A Mostly British Obsession

Exhaust Manifold Helicoil Repair

Warning: Unless you like mechanical stuff, this will be the most boring post ever.
[flickr id=”6213864834″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]The exhaust on my “new” XJ6 leaked pretty badly where the manifolds joined the downpipes, and for good reason: The studs that were installed (in one of the few bodges on the car) were at least an inch too long. There was no way for the “donut” gaskets to be clamped tightly, and (surprise!) they leaked. In the process of removing the too-long studs, I discovered three of the tapped holes were stripped, one on the front manifold and two on the back. Shown above are the repaired manifolds with new studs (temporary, see end note). Below we’ll take a peek how I repaired the threads.

I used “Helicoil” brand inserts to repair the stripped threads. The repair consists of drilling out the hole to a slightly larger size, tapping it with a special tap, and then screwing in an insert, which brings the thread back down to the original size. The holes in the manifold of an XK are threaded at 3/8-24, not a common size insert to find on the shelf. My local NAPA was able to get me a kit in the right size for about $60. The kit contains a dozen inserts, the special tap, and a special tool for installing the inserts. You need to provide your own drill bit (in this case 25/64, I think), but they specify what size to use.

Fixing the two manifolds took me about three hours of leisurely work. (I had abdominal surgery last Tuesday, so I’m not working at demon speed–not that you can tell the difference!) Removing the manifolds from the car took about an hour and a half, and the repair took the remaining time.
[flickr id=”6213351621″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]This manifold required two inserts, on the bottom two holes as seen in these photos. First thing I did, though, was run a regular 3/8-24 tap through the top “good” holes to clean them up. I used WD-40 to lubricate the tapping and drilling in all of these photos which worked well enough, though I’m sure there are better fluids for both. The important part is to use something!
[flickr id=”6213351577″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]Here’s a close-up of the first thread we’re going to repair. You can see that there is still a bit of the original thread visible, though any stud or bolt threaded into the hole rattles and moves around, as there is very little “meat” left. Better to fix it than try to finesse a fastener in and hope it stays in place.
[flickr id=”6213351551″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]First step is to drill the hole clean with the recommended bit. This is actually quite easy as the bit removes only the remains of the old threads and is self-centering, for the most-part. This takes all of a few seconds to accomplish.
[flickr id=”6213351525″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]Here you can see the “cleaned up” hole. There is still the faintest vestige of the original threads; you can imagine how easily the bit went through here.
[flickr id=”6213351499″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]Next step is to run the special Helicoil tap through the hole, remembering your best practices: Use plenty of lubricant, cut a quarter turn and then back off a quarter turn to “clear” the tap, repeat. This takes a few minutes but it isn’t hard.
[flickr id=”6213351439″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]This is the special insert tool for placing the Helicoil. You can see I already have an insert loaded into the tool (there is a loose one on the vise to show you what they look like). The bottom of the coil has a tang which the tool engages to screw the new threads into place.
[flickr id=”6213864560″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]Here I’ve completed screwing the coil into place; I’ve lifted the sleeve clear.
[flickr id=”6213864510″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]And here’s that same shot from below. It’s critical you place the insert slightly below the surface of the part you’re repairing to allow good mating of the piece being attached, as seen in the preceding photo. The inserts are made in different lengths, and in our application here it’s not a problem for it to stick up a bit on the non-gasket side of the manifold.
[flickr id=”6213351337″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]I’ve backed the tool out, and all that remains to be done is…
[flickr id=”6213351307″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]…break off the tang. This is done by using a punch or other tool with a reasonably sized flat face…not a screwdriver.
[flickr id=”6213351277″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]Here’s a punch, held against the side of the insert where the tang is attached. A smart blow with a hammer breaks the tang off cleanly.
[flickr id=”6213351241″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]And that’s it. I repeated the repair for the other stripped hole and test fitted the new gasket.
[flickr id=”6213864330″ thumbnail=”original” overlay=”false” size=”original” group=”” align=”none”]You’ll note these studs aren’t quite right; a consequence of living on an island without a specialty hardware store. The fine-threaded sides really should be screwed in all the way so the unthreaded shoulder is up against the manifold. If I do that with these particular pieces, the studs are then too short.

Ideally, the lengths of the coarse and the fine threads on these would be reversed (as otherwise these are the right length), but that particular type of stud cannot be found on-the-shelf around here (the coarse side is always short, the fine side always long). I’ll see if I can find some slightly longer studs without increasing the amount of unthreaded shaft, which was the problem that led to the leaking exhaust. Worst-case scenario is having to run the appropriate die down new, longer studs to reduce the length of the unthreaded bit.

And that’s it. Don’t forget to use brass nuts when assembling the down pipes to the manifold, otherwise, you’ll never, ever, get this joint apart again.

This task, incidentally, would have been a nightmare to do on the car. It’s always tempting to see if you can leave something assembled and in-place, but I’d still be out there if I had done so, probably without any tools left within reach to throw angrily!

3 Comments

  1. Allie

    How neat. I had no idea you could fix threads at all.

  2. ed

    is a helicoil better than merely drilling and tapping for a larger hole?

  3. mark

    Nice work. To answer Eds question, a helicoil will allow you to retain the original size stud whereas tapping a larger thread necessitates going up in stud size. Not always possible or desireable. BTW, Cast Iron taps quite well without lube of any sort.

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