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post #1 of 6 (permalink) Old 11-08-2005, 10:45 PM Thread Starter
 
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Question on TTY bolts

I understand that TTY bolts are designed to be used only once, but was just wondering exactly when they became unusable again.

I was told that after you torque them down to specs you could unbolt them, remove heads, and reinstall them as long as they were not heated up by engine. It was explained that the actual torquing process was not complete until the engine had warmed up, causing the bolts to expand and then cooled off causing the bolts to contract and tightening even further and completely sealing the head. The yielding part is the process of stretching and contracting the bolts during the warming and cooling off of the engine.

I always thought that the bolts stretched as they were torqued down on the engine and were useless if torqued and then removed.

Thanks for the input.
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post #2 of 6 (permalink) Old 11-08-2005, 11:37 PM
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In most application fasteners at torqued to ~80% of Yeild strength. Now personally I do not know the PSI strength of our head bolts, but consider this. They are approxiamatly the same diameter as the wheel studs but at Torqued to a much higher spec. Therefore I'd say that you might be able to remove them before running the engine you run the risk of reusing at least one bolt that did reach Yeild prematurely, coupled with the fact that torqueing that removing does produce a good amount of heat on the bolt anyway.

To sum it up if you have complete the final torque squence and have to remove the head get new bolt no matter what IMO.

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post #3 of 6 (permalink) Old 11-09-2005, 12:16 AM
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I had a guy at ford explain to me that tty bolts can sometimes be reused. I'm not sure if I believe it but here goes. If you get the required torque you're looking for, then it's good. But you may not get it. The problem as I understand it is that the threads of the bolt that don't get used get stretched. So for example a bolt with two inches of thread is threaded into a hole one inch deep. The 2nd inch of thread which is unused will stretch and and the thread-pitch is now off. But now that the bolt is stretched and thus longer, the messed up portion of thread will now be used. This will cross-thread an aluminum head. This plus the obvious part where a stretched bolt is weakened.
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post #4 of 6 (permalink) Old 11-09-2005, 03:08 AM
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you should only use the tty bolts once, no matter what, if you are having to remove the head frequently after installation you need to go with studs, they will be cheaper in the long run, if i ever do a set of hg's on my 3.8's again i'm going with studs.

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post #5 of 6 (permalink) Old 11-09-2005, 08:48 AM
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Quote:
Originally Posted by nickmckinney
All bolts stretch between the head surface and the engaged threads when torqued. TTY stay stretched some when removed. They are designed to not loosen on their own compared to regular bolts, which is why manufacturers love them. Replacing them is alot cheaper than losing another head gasket from a bad bolt.

That's pretty much the jist of it - but it depends on the torque value not the bolt.


Bolts have a failure rating and a yield rating. (Yield meaning Plastic Deformation)

When torqued to yield it means they are torqued to the point where they yield, or stretch and deform. Which makes them into a spring for all intents and purposes. This locks them in place and they don't loosen from many heating/cooling thermal expansion cycles like with aluminum heads.


But once torqued to this yield point they will never be able to tighten again, they will just keep stretching and not hold their tightness, you can keep trying to bring them to torque until they snap.


This is why you don't want to overtighten bolts - tighter is not better. Once you stretch a bolt it will never hold it's tightness causing leaks etc. Then you will most likely snap it trying to get it to tighten up.

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post #6 of 6 (permalink) Old 11-09-2005, 05:45 PM
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Yeah, I wasn't at all clear on this but I would never re-use tty bolts. And I didn't mean to suggest anyone else should try it. Sorry if there was any confusion. I
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