Question about drilling and tapping a head??

[dznucks]

I have a helicoil that has broken off in the passenger side head. The bolt is the lower alternator bolt and the helicoil pulled out the threads in the head. There is still a portion of the helicoil in the head and i am able to grab a few threads but it is working itself loose.

I want to replace the helicoil with a threaded insert since the original oversized threads in the head are gone. Its a 3/8-16 helicoil and the head i am guesing was taped at 7/16. The hole mic'd at .397

My question is...do i have enough metal in the head casting to drill the hole out to a 31/64 (.484) without weakening the head? the external threads of the insert are 9/16-12.

its a '69 with a jasper 350. jasper helicoiled every hole in the block.

[OLDCHEV4X4]

The thread cant be tapped for 7/16-14. The 3/8-16 would not screw in. STI taps (standard thread insert) are special taps.
A thin wall key locking insert would require a 1/2-13 thread , 5/8 deep or more. The insert is 1/2" long.
Hmmmm, I never drilled a head in that spot and i dont have one infront of me.
Do you have a junk head to try it on?

[dznucks]

No i dont have any spare heads laying around.

The ID of the helicoil is 3/8-16. the ID of the hole in the block is .397".
The manufacturers specs for a 3/8-16 threaded insert call out a 9/16-12 external thread and the hole needs to be drilled to 31/64.

[OLDCHEV4X4]

There is a thin wall key locking insert in McMaster Carr catalog with a 3/8-16 ID and a 1/2-13 OD. I would use that if you can, so you that you can drill a smaller tap drill hole and leave more meat in the head. The tap drill size for the thin wall insert is 29/64.

[70rs/ss]

It goes into a water jacket, do not ask how I know!? You can try it but you will hit water, I plugged the hole and unbolted the heads and swapped them from side to side, tada, new hole with threads. PIA but better than leaking water into the alt!

[dznucks]

Nice!!

so their isn't any room to go to a bigger size hole. i wasn't going to go deeper just larger diameter. i would like to think that they designed alittle meat into the bolt holes of the casting.

I might get a half inch longer bolt and shim it out with washers. i still have about 1/3 in of threads left that the bolt cant reach.

I see a similar thin walled bushing in Grainger.

[Sport/Truck]

I think it still will work loose on you. But it’s worth a try.
I like the idea of swapping sides like 70rs/ss said.
s/t

[OLDCHEV4X4]

There ya go. Darn good idea!
Swap heads side to side.

If ya want to try the longer bolt idea, use some gr 5 all-thread, and some thread locker, bottom it out in the hole and use a nut to tighten the alt.