Is there a "useful" life for head bolts? When to re-use them and when not to???

[Jeffu]

I am just curious to know if there is a rule of thumb on the "useful" life of the headbolts??

Let me explain a bit further. I am about to pull the both heads off of my motor to have them cleaned and re-worked. I am doing this because I have one head gasket that is weeping a small bit of coolant into the VOD. Since I have to pull the one head I figured I should pull both and replace both head gaskets since I am there.

I am the 2nd owner of this car. Shortly after the orginal owner bought the car in '81 he put an aftermarket BAE turbo on. He drove with new BAE unit for a small bit until he burnt a piston up. The motor was pulled and he had forged pistons put in.

My Question: Since this would be the 3rd time that these head bolts will essentially be re-torqued, is there anything I should be concerned about with re-use? Could using them on a 3rd install weaken them? Is it better not to re-use them in this case? Aside from any visual defect on the threads, etc that would be a given for replacement. I would also be taking the time to thoroughly clean & chase the threads in the block.

Any advice or comments would be greatly appreciated. Thanks!

[deloumis]

I am just curious to know if there is a rule of thumb on the "useful" life of the headbolts??

Let me explain a bit further. I am about to pull the both heads off of my motor to have them cleaned and re-worked. I am doing this because I have one head gasket that is weeping a small bit of coolant into the VOD. Since I have to pull the one head I figured I should pull both and replace both head gaskets since I am there.

I am the 2nd owner of this car. Shortly after the orginal owner bought the car in '81 he put an aftermarket BAE turbo on. He drove with new BAE unit for a small bit until he burnt a piston up. The motor was pulled and he had forged pistons put in.

My Question: Since this would be the 3rd time that these head bolts will essentially be re-torqued, is there anything I should be concerned about with re-use? Could using them on a 3rd install weaken them? Is it better not to re-use them in this case? Aside from any visual defect on the threads, etc that would be a given for replacement. I would also be taking the time to thoroughly clean & chase the threads in the block.

Any advice or comments would be greatly appreciated. Thanks!

Head bolts really should be replaced each time the heads are off, but can be reused one time. If your at the point where your having your heads off for the third time on your block, I would say if possible replace them. I'm sure you can reuse them and get away with it. But if the very last bolt snaps on the last head you bolt on, you will have to remove both heads to replace all bolts. I would say replace them if possible.

[opethmike]

Seriously, replace them. They're a bit of money, but do you really want to risk breaking one, or having to go back in yet a fourth time?

[David T]

The head bolts on a Delorean are not tightened anywhere near their yield point so they can be reused. All of the usual cautions are that they can't be bent, the threads distorted, the lengths checked, etc. You will pull them out of the head before you overstress the bolts. More important then the bolts, all of the threads in the bock should be checked.
David Teitelbaum

[Jeffu]

Thanks for the input guys, I appreciate it.
I have to say I am kinda on the fence with this one...at least at the moment. New bolts might be the best approach?? However, if what Dave is stating is true it would appear to me that the true "weak link" just might be the cast aluminum threads in the block and not the bolt. If the tinsel strength of the bolt outweighs the block threads then that is a whole other issue. Nonetheless, I am not a metals expert by any means! It's just that I would rather spend the $112 dollars on something else if the real issue wasn't the bolts. Please don't get me wrong, I have spent Hundreds of $100 bills on this little car and will continue to spend more:cheers: I just don't want to spend it if it really would not be necessary.

[tgraham]

Unless your bolts are damaged, you'll rip the (aluminum) threads out before you break the (steel) head bolt. Re-use the old ones unless they're bad (probably bad threads).

One thing that's worth noting: since the head bolt holes aren't blind, you can actually tap/chase the threads all the way through and you'll end up creating more threads towards the outer edge. Use some cutting oil and a tap (not just a chaser, if you can find an M12-1.5 chaser), because you will end up cutting a few more threads in the heads. IMO, this is a good thing to do to ensure that you're not "bottoming out" the bolt on the non-threaded end of the bolt hole. Perhaps your bolts are slightly stretched and this is your issue? Just be sure to clean the holes out thoroughly to get all of those shavings out. You don't want to deal with damaged head bolt threads if you don't have to (ask me how I know).

Also watch out for one more thing: the PRV came with at least two sizes of head bolts, one with 27mm of threads and one with 30mm. IIRC, the DMC-era PRVs use the 30mm variety, but if you find some old NOS head bolts collecting dust at some Volvo warehouse, they may be the other size (and less threads could matter).

Travis

[Jeffu]

Travis good to hear from you! Thanks for the input. I have been creeping around your website so much over the last few days I feel like a criminal voyeur :reading:....ha,ha. I have actually been checking out all of your restoration photos and preparing myself for the work on my motor as well as replacing the dreaded first & second gear roll pin in the tranny. You did a great job in documenting those things on your site.

I have a Metric set of taps at home just not sure about the M12-1.5 size. This is the bad boy I would need, right?

http://www.grainger.com/Grainger/OSG-Spiral-Point-Tap-4ANW9

[David T]

The proper way to chase the threads is not with an ordinary tap. You could wind up actually removing materiel and that would make the threaded hole weaker. If you suspect damaged threads the best way to go is to just replace ALL of the threads with a S/S threaded insert. For the bolts get a dial caliper and measure all of them for length and thickness on the shank. You will quickly find any that have been over-stressed, they will be longer and thinner. Part of blue-printing a motor is to make sure ALL of the bolt holes are perfectly perpendicular, the correct depth, and clean. The hole should be countersunk slightly so the first threads are not raising the surface. There are books on this kind of stuff that can explain it better than me. Preparation is KEY to building a long-lasting, reliable motor.
David Teitelbaum