I just picked up a 68 427 block that has helicoils in all of the threads for the head bolts. Anyone have any suggestions of a better method of thread repair. Can they be removed and something else done?

Mark

The only other option I have heard of and I don't remember the name of the product, but have seen pictures is a threaded steel insert. Its kind of like a sleeve, not like a spring piece you typically see with a helicoil. I really don't believe the helicoil will be an issue if they were done correctly. Its becoming almost standard practice now to install helicoil threads inside aluminum cylinder heads just so the manf. don't have to deal with warranty or customer complaints on stripped threads. Not that this is their problem, its ususally caused by the customer.

The only other option is to get a new block.

I agree, there is no problem with a properly done helicoil. And the solid thread inserts are also great. You drill the hole larger and put an insert that is like a bushing into you new drilled and threaded hole. Then small stakes are driven in to stop the insert from turning.

But if the helicoils are good, I would just use them. There are cases where they are used as OEM in alum, just to make a stronger thread.

Thanks for the help. Sounds like I'll leave them alone.

Mark

The solid inserts you're looking for are called EZ Lok, I found them on the internet when searching "threaded inserts". Helicoils are supposed to be strong, but personally, I wouldn't be comfortable using them for headbolts, but that's just me.

Matt

Matt, I understand your concern. But I have used Helicoils in head bolts many times. They will do the job.

Any bolt only grips at a maximum of 1 1/2 time the diamiter along the thread length. Harder bolts are less. Most of the gripping is done in the first 4 threads. That also why nuts are made to the thickness they are. Beyond that, you just have a extra threads. While this helps to start and run down bolts, it does nothing to increase the strength. Heilcoils are designed to use the functional area of the thread.

So why only that area? When a bolt is tightened, the metal stretches. This is called the elastic properties of the bolt. (or nut) The top thread deforms most, the next thread deforms less, and so on, until there is no tension on the threads.

The stretching keeps tension on the surfaces, and creates friction to keep the bolt tight, and not turn out. The softer the bolt, the more thread are stretched. Over stretch the bolt, and you have a thread failure.

This would tend to make you think a harder bolt is stronger. It is. But to use too hard of a bolt means you have a brittle bolt. Instead of stripping, it will snap. Hardness is the key for any application.

I agree.. Ive used both... I dont like doing it on general principle.. but it works...

Also... while on the subject, I have reused head bolts many times.. but ive always had fewer problems when I replaced them...

I had a 302 (ford) that used to blow head gaskets like they were going out of syle. Changed the bolts.. it quit..

one of my employees has a 92 ford taurus wagon.. someone did a vavle job on it about 6 months before he bought it. (he had receipts) but they reused the bolts..

blew the gaskets.. I pulled the heads and replaced the bolts.. he hasnt had a problem since..

Replacing the bolts is always a good idea. If they are torque to yeild bolts, it's necessary. The torque to yeild are the bolts that have a torque spec, then say to turn then an additional degrees of rotation. These bolts stretch as tightened and do not flex back. Each time they are used, they stretch more, and get thinner and weaker. The supply a more even clamping force, and thus do a better job, as long as they are used only once.

Regular bolts do stretch, but flex back when released. Unless they were overtightened. We use torque as a guide to tighten them. But that is twisting force, and many things affect the actual clamping force. Clamping force is what we are interested in, not really torque. It's just that torque is the best we have to judge the clamping force. Worn threads, liquids, rust, anything affects the torque, and thus changes the clamping force.

Torque to yeild bolts stretch at the correct clamping force. So even if the rotation and or torque is a bit off, the clamping force is the same. Many cars in the 90 and up use these.

After so many times, any bolt will not fully come back to it's original. And no matter how clean it is, the threads do wear. Sooner or later, they fail.

|UPDATED|9/11/2004 7:28:21 PM (AZT)|/UPDATED|