Why Do you Back-off to Torque ?

marc hildebrant

marc hildebrant

Well-known member
Group,

I have just replaced the head gasket on the Model A. As such, I have been told to re-torque the head bolts after a short time, and then later after 500 miles.

The instructions from the garage was to re-torque, by first backing off 1/8 turn, and then torque to 55 lbs. per foot.

I have been doing this, but why do you first back-off the nut some ? Why not just re-torque to the value directly ?

I re-torque the head cold.

Marc
 
The idea behind it is so you don't have to overcome the "break away" torque meaning itll take more work to get the fastener to overcome initial friction to move it. That said I think its horrible advise to loosen and retighten the head bolts. every time you do that you stretch the studs possibly leading to early part failure. Torque is essentially a measure of stretch of the fasteners.
 
Believe what ever you want just because its a model a doesn't mean its somehow any different than industry standards. I dare you to find any other manufacturer that requires backing off head bolts. Or any other fastener for that matter. Any good machine shop will replace stressed fasteners (rod bolts, head bolts ) with new because they eventually reach an elastic limit where further cycling weakens the fastener. Also 25 years in aerospace field I have never heard of such nonsense. As for his sarcastic comment on forum experts..gotta agree because he falls into that category
 
Check out the procedure recommended for the Continental 1790 Cu. in. Engines In the mid 1950's. These were the water cooled versions used by the Marine Corps in their tanks and amphibs. Not arguing, just curious why they would have taught me that in the mechanics class?
 
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I admit, in light of contrary opinion, that I do slightly loosen a head stud nut to remove the "break away" force and then immediately tighten to my goal torque value. I can't say I even go close to 1/8 turn. Just enough to hear the "tic" as the static friction is broken and then immediately tighten. I do this in the suggested sequence just because I am used to doing so. Why?
First: I believe I am more accurate in application of consistent force using this method. JMO.
Second: I have a Brumfield head and that is what Mr. Brumfield suggests. **
I also use chrome moly studs and nuts and I do replace them if I am going through/freshening an engine so I don't think I will ever stretch one beyond its limit of functioning properly.
** Larry Brumfield has detailed instructions as to installation and maintenance of torque. Back when I purchased the head, I wanted his warranty to be honored so I followed his instructions and still do.

Applying and maintaining consistent pressure to all areas of the assembly is the ultimate goal of re-torqueing a head. The measuring of individual fastener torque assures us of meeting this goal. I typically re-torque a head two times after initial installation as the torque value changes very little after that. The type of head gasket will dictate variations to this procedure as to torque values and the number of times re-torqueing. For my own touring car, if I remember, I may check the torque at the beginning of the driving season (I live in Minnesota) or more likely, I will check it prior to leaving on a long road trip.

As to whether it is good mechanical practice to back the nut off prior to re-torqueing, I won't debate the issue as I don't think it much matters with a stock Model A head and block. Either way you execute the process will likely get you close enough for success. Just keep all of them torqued to around 55#.
Good Day!

www.durableperformance.net
 
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When re-torqueing, I NEVER loosen the nuts!!! Once it's "STUCK" down, LEAVE WELL ENOUGH ALONE!
IF, Model As had 27 head bolts, you "might" get away with it.
Bill Adimate
 
Hi Marc,

One humble opinion after (20) years of reading Model A Forum Messages:

There is usually a deeply embedded personal, mental and physical thought process for the 1/8 Back-Off reasoning which more than likely always coincides directly with one's individual mental choosing of Model A oil, air filters, oil filters and Model A whitewall tires.

Appears so simple.

For example, why chew tobacco if you prefer to chew Bubble Gum; and/or why chew if you do not like chewing? Just chew or don't chew and try to be happy.

Seems after we pass (65) our lives shift into Model A high gear where it seems Christmas happens at least every three(3) months ..... likewise, if we wait a little longer to further mature, the subject of chewing and the 1/8 turn will soon appear to be insignificant.

Hope this helps to stay calm and further analyse different Model A decisions.
 
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Group,

Didn't mean to stir up trouble....

From first hand experience, I have to say that some of the head bolts seemed to move much more than others when I re-torqued the bolts.

Since the garage that replaced the head gasket told me to first "back off" 1/8, I did.

Marc
 
marc hildebrant said:
The instructions from the garage was to re-torque, by first backing off 1/8 turn, and then torque to 55 lbs. per foot.
I have been doing this, but why do you first back-off the nut some ? Why not just re-torque to the value directly ?
Marc
Click to expand...

