1969 427 motor mount problem

[Joe T.]

I have a stripped thread in the block on my 427 for the motor mount. The bolts are 3/8 x 1 coarse thread with a very short or no shoulder. Are these the correct bolts? Can anyone recommend the preferred method for repair? The engine is out on a stand, so access is no problem. Will Heli-coil fix this? Shall i tap it to next size bolt (25/64; 13/32/ 7/16)? I don't want to ignore it, since that seems to be asking for trouble..Thanks and regards...joe

Also: Is there a manual or other document that describes all the fasteners/bolts/nuts etc/? Someone else disassembled this motor and I have a box of bolts with little idea of what goes where. thanks again...jt

[Dick W.]

Re: 1969 427 motor mount problem

Helicoil will fix the mount problem. The closest thing to a book on bolts would be Paragon's. And then there are many bolts used on the engine that would not be on that chart. The 1968-1969 Technical Information and Judging Guide will give you head markings on some of the bolts. Best guess is to find an original car to use as a guide

[Joe T.]

Re: 1969 427 motor mount problem

Thanks Dick. Helicoil it is! I appreciate the info. Happy Thanksgiving! regards..joe

[Terry F.]

Re: 1969 427 motor mount problem

Heli coil will work fine. I am fonder of thread inserts. They are stronger than a heil coil and most fastener stores will have them and the tools to insert them. Just cut and grind them flat to the surface. They are hard to detect once properly installed. I don't like the heilcoil because it is simply a spring coiled into a specially tapped hole. Thread insert is similar but solid. You have your choice of what it is made out of. Infinitely stronger and a true good as new repair. Further, I would never ever use a heli coil on a head bolt location or high pressure application or situation where there is fluid behind the threads. The chances of leakage or failure is greater. A insert might be a little over kill in your situation, but I would still do it. Just my opinion though. People like the heli coil because it is so easy to use. Terry

[Joe T.]

Re: 1969 427 motor mount problem

Thanks for the info Terry. I was just talking to a machinist in training, and he indicated a helicoil can have some installation issues. I am looking further into two options now. I just found a product TIME-SERT. I will look further. The helicoil is not eliminated yet and I appreiate the information...Happy Thanksgiving!...regards...Joe

[Terry F.]

Re: 1969 427 motor mount problem

Yes, the item you have indicated is probably an insert. The neat thing is that you can get them made from some incredibly durable stuff. A true 100% or 120% inprovement depending on application. This type of insert also bottoms out in the hole differently than a heli coil. When you get ready to install the insert, you will use some type of lock-tight to make it a permanent installation. Sometimes, people will stack heli-coils on top of one another to obtain the thread depth. That is a joke with trouble waiting to happen. Incerts can be purchased in different lengths. They also have great application on cylinder head bolt holes between the intake and the cylinder head. That is another place I would never ever use a helicoil. You are on the right track. Good luck, Terry Happy holidays everone!

[Jim T.]

Re: 1969 427 motor mount problem

Some things can be done at a engine rebuild machine shop that are reasonable in labor and parts. With your engine out you may be able to coordinate one day service.

[Clem Z.]

i have installed heli coils in lots of head bolt

aplications with no problems. i have install a lot of heli coils in aluminum and cast iron parts on engines with never a failure. the disadvantage of heli coils is you need their tap to install them BUT with the insert type where you use a larger standard size tap you need to use a much large diameter of the material to install them and in some applications that is not possible. JMHO

[Terry F.]

Re: i have installed heli coils in lots of head bo

I agree, they may work in most applications. I have heard of failures in head bolts and leakage in some applications. The machinests that I talk to from time to time inform me there is no comparison and they laugh at heli-coil. They have sited to me the strength differences and they are significant enough to use one over the other. Much of there work is done on hylraulic applications with considerable forces. Heli-coil doesn't hold up for them. They like to do it once and call it good. Granted, not all applications are the same and heli-coil may pass or have to be used. As you mentioned, wall thickness is a consideration. If you put a heli-coil and an inset next to one another and compare them and then ask yourself which you would rather have installed. I think the insert will win over the spring. Always good to here from you Clem. Take care and have a great turkey day! Terry

[Joe L.]

Re: 1969 427 motor mount problem

Joe-----

A Heli-Coil will work perfectly. In fact, this sort of thread problem with engine mount bolts is not really too unusual. I've repaired several that suffered this malady.

The original bolts were just the size you mentioned: 3/8-16 X 1 (actually, they were officially 15/16" long, but they'll usually measure 1"). They were non-indented head and of all thread design with no unthreaded shank or "shoulder" of any kind. They were of GM-280M (SAE Grade 5) and phosphate-finished.

Want to use "Genuine GM" bolts catalogued for the application? Order GM #331246. These are the same configuration and size as originals, although I think that they're of GM 300M (SAE Grade . The GM list for $4.25/each, though.