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.

[ARHarh]

I would helicoil. Or if not easy drill and tap to the next size up. Helicoils work well if fitted correctly. They are easy to buy and fit.

[Richard-D]

What size thread is it that you're repairing?

[geeman237]

Richard-D said:

What size thread is it that you're repairing?

5/16 UNF

[Richard-D]

Reasonably large then so shouldn't be too difficult to helicoil despite being a fine pitch. I've found the cheap helicoil copy kits to be pretty good. They're only £8 or £10 on ebay.

[V10Mike]

If it's only to take a fixing bolt, you might as well drill and tap the next size up. You could always put a stepped stud in if you want to keep the same size hole in whatever is attached to it.

[gazza285]

You could also Locktite a stud in to the full depth, then use a nut.

[LimSlip]

A helicoil will do the job very nicely, but you need to get the drill and the tap straight down the center of original hole which can be tricky is access is poor.

[PaulKemp]

You’ll need a bottom tap if the helicoil kit does not have one, tapered taps will only cut the first few threads in a blind hole

[tapkaJohnD]

It's easy to cut new threads in your cylinder head, as the cast iron is self-lubricating. A 3/8" taper tap will start in the old, worn 5/16" hole, you may need a bottoming tap unless you choose the length of bolt you will use to be short enough to sit in the length of fully formed thread from the taper.

You will need a tap holder, to drive the tap, liberally grease the lands of the tap, to capture the swarf, and withdraw and clean it frequently, replacing the grease. When sufficiently deep, clean out the hole and threads with a solvent, and if possible an airline blast, or just squirt WD40 into the hole repeatedly!

The hole in the fitting, the thermosat/water pump housing, will need to be drilled out too, to accomodate the larger bolt, but there is plenaty of 'meat' in it for that!

Good luck!
John

[Krikkit]

I think I'd helicoil it, but I'd be tempted to get a pro in to do it right - saves buying the kit and a bottom tap (as noted above, you'll need one) just for one hole, and averting the risk of it going wrong!

[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

[Evoluzione]

You won't need a different tap if the hole is deep enough, if it isn't then drill it deeper if safe to do so.
As you're buying a kit you'll have a few so if the first doesn't go well rip it out and do another. For future ref you can get the inserts in many different lengths.

[Legacywr]

You may find that the diameter of the hole where the thread is stripped, will accept the wire insert tap without having to drill it

You’ll also need to know what depth of wire insert you need? They should be listed as 1D, 1.5D, 2D etc. The ‘D’ signifying diameter, for instance, a 10mm x 1D, is a 10mm deep/long insert.

[ARHarh]

If you need a bottoming tap. Start the thread with the tap, then grind the end flat removing the taper and carry on tapping. It's a bit one way for the tap, but it will get it sorted

[neill]

Hello,
I would be very surprised if your thread is 5/16 UNF, just about all threads into cast iron are UNC
Neil.

[GreenV8S]

If you decide to go the insert route, consider timesert rather than helicoil. YMMV, but I get on better with them.

[gazza285]

I'd go for the Locktite a stud in route, using Locktite 271. In the unlikely event that it does fail, you still have the Helicoil/Timesert options available.

[tapkaJohnD]

neill said:

Hello,
I would be very surprised if your thread is 5/16 UNF, just about all threads into cast iron are UNC
Neil.

Not in Standard Triumph engine!

[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.