The answer is "stiction". That is, it takes more force to start the nut/bolt moving than to keep it moving. Torque specs ae based on taking the reading while the nut/bolt is moving. That is, pull until the wrench clicks (or needle reaches the right number) then stop. Thus, when you retorque without loosening, the wrench may not move and you think it's tight. But back the nut/bolt off a bit (like the 1/8 turn mentioned) and then retighten. The nut/bolt may move further.

Before you retorque, try marking each nut/bolt first with a marking pen, then see if it moves. If not, try loosening first, then retorque. See if it moved. If the nut/bolt is close to spec before retorquing, it probably won't move. If the gasket has relaxed a bit, the nut/bolt will probably move without loosening.
Unless you overtighten beyond the bolt/stud's elasticity limits, you will not damage it by loosening before retorquing.
 
try this on your head bolts. Mark the nut and head on the two center head bolts between 1 and 2 and 3 and 4. to see how much they move. Back off one and just torque the other with out backing off. which one moved the most, or were they the same.
 
marc hildebrant said:
Group,

Didn't mean to stir up trouble....

From first hand experience, I have to say that some of the head bolts seemed to move much more than others when I re-torqued the bolts.

Since the garage that replaced the head gasket told me to first "back off" 1/8, I did.

Marc
Click to expand...

I'am thinking thats why I was instructed to torque first time at 55# and there after torque to 60# ? I just following my rebuilders instructions :-)
 
Hi Marc,

Per your comment in #9:

"From first hand experience, I have to say that some of the head bolts seemed to move much more than others when I re-torqued the bolts."

Why not move much more? The head gasket is not solid steel and can be compressed.

Think about a building foundation.

If 6,000 pounds is placed on a 12" square footing and 6,000 pounds is placed on a 60" square footing on the same type of soil, which footing will sink more.

Next, look at your head gasket to try to visualize how many square inches of head gasket each nut is compressing with 55 ft. lbs. of torque.

Similar to chewing sweet potatoes with two teeth or chewing sweet potatoes with twelve teeth with the same jaw force.

Nothing too complicated when Model A head gaskets are compared to sweet potatoes.
 
Hey jetmek,
I go along with some of your thoughts written here.
I'm just wondering how any business/work can get around the laws of nature.
In this case INERTIA, that is....a body at rest tends to stay at rest, whereas a body in motion tends to stay in motion.
 
Humble Opinion:

Model A Forums are often read by the very same exact people who are interested in watching the news and/or reading the newspapers.

For example, like the news there is always:

1. A Model A opinion fight on the headlines about non-detergent oil, filters of all sorts, whitewall tires, and both mechanical types of torqued nuts and non-sense ideas expressed by human types of Nuts ; and,

2. A few Model A sob stories like we see in Dear Abbey where somebody maybe has water in the oil pan, or Babbitt fragments in the oil pan; and,

3. The Model A funnies are loaded with the exact same types of characters we see on this Model A Forum where Model A comedians get the most laughs when they are adamant and most serious with their know-it-all solutions ; and,

4. Don't forget that there is a Model A Classified Section with Model A wanted and for sale items; and,

5. A Model A "Search" feature where one can obtain prudent Model A advice and/or read some of the past amusement articles addressed in (1) through (4) above.

6. Like with the news or newspapers, never a reason to get our underwear knotted up; just get a cup of coffee, relax, sit back and either enjoy reading the newspapers or a Model A Forum ........ the day after we cross the Rainbow written articles will not matter .... but they will continue to be written.
 
Marc is a great gentleman, like so many here, who has great Model A questions; and that as witnessed above, has multiple sincere Model A answers.

None of us will ever have the same Model A answers; but taking time to offer different Model A opinions will always make this Forum interesting to enjoy reading.
 
40 Deluxe said:
The answer is "stiction". That is, it takes more force to start the nut/bolt moving than to keep it moving. Torque specs ae based on taking the reading while the nut/bolt is moving. That is, pull until the wrench clicks (or needle reaches the right number) then stop. Thus, when you retorque without loosening, the wrench may not move and you think it's tight. But back the nut/bolt off a bit (like the 1/8 turn mentioned) and then retighten. The nut/bolt may move further.

.
Click to expand...


This is the reason!
 
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