Blind hole thread repair options

[geeman237]

I've got a Triumph TR6 and a blind tapped hole in the cast iron cylinder head that is used to mount the thermostat and waterpump housing is damaged from a previous owner's ham-fisted efforts. The blind tapped hole is about 12-15mm deep but has been 'wobbled' out so that when you turn the bolt in, the bolt wobbles in the threaded hole and doesn't tighten until the last couple of turns. The threads aren't too bad form-wise, but I think if I try and apply the correct torque then the threads may strip.

The cylinder head is still on the engine block.

Would a 'heli-coil' insert work in this case?

I also thought about replacing the bolt with a threaded stud Red Loctited in.

Words of experience welcome.

[geeman237]

Richard-D said:

What size thread is it that you're repairing?

5/16 UNF

[geeman237]

Thanks for all the comments. Here’s a photo of the hole. Not much to see really. The engine is out of the car and on a stand so access isn’t a problem.
I’m actually in the US. A 5/16 UNF helicoil kit is about £30 it seems. Thanks for the heads up on a bottoming tap too, but can I use a regular bottoming tap with a heli coil because the heli coil tap is a slightly different size than a standard 5/16 UNF tap correct?
So, it’s heli coil vs open up to 3/8 UNF. Hmm. For no reason other than the sake of originality I’m leaning towards a helicoil for the same thread size. I’ll have to see on the availability and cost on the options.

Edited by geeman237 on Friday 23 October 14:32

[geeman237]

Thanks for all the top tips. I went down the Heli Coil route as that was readily available to me. I ended up having to do 2 holes. The first one went well, the second I had to carefully re=tap for the insert as I went in a little off centre.

Waterpump/thermostat housing now fitted. Fingers crossed. Now to re-install the engine and get it running again.