Stripped head bolt hole

raylee · #1 08-09-2022, 04:26 PM

Hey guys,

I have a K-301 with one stripped head bolt hole. Has anyone ever tried to drill and tap a stripped bolt hole on the block? Can it even be done?

Thanks in advance

finsruskw · #2 08-09-2022, 06:25 PM

Stripped as in the bolt is gone and the threads are messed up??

Or broken off in the hole?

If stripped, you could carefully chase the threads with a tap but I would not want to torque it back to spec as that's what probably caused it to strip in the first place.

If bolt is broken off in the hole. best to take it to a machine shop and have them drill, tap and helicoil for a proper repair.

I WOULD NOT try to use a hand drill to accomplish this repair. and wind up breaking off a bit or worse yet a tap in the hole and then you really will be shafted.

The block needs to be secured on a mill table, indicated and machined square and plumb to affect a proper repair IMHO.

raylee · #3 08-09-2022, 07:01 PM

Sorry should have made myself more clear about the problem.
the bolt is out, the threads are stripped It's a 3/8 bolt was wondering if I could drill and tap for 7/16

finsruskw · #4 08-09-2022, 07:29 PM

7/16" tap requires a 25/64" bit for 14tpi
and 13/32" for 20tpi

Maxx Catt · #5 08-09-2022, 08:09 PM

Will a 7/16 go through the head?
If it was me I'd Heli-coil it, but if you don't have the set already it will set you back a few bucks.

raylee · #6 08-09-2022, 09:21 PM

Well I thought maybe I could drill the head out too if I had to I'll just check and make sure the bigger hole doesn't interfere with the fire ring on the gasket. Never used a helicoil before but I think there not too difficult My main concern is if there enough meat on the block itself so I'm not drilling into the cylinder wall and I also thought about dropping down to a 5/16 bolt if I used a helicoil but not familiar with drill sizes for the helicoil it may require a bigger bit for a 5/16 helicoil than the bit for a 7/16 bolt I just don't know but I will find out and I appreciate all you all's help

raylee · #7 08-09-2022, 09:52 PM

here's a picture center right hole

finsruskw · #8 08-10-2022, 07:56 AM

Impact wrenches will do that Ya know!!

raylee · #9 08-10-2022, 08:36 AM

I'm aware of that and you would never see me use an impact wrench on something like this. 50 year old engine I've only had about a year but I can't speak to what was done previously by previous owners

Ambush · #10 08-10-2022, 09:48 AM

Definitely Helicoil it to the original 3/8". You can also reuse the stock bolt that way and keep everything stock. If any of the other holes look suspicious, I'd do them at the same time. Heck, I'd probably just do them all if the engine is a keeper that will see some time with you.

Because you have the holes as a guide, you can do the job under any decent drill press. DEFINITELY not with a hand drill. You may get lucky, but Murphy says otherwise.

Helicoils are a huge asset to threaded holes in aluminum. Puts the holding strength of the thread close to the tensile strength of the bolt.