Torque To Yield Head bolts. Can they be reused or not on a 3.1/3.4 L GM?

It's all over the internet like a bad rash...plenty of controversy on that topic.

 

Devon,

I am well aware that you are our "resident" engineer and knowing this I have tempered my comments above accordingly. Also realize that I spent 5 years at GMI Engineering and Management Institute in Flint, with Rochester Products as my sponsor, back in the early 80's. It is common knowledge that engineers will point the finger at the acountants and managers. Many managers come from an engineering background. What confounds me is I see things on a daily basis that are completely devoid of any sound engineering. "If someone would have put 5 more minutes of thought into this" seems to echo off the walls of my shop now. 2007 Buick Lucerne w/30K miles came in with the rear suspension jacked to the moon. Seems that "somebody" thought that using the rear air leveling compressor assembly as a mudflap for the right rear tire was a good idea.

Contrary to what some might think, people don't forget the amount of repair expense they put into a certain brand of vehicle. Hence the recent demise of Olds, Pontiac, Saturn and Saab. In a true capitalistic society, GM in its entirety would be a memory too. People vote with their wallets.

But back to the topic. I wonder why if head clamping loads are so critical and TTY bolts have no business being re-used then why many torque specs call for a certain torque then a "range" for stretching?

 

Sheesh, is this beginning to turn into a pissing match?

 

Like you, I wish I knew the answer to that question.

Devon

 

WOW, TTY bolts should not be re-used! Sure you can re-use them but is it worth the risk of doing it again AND buying new bolts at that point? Way too much work for me to consider NOT replacing the bolts.

Just because there are few examples of cars/engines that turned out to have pitfalls does not mean that "Engineers" in general are at fault. I can see how an engineer could design something and have the final product come out with flaws after other people made changes. I can only imagine the compromises made when the various types of engineers, bean counters, etc get together to cooperate on a vehicle.

 

http://www.acl.co.nz/Tech/Torque To Yield Headbolts.pdf

 

The problem is that the production tap is sized to make new, correct diameter threads in a single pass in a drilled, non-threaded hole which produces some amount of diametrical interference with the tap (the hole is trying to squeeze the tap to a smaller diameter). When you run the tap through a second time, there is less diametrical interference, so the tap gets less "squeeze", and more metal is removed on the second cut.

Thread chasers are sized for zero diametrical interference; they're sized so that they fit their thread class without removing more metal no matter how many times they're run.

No offense to Larry Carley who wrote the article for that magazine. Either he misspoke, didn't know better or fell prey to the editors.

Hope that helps.

Devon

 

Trust me Sean, there are plenty of engineers who screw things up all by themselves, thus my first comment here about my loathing for "engineering people with no hands-on understanding". Look how many years it took for us to get the awful air conditioning quick-connects off the Ford fuel rails for God's sake. The only way we made it happen in the end was to sell it to production as a cost savings.

Devon

 

I wish the answer wasn't as simple as "cost", though that may be the case according to that writeup.

Devon

 

Well we all know there is a large gap between the "engineering world" and the "practical world". If people keep both in mind then things can work out well.

 

A couple things come into play here. The reason TTY or 'spring' bolts are used is because of aluminum's expansion/contraction co-efficient. A normal steel bolt cannot expand and contract as much as aluminum does. Think about it. A tty bolt or "spring" (which is what it really is), can keep a constant tension on the part by being able to stretch and contract a certain amount (dictated by the thickness and type of material it is clamping) with each heating/cooling cycle.

The reason for head bolts to be the ones determined to be replaced mandatory is because the outcome of re-use is much worse than that of other engine fasteners. Not many bolts on an engine that use tty bolts take as much punishment as the heads, thats why upper/lower intake bolts can be re-used, but not head bolts. Take an all aluminum engine apart some time. ALL the main bolts have to be replaced, they have been stretched to the max, their "spring" is gone, and if one fails, you loose the engine. If an intake bolt breaks, you MAY have a small vacuum leak...

 

I don't want to stray off topic, but I have to say the main reason people have problems with Dexcool is because they're mixing it with the regular "green" stuff. As soon as you do that and it touches aluminum, it starts to eat parts and gel into a pudding.

 

Fel Pro

 

I' going to take back what I said, to a degree...after reading about how bottoming taps are designed, I don't believe the danger of removing more parent metal is as great as it is with taper taps; the bottoming taps are designed to be used as a secondary operation after the initial tapping has already been done.

So I'll correct my comment by saying that for cleaning threads a thread chaser is still ideal, a bottoming tap a good second choice for a blind hole, and by all means avoid using a taper or intermediate tap.

Devon

 

No! Keep reading!

 

Dont use a conventional tapered or bottoming cutting tap to clean threaded holes, since a cutting tap can remove thread material, which can weaken the threaded connection. A chaser tap is designed to follow the existing threads, cleaning and removing burrs or other deformations. Essentially, a chaser tap cleans and re-forms the threads. Yes, this means spending more money on dedicated chaser taps, but its the only way to do the job right.

Arp makes a chasing tap comes in a set of 3 for around $45 Part number 912-0005 Expensive, maybe you can borrow from an engine rebuild shop.

I would probably go with the bottoming tap and inspect the hole closely to make sure you do not remove any parent material.

 

Found this site today that suggests either bottoming or chasing is okay:

http://www.milodon.com/2012 Milodon Catalog/Head Bolts & Head Studs.pdf

Dave's another mechanic I know personally and respect greatly. Thanks guys

 

You want to be really clever, use a bottoming thread forming tap. They ONLY displace material to create threads even from a drilled hole.

 

Brian,

Do you think this would work? $3.90

http://buy1.snapon.com/catalog/item.asp?item_ID=56051&group_ID=1145

 

Ahhh, yeaaaaaou: Tapped the holes and torqued the heads down tonight. No problems. I even reused one bolt because nobody offered the correct one with a drilled/threaded head for the a/c bracket.