BACK to TECH TALK

Heli-coiling an Engine Block

How annoying is this. You spend countless hours assembling your engine, you do everything right, even running a thread chaser through all the head bolt threads in the block. All looks OK during assembly but as you start to tighten the cylinder head you strip a thread before your even at the third bolt into the second tightening sequence. You heli-coil it, refit the head and then strip another one a few bolts later on the same torque setting. What are the chances that you will strip another one after repairing that one. EXTREMELY HIGH is the answer. It is best to bite the bullet and heli-coil all the bolts. If you get to the third or final torque setting and strip another one then you will need to also replace the head gasket not to mention the countless hours wasted taking the head off and on during each repair. This happened to me not long ago and I thought I would share how I did the repair.

        Doing heli-coils whether it be one or all of them is extremely messy. The last thing you need is a small piece of metal from the drillings to enter your new engine. It could end up jammed between your piston and bore or enter through an open hole somewhere else in the block and get caught up in the crank when you first start the engine destroying the bearings in no time. Not a desirable result after all your hard work. There is a way to do this job cleanly and with absolute safety to your engine internals. So let me start at the point where I have removed the cylinder head, removed the head gasket and am now wondering how can I do this without getting metal into my new engine. This is all done with the engine removed, so instructions are written accordingly.

The first thing that is obviously needed is a Heli-coil kit of the correct size. A kit to do one specific size will cost about $60 and will include 10 heli-coils. Extra coils can be purchased for around $4 ea.

Now I use a sheet of clear contact and stick it over the entire deck of the block ensuring the block is thoroughly clean before applying it. The contact must stick and have no air pockets. If the contact does not stick then it will not seal and therefore will allow metal to slip under the contact and into the bores and other openings.

Now I check the rest of the block for any possible opening and block them off. There is no way any filings can now get in. The only place they can go are into the water jackets if the threads continue into them. At the end of this process I will air blow the filings out of the water jackets with compressed air.

I have a home-made jig to drill the hole perpendicular to the deck and I used it to do the first thread, but when doing the whole lot I changed to a quicker method. If you do not have a jig to set the drill exactly perpendicular to the block then give up, you will not be able to keep the drill straight enough for a successful repair. If you are only half a degree out of alignment, you will stress the thread when tightening the bolt, not get the correct torque on the bolt and possibly won't even get the bolt to start threading in. All this leads to early failure of the head gasket.

The method I used was to drill all the holes in one go was to use an old cylinder head I had laying around. If you don't have a jig or a spare head then you will need to strip that nice reconditioned head and use it as the drill guide. Then after the job, clean it thoroughly and re-assemble it before fitting back on. Extra work I know, but unavoidable. Perhaps making a jig would be less time consuming. First check that the drill supplied in the heli-coil kit will fit through the head bolt holes of the cylinder head, they usually do. If not then a jig manufacture will be required. I simply placed the cylinder head on the block and used one bolt to hold it down. I then removed the bolt and drilled out the last hole. Warning: The drill will bite with only light pressure. Make sure your ready to hold the drill when it does and you will need a high torque drill to handle the large drill size.

This hole was drilled with my home made jig. The contact does a marvelous job of stopping the metal filing from entering the engine.

I used an old head to do the rest of the threads. As you can see from the photo, metal filing will come out of the bolt holes and make a huge mess. Hence if using the current head, then strip it to bare before you start drilling.

Once the holes were drilled, I removed the head, then cleaned any excess filing from the contact sheet but kept the contact in place and ensured it was still stuck to the block. Using the tap supplied with the heli-coil kit, I then cut all the threads. Before removing the contact, I cut out all the water jacket holes through the contact, then turned the engine over and blew out any filing from the water jackets. This took some time as I put the air gun into every hole that enters the water jacket several times to ensure I got all the filings out. When I was satisfied it was all out, I left the engine upside down to remove the contact. There will be small amounts of metal filing stuck to the contact around all the bolt holes. They should stay attached to the contact as you remove it. Never the less, remove it with caution. I removed my contact when the engine was upside down to ensure if any did come loose, they would fall down. To make it easier to fit the coils I flipped the engine on its side.

Now I just screwed in the heli-coils into the threads ensuring that when it was flush with the deck that I continued a further 90 deg as stated on the instruction panel.

 The finished product, looks better than new.

The little tag that needed to be snapped off was easily removed by flipping the engine upside down. When I broke off the tag, it fell straight out of the thread hole. Heli-coils are actually stronger than the original threads when fitted correctly. There you go, plenty of mess made but not a grain got into the engine internals.  The last word on the head bolts. Would you fit the bolt below? I hope you said NO. It is badly pitted and missing about 2 full threads at the bottom. Less thread contact means more load is placed on the remaining good threads. Ensure your bolts are in good condition. If a new set proves expensive then another option is to purchase an ARP stud kit. Studs are far superior to using bolts. Well that's it. For approx $70 outlay you can replace all the threads in your block and get to keep the helicoil kit for future use. If you paid a mechanic to do it then bank on a charge of $300 minimum.