Torque to yeild bolts

P

PEI Rob

New member
Here's one for you guys. Take a properly torqued bolt, say 100 ft lbs then turned 90` and turned 90` again so it stretches. Clamping force is easier to determine because the bolt material strength is known and the bolt body is stretched. For the sake of arguement,say it has a 20,000 lb clamp force.

Indicate the bolt position and remove it. Reinstall it to the indicated mark. Will it still have the same 20,000 lb clamping force? Please explain.
 
Wag

PEI Rob said:
Here's one for you guys. Take a properly torqued bolt, say 100 ft lbs then turned 90` and turned 90` again so it stretches. Clamping force is easier to determine because the bolt material strength is known and the bolt body is stretched. For the sake of arguement,say it has a 20,000 lb clamp force.

Indicate the bolt position and remove it. Reinstall it to the indicated mark. Will it still have the same 20,000 lb clamping force? Please explain.
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Just a WAG (wild a** guess) but the material would be the deciding factor. Maybe one of the metallurgists that frequent the site can elaborate. If you are within the elastic range on the stress/strain curve, the material should go back to it's original dimension, once the force is removed. If you exceed the elastic range and get into the permanent deformation area of the curve, the material has elongated. Every steel and alloy has a specific stress/strain curve designers use when selecting materials. There are gigantic studs that hold nuclear reactor heads on reactor pressure vessels and turbine studs that are heated along with hydraulic tensioning machines to elongate the studs before the nuts are fastened. They re-use these studs after subjecting them to this because the material used in their design was specifically selected for this use.

Lou Baccino
 
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I though the same until recently .
I replaced a cylinder head on a 2007 Chrysler and the torque to yield bolts were re-usable if they did not show signs of stretching. (visual check with a straight edge) Per Chrysler.
I would not apply this across the board though as the metal composition between this application and others may be different.
 
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Yield

Keep in mind, at 100 ft pounds, the bolt has already been stretched a certain amount. Another 90 degrees, and then another, just might overcome the yield point of the material, which is that point where the material does not come back to it's originol configuration when released.

Like was said before, it depends on the material. A bolt made from an alloy steel with a 150,000 pound yield will stretch a lot less at 100 ft lbs than a bolt of the same size made from material with a 80,000 yield. Also, If you stretch a 150,000 bolt .008, and a 80,000 bolt ,008, the 150,000 will have more actual clamping force.

As long as you do not exceed the yield strength of the material, the bolt should come back to it's exact length when you loosen it..........jackie
 
PEI Rob said:
Here's one for you guys. Take a properly torqued bolt, say 100 ft lbs then turned 90` and turned 90` again so it stretches. Clamping force is easier to determine because the bolt material strength is known and the bolt body is stretched. For the sake of arguement,say it has a 20,000 lb clamp force.

Indicate the bolt position and remove it. Reinstall it to the indicated mark. Will it still have the same 20,000 lb clamping force? Please explain.
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No, as a fastener is torqued and removed, there is wear. Every time you return to a marked position, the clamping force is reduced. That is why a fastener should be retorqued every cycle, or stretched to a specified force, if force is to stay constant.

To determine if a fastener is yielded, you measure its change in length.

We do this with large diesel engines as we stretch the staybolts, we measure the increase in length with a dial indicator. If the bolt is too long or..if it increases in length beyond a specified change in tolerance, as we stretch it hyraulically, we know there is a problem with the bolt/material and we replace it.

Ben
 
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I have replaced literally buckets of head bolts and wondered why they are not reusable. I can tell you the answer is a definite no, but I have no idea why. I'm talking about bolts that are specifically designed to get stretched past their yield strength and remain stretched after removing, visibly. The only thing I can think of is that they are stretching when being removed. Remember they are stressed past their yield strength and then untwisted, possibly this stretches them some more?
 
On head bolts I think the manufacturers are trying to avoid having to specify a retorque as part of the first service. I used to build a few performance engines and I built a test stand to allow me to break in the cam for 45 mins , set the timing and check oil pressure, etc before installing or delivering the engine.

The day after running in the cam I would come in and drain the coolant, remove the exhaust and valve covers and retorque the heads. My method of retorquing was to loosen the fastener until it's torque face was not touching it's seat and then torque it to about 50% of it's full torque. Do all the head's fasteners this way then start doing the normal incremental torqueing of the fasteners in the order in the manual.

What I found when loosening the fasteners was that there would be some with nearly full torque on them but others would have noticable less preload on them and some had much less torque than they had been originally installed with.
 
Torque to yield bolts

I'm no Bullwinkle, but when I was still working, I dealt with a lot of engineering principles. This is one of them. Steel works with two limits. So long as you stress it below it's "elastic limit" it will return to it original shape and configuration when that force is released. If you stress it beyond it's "plastic limit", it is permanently deformed and when the force is released it stays close to where it was at max strain.

Torquing the bolts beyond their "yield" is in effect somewhat of a failure of the material passed it plastic limit.
 
