So long stroy short I had one of my engine mount bolts snap off in the block of my 351 Windsor. After spending an hour and a half with a pair of vise grips I got the broken bolt out of my block, but it brought the treads with it. Looking in the hole there are some threads not damaged about a 1/2 inch in but the first 1/2 inch are completely gone. I don't know what the "correct" fix should be since it is in the block and part of the engine mount and under a decent amount of stress (ie; drill it out and tap it for a larger diameter bolt or helicoil the first part of theads and get a longer bolt).
The correct fix would be a Helicoil. They come in a long version for most sizes and is what I would use myself. That's a 7/16" bolt and I would not tap the boss to 1/2". It would make the cast boss pretty thin on the outside wall. The Wizzard
Heli-coil- I second that! If you are not sure what size drill and tap to use go to your local auto suppy or machine shop supply and get a chart showing what drills and taps to use.
Heli coil is surely the correct answer, but IF this is in-car and impossible to reach for all the drilling, there are kludges to explore and evaluate.. Is the HC job do-able, or does it require major dis***embly of a working car?
Should be able to pull off the header and jack up the engine a little to get a strait shot, but should be doable.
The tap will be a 7/16-14 STI tap. Some people think you use a 1/2-13 tap for a 7/16 heli-coil. never happen. Then those same people think Heli-coils are junk. Wounder why. If you buy the little, cheap, kit at the store it will tell you what drill to use.
Aproperly installed Heli Coil makes a top notch repair, usually, every bit as good or BETTER than the stock threads.... Go figure!
Doing the Heli-coil dosn't worry me to much (I think I even have the correct Heli-coil kit on the shelf), I just didn't know if it was okay to Heli-coil something with that much stress. Thanks for the responces/help
Seems to me with HC you are essentially tapping into the casting with BIGGER threads while retaining same bolt...you are making the joint stronger both by lining the weak part, the casting, with steel threads and by using the larger base represented by the outside of the HC. I believe many road racers drill out and helicoil everything in the aluminum bits, and of course other racers helicoil all the high maintenance stuff so casting threads don't wear out.
HeliCoil Kit #5521-7 for a 7/16"-14 bolt. The Helicoil kit consists of a special 7/16"-14 Helicoil tap, six Helicoil inserts, installation tool and plastic tool guide for about $22. You must purchase a 29/64" HS drill bit separately in order to drill out the old threaded hole. Always use a new drill bit and drill out the damaged bolt hole perfectly straight! After tapping the drilled hole with the specaul helicoil tap, clean the threads with brake cleaner and put a little dab of Permatex #27140 Permanent Red thread locker on the outside of the helicoil just before you install it. Install the Helicoil with the installation tool using the plastic guide until the Helicoil is barely countersunk, snip off the excess. A light tap with a screwdriver will break the drive tang in the bottom of the hole. You can leave the tang in the hole or try to fish it out. A magnet will not fish it out because a Helicoil is stainless steel. Let the Permatex Threadlocker set up 24 hours before installing a bolt. Most of these instructions come with the kit. I recently used the 7/16"-20 HeliCoil Kit #5528-7 on the caliper bolt hole in the steering knuckle of my S10 Blazer. It also uses a 29/64" drill bit. I have a pair of 3/8" drive adjustable tap sockets which makes it easy to tap the hole using a ratchet. Using a ratchet to tap a hole is easier than spinning a tap handle. Helicoils are stronger than the original threads.
You say you can still see some undamaged threads in the hole. While I would personally use a Heli-Coil if I could get at the hole without too much trouble, I also wouldn't be too concerned about the integrity of the threads if there were enough of them left to get x3 full turns on the bolt. A thread -form usually achieves it's full strength rating at x3 full turns. Having said that, if I did use the 3 turns method I would always have registered in the back of my mind 'that bolt repair' the next time that engine came out.
Helicoils are fine, once worked on a 283 bored out to 302, head gaskets had gone as bolts could not be torqued up enough. Helicoiled half the block and it was all good from then on. It later had a 4.71 fitted with same 9.5:1 compression and it still was fine.
Heli-coils are used in aircraft manufacturing as well. The exception would be to install a bolt into a "non" heli-coil hole
The Jalopy Journal
