veefre
Plastic
- Joined
- Feb 5, 2011
- Location
- california usa
Saw a post about trouble helicoiling a V8 head in place... wrote a reply and then found I couldn't post it because the original post was archived.
Anyway, here's my reply for what it's worth. I imagine someone might find it of amusement or even value...
The proper way to renovate threaded holes on a V8 head would likely be to pull the head, hold it securely on a big drill press or Bridgeport type mill, and then drill out the holes precisely with the correct size drill for the STI tap, and follow that with a tap in exactly the same position.
I'm a machinist and have installed a fair number of helicoils, albeit usually in aluminum or other non-ferrous material (sometimes in titanium). There is a certain feel you need to have to know when you've loaded the insert into the insertion tool properly, and again when you've entered the STI tapped threaded hole properly.
I agree that a little deeper on the insertion is better than too shallow. If you install too shallow, and subsequently the installer puts tension on the fastener as it's being installed, it could pull out the first lead of the helicoil and then you gotta start over.
And of course you must break off that tang cleanly and make sure the tang remnant is blown out of the tapped hole before trying to thread in the fastener.
For exhaust studs I usually use some anti-seize compound, because steel exhaust studs are notorious for corroding in place and can be a real PITA to remove after they break.
Obviously a paint type primer isn't going to work in an exhaust stud
application. The paint would just burn away. Although I've seen a type of Loctite used on stud threads. Probably high temp stuff. From experience I'd rather have the stud come out of the head when pulling the header, than have the stud frozen to both the nut and the head and break off somewhere in between.
And sometimes the helicoil tang fails while you are installing the damn thing, in which case you gotta ease out the helicoil and start all over.
I like the inserts with the pins you tap in... I think one brand is "Keensert". These also require a different size tap than helicoils, if I'm not mistaken.
A little off topic, but for a stripped out spark plug hole on one of my cars (aluminum head) I used a nifty insert called a Timesert. It's solid, not wire wound. Same general concept as any insert, except that you must use the tools provided to put a counterbore on the tapped hole. This allows the spark plug head to contact the cylinder head instead of the insert, which makes it hold fast. Otherwise the insert will get loose, leak exhaust gases, bind to the spark plug, and undo itself the next time you try to pull the plug. There's also an insertion tool that tries to flare out the bottom of the insert to help it hold, but I know from experience that putting enough of a counterbore on the tapped hole is more important.
Back to the helicoils... it doesn't really matter, I think, what angle you put on the countersink, or even how deep it is. But you must make sure that the top thread coil on the helicoil is firmly embedded in a full thread of the tapped hole. Too shallow, and it will only partially engage in the countersunk chamfer and that may not hold as I explained above. The strongest would be to have very little chamfer and then turn the helicoil in one to one and a half turns below the surface of the material. But no chamfer makes the helicoil more tricky to start. If there is a deep chamfer, then you might need to thread in the helicoil two turns below the surface of the material to make a good fit. And this is assuming you have enough material to go deeper, and that the fastener is long enough to engage enough threads of the helicoil to achieve the required strength.
Anyway, here's my reply for what it's worth. I imagine someone might find it of amusement or even value...
The proper way to renovate threaded holes on a V8 head would likely be to pull the head, hold it securely on a big drill press or Bridgeport type mill, and then drill out the holes precisely with the correct size drill for the STI tap, and follow that with a tap in exactly the same position.
I'm a machinist and have installed a fair number of helicoils, albeit usually in aluminum or other non-ferrous material (sometimes in titanium). There is a certain feel you need to have to know when you've loaded the insert into the insertion tool properly, and again when you've entered the STI tapped threaded hole properly.
I agree that a little deeper on the insertion is better than too shallow. If you install too shallow, and subsequently the installer puts tension on the fastener as it's being installed, it could pull out the first lead of the helicoil and then you gotta start over.
And of course you must break off that tang cleanly and make sure the tang remnant is blown out of the tapped hole before trying to thread in the fastener.
For exhaust studs I usually use some anti-seize compound, because steel exhaust studs are notorious for corroding in place and can be a real PITA to remove after they break.
Obviously a paint type primer isn't going to work in an exhaust stud
application. The paint would just burn away. Although I've seen a type of Loctite used on stud threads. Probably high temp stuff. From experience I'd rather have the stud come out of the head when pulling the header, than have the stud frozen to both the nut and the head and break off somewhere in between.
And sometimes the helicoil tang fails while you are installing the damn thing, in which case you gotta ease out the helicoil and start all over.
I like the inserts with the pins you tap in... I think one brand is "Keensert". These also require a different size tap than helicoils, if I'm not mistaken.
A little off topic, but for a stripped out spark plug hole on one of my cars (aluminum head) I used a nifty insert called a Timesert. It's solid, not wire wound. Same general concept as any insert, except that you must use the tools provided to put a counterbore on the tapped hole. This allows the spark plug head to contact the cylinder head instead of the insert, which makes it hold fast. Otherwise the insert will get loose, leak exhaust gases, bind to the spark plug, and undo itself the next time you try to pull the plug. There's also an insertion tool that tries to flare out the bottom of the insert to help it hold, but I know from experience that putting enough of a counterbore on the tapped hole is more important.
Back to the helicoils... it doesn't really matter, I think, what angle you put on the countersink, or even how deep it is. But you must make sure that the top thread coil on the helicoil is firmly embedded in a full thread of the tapped hole. Too shallow, and it will only partially engage in the countersunk chamfer and that may not hold as I explained above. The strongest would be to have very little chamfer and then turn the helicoil in one to one and a half turns below the surface of the material. But no chamfer makes the helicoil more tricky to start. If there is a deep chamfer, then you might need to thread in the helicoil two turns below the surface of the material to make a good fit. And this is assuming you have enough material to go deeper, and that the fastener is long enough to engage enough threads of the helicoil to achieve the required strength.