Young's modulus

jackie schmidt said:
A bolt made from an alloy steel with a 150,000 pound yield will stretch a lot less at 100 ft lbs than a bolt of the same size made from material with a 80,000 yield. Also, If you stretch a 150,000 bolt .008, and a 80,000 bolt ,008, the 150,000 will have more actual clamping force.
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In the elastic region (lower than the yield stress), the stretch of the bolt for a given tension depends on Young's modulus, not the yield stress. Young's modulus is surprisingly similar from cheap mild steel to high tech alloy steels, only about 25-30Mpsi (http://www.engineeringtoolbox.com/young-modulus-d_773.html). So for the example above, the bolt with half the yield stress may stretch a few percent more, but only if it is made from a different alloy. Just a clarification.:)

Cheers,
Keith
 
I have TTY head bolts on my Ford 6.0 diesel engine along with a few million other owners. When the heads are to be removed new bolts have to be used to avoid lifting the head with normal turbo pressures.
 
Torque to yield head bolts came about as a result of using aluminum cyl. heads on cast iron blocks. The different expansion ratios of the dissimilar metals caused head gasket failures. The TTY bolts gave enough "preload", for lack of a better word, to keep the head gasket under enough tension to hold under the stresses of the two different materials expanding and contracting at different rates. The difference creates a shearing effect on the gasket as the top half migrates with the head, and the bottom migrates with the block. The bolts also go through these expanding/contracting cyles. The TTY bolts address this as well and will keep good, constant tension--Mike
 
mwezell said:
Torque to yield head bolts came about as a result of using aluminum cyl. heads on cast iron blocks. The different expansion ratios of the dissimilar metals caused head gasket failures. The TTY bolts gave enough "preload", for lack of a better word, to keep the head gasket under enough tension to hold under the stresses of the two different materials expanding and contracting at different rates. The difference creates a shearing effect on the gasket as the top half migrates with the head, and the bottom migrates with the block. The bolts also go through these expanding/contracting cycles. The TTY bolts address this as well and will keep good, constant tension--Mike
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Unfortunately, the Navistar 6.0 diesel with cast iron heads and cylinder block are plagued with problems because of these TTY bolts. The solution for many was to go to a aftermarket high strength stud.
 
Ljutic said:
Unfortunately, the Navistar 6.0 diesel with cast iron heads and cylinder block are plagued with problems because of these TTY bolts. The solution for many was to go to a aftermarket high strength stud.
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I wasn't aware of that as I've been out of that line of work for quite a while now...but being a diesel, I can see the problem being multiplied with the much higher compression of the diesel.---Mike
 
My 670+hp blown/nitroused LS1 engine in my GTO uses TtY bolts for the cylinder heads.

If they ever need to come off they go straight to the trash and get replaced with new ones according to the book.
 
These types of fasteners are probably extremely abusive to the threads in the block. Any serious engine I ever built got equipped with ARP studs.
 
on land rover v8's they recommend tty headbolts on composite gaskets but not on steel shim type gaskets. they also dropped the outer row of head bolts on composite gasket engines. when the headgasket on my discovery blew it was right where a head bolt would have been if it was an earlier engine.
 
Agree studs are the far better way but if a guy ever has to pull the heads on a motor from a modern V-8 street car good luck getting them off without tearing the whole engine out. My GTO has a good portion of the engine under/behind the leading edge of the firewall.

So far (over 10k miles on a blown/silly juiced engine) I've had no issues with cylinder heads/gaskets. Fingers crossed. . . .
 
NesikaChad said:
Agree studs are the far better way but if a guy ever has to pull the heads on a motor from a modern V-8 street car good luck getting them off without tearing the whole engine out. My GTO has a good portion of the engine under/behind the leading edge of the firewall.

So far (over 10k miles on a blown/silly juiced engine) I've had no issues with cylinder heads/gaskets. Fingers crossed. . . .
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How much boost from your blower 10-12 psi.?
 
I'm using a Magnuson 122 supercharger. I run at 7lbs on a cool night.

Yes, quite conservative for squeeze I agree. Boost isn't everything IMO and often times running more just means added heat and drivability issues. Having free flowing heads, the right cam, and the right exhaust to purge all that extra heat helps too. Also makes for a very nice daily driver.

I went about this a little differently. The Maggie uses a jack shaft that runs to a pair of drive pulleys on the back of the blower housing. The pulley up front that ties into the serp belt isn't real big. They make smaller ones to crank up the boost but the surface contact with the belt is borderline already IMO. So, I bumped up the OD of the balancer pulley to add a bit more RPM instead (ATI super dampener). The bad side to this of course is added wear/tear to accessories. This thing loves to eat alternators now. I have to make a larger pulley for it to keep from frying the regulator. My next plan is to swap the rear pulleys on the blower. This'll overdrive the screws a bit more and should make the car "super duper" snappy. (as opposed to now only being "super snappy.":D) I should still have enough fuel pump to feed this pig. Hopefully anyway. I have a Ken Bell pump booster sitting in a box but if I can avoid using it I will. I'm about out of room under the hood!

Right now I'm fighting an intermittent miss. I have MSD everything and put new plugs in it, but I'm still getting a miss now once the car warms up. Cold its fine. I think I may have cooked an O2 sensor. I keep getting "lean bank" trouble codes.

Musclecars; more maintenance than trophy girlfriends!
 
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