above. The theory makes sense, that is, for pistons that may be real close or a little too close in fit. However, like I said I've seen no evidence of this or a problem re-torquing when cooled off.

The reason I tell people to re-torque when cooled off is because the high compression heads, especially those of higher ratios, are not made for the Model A with its only 14 stud and nut clamping power and the head needs all the clamp it can get.

Why would anyone want hot or warm threads expanded against one another? How many of you have seen a torque procedure that says, "Always heat up your fasteners with a torch for maximum torque accuracy." ????

Now if the Model A had 21 or more studs like the early Ford V8 (same size, 7/16"), installation could be a little sloppy and there's still plenty of clamp.

I remember something I witnessed in my youth. A group of younger men were trying to remove a large nut on a shaft that was so tight they couldn't get it to move. So they decided to heat it up with a torch. After they got it good and hot, one of the men said, "Quick... turn the wrench while she's still hot!" They pulled and pulled and the nut wouldn't budge! An older man, who worked at the shop, was witnessing the event and yelled out, "Let that thing cool off ... or better yet, look here." He grabbed a cup of water and poured it all over the nut. A few seconds later he reached down and unscrewed the nut with his fingers!


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Old 06-16-2010, 07:27 AM   #8
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Default Re: Head Bolt Torque-Hot or Cold?

Larry - What an amazing analogy. Your story could not have illustrated an exmaple of torque any better. The only thing better would have been a video of the older man doing his demonstration.

Thanks,
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