My efi TP sedan has an exhaust leak at the outer (upper) stud of No. 1 cylinder. The stud was present but loose because the thread in the head was stripped. Consequently the rear surface of the gasket at that end has been eaten away. Fortunately it is the easiest stud to get to without removing the head.

The studs are 7mm. I tried to tap the hole with an 8mm tap but it hit bottom before the tap could bite. I expect the same might happen if I go to 10mm. Does anyone know how far the hole can be deepened without breaking into the water jacket? Alternatively, what are my other options?

I haven't heard a good word from anyone about helicoils.

Other options: Use a bolt instead of trying to replace the stud. Pick the right sized bolt (experiment) and it will bite into the alloy and cut its own thread. Be gentle though. Use oil and work the bolt back and forth. Use washers to space up the head of the bolt if too long.

Important to fix soonish, my V6 TS burnt an exhaust valve because the stud had broken flush with the head and it was sucking air at that exhaust port. Head had to come off to fix. Yours still might do this if the gasket is too damaged.

Thanks. I already tried with an M8 bolt so will have to buy a selection of slightly larger imperial sizes and give those a go. I might even have to replace the manifold but haven't taken it off yet - need to have gaskets ready.

Think what you need is a bottom tap. These taps are flat on the end and are used for tapping blind holes.

Closest City mechanic
And Helicoil
Stud depth to work it out
Take out a stud from your parts car head

Sorry about the delay in replying. I'm not well at the moment.


Think what you need is a bottom tap. These taps are flat on the end and are used for tapping blind holes.

I tried to get one at Bunnings but they didn't have any - only intermediate taps. Complete sets of starter, intermediate and final (bottom) taps are available on eBay quite inexpensively. However they need a bigger drive handle and I doubt I could put enough pressure on it even if I dropped the alternator and removed the radiator.


Closest City mechanic
And Helicoil
Stud depth to work it out
Take out a stud from your parts car head

I have a spare original stud. The parts cars are gone now although I kept a few bits. (distributor, air cleaner (for the maf sensor), ecu etc.)

Money is always a problem so I intend to try a low-cost solution. The cylinder head end of the stud is only 12 to 13mm long. It's too risky to deepen the hole in the lug on the cylinder head but if I insert the stud and then drill a hole through the side of the lug and hopefully also through the stud, I should be able to insert a pin to act like a grub screw. Once the outer nut on the stud is tightened up, the tension should hold both the pin and stud in place. With a 7mm stud, I figure 3mm is about the maximum diameter of pin that could be used. It shouldn't matter if I don't drill though the stud dead centre so long as I don't miss it altogether.

Update:

It worked, though I made a messy job of it. My first attempt was too far in. The hole needs to be drilled only a few millimetres in from the outer front surface of the lug. Otherwise it will only catch the end of the stud. I broke a couple of drills of course but used the shaft end of one of the broken ones as the pin. I was able to get full tension on the outer nut of the stud.

Hi Coldy,
Just picking up on your bad experience with Helicoils.
I have used them for years and have to say that I am a firm favorite of them.
I first used them with the aluminium castings on a Morris 850. Since then I use them for everything. I have 5/16 Whitworth, 1/4 BSF, 6 mm metric, and last year I fixed the wall oven door with a 3/16 whitworth. I have a house water pump and the cover is fixed into the pump body moulding with self tapping screws about 1/4 inch. The plastic threads sheared and I repaired the self tapping threads with 5/16 helicoils. My fergy 35 tractor had a sheared thread on a water pump fixing bolt and this was repaired with the 5/16 helicoil.
Never had a problem.
In your case with the cylinder head, the biggest problem is holding the drill square to the hole when drilling out the hole; or even worse, when drilling out a broken stud. I find it is almost mandatory to use a drilling guide to hold the drill central and square. Its the sort of job where it takes a week to set it up properly and 10 minutes to do the job.
Cheers,
rumpfy

It worked, though I made a messy job of it. My first attempt was too far in. The hole needs to be drilled only a few millimetres in from the outer front surface of the lug. Otherwise it will only catch the end of the stud. I broke a couple of drills of course but used the shaft end of one of the broken ones as the pin. I was able to get full tension on the outer nut of the stud.

I'm trying to visualise what you did there.

Drilling a hole into alloy close to a surface, and then putting a pin into it and applying a pull to it, as I think you have done is going to end in disaster.

Not an ideal engineering solution (if my mental picture is correct)

Keep an eye on it and don't be surprised if one day you find chunks of alloy missing.