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Broken Manifold Stud - Helicoil

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While removing the manifold studs on my rebuilt head to replace with new studs from MotorSports, the front one broke off below the head surface!  :angry2:  The one that broke was already broken to where only a 1/4" worth of thread was usable, and I couldn't double-nut it, so it had to be replaced.  The others came out okay after a good soaking with Kroil, a bit of heat, and double-nutting to loosen.  But the front #1 exhaust wouldn't budge and broke while trying to remove with vice grips.  Lucky for me the engine is on a stand, easier to work on.  I figure I'd post some pics of the Helicoil job since I didn't see one around here, in case someone else has the "joy" of doing this in the future.   I've done a few, but definitely not an expert.  Feel free to add your thoughts or tips to the thread.


Above shows broken stud.  I've already center punched it to help get the drill bit centered. 


Here was my first choice, screw extractor and left handed drill bits.  Tried 2 different sizes, no luck.  Plus I'm always afraid the extractor will break off and make further drilling a real pain, so I didn't crank on them too hard.


Since I already had the stock M8 1.25 stud kit, I opted for that size instead of going to a stouter 10M stud.  Ordered a real Helicoil kit instead of cheaper knockoff.  Kit includes some threaded inserts, a tap, and tool to install inserts into newly tapped hole.  Didn't include the 21/64" drill bit, which I had.


Carefully drill, keeping hole perpendicular to surface, and no deeper then existing hole, and centered.  I started with 1/8" hole, 1/4", 5/16" then the final 21/64".


Tap the hole, using oil to help get a clean cut.  Work tap back and forth and remove a few times to clean out debris.


Insert is threaded in using the tool.  There's a little tang at the bottom of the insert that you break off after you get it in place.


Done.  Be sure to use anti sieze on the threads when installing new studs.   I'm not sure how well this will hold up in the long run, and the repeated hot/cold cycles.  I guess I'll find out.


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Nice write-up.  Good that the engine was out of the car and already stripped, because the drilling-and-tapping job would have been more difficult otherwise (canted engine doesn't help).  That said, I'll bet the engine-in-situ helicoil job has been performed hundreds, if not thousands of times in Nissan service bays around the world.  The front one is certainly easier to get at than the back one.

Interesting that the reverse-rotation drill didn't provide a solution.  I've heard others swear by that technique.  I could never quite understand why, though.  The direction of rotation should have no effect on the amount of heat generated (to help break down the thread corrosion) and the reverse torque generated by the drill's cutting action would seem to be modest at best.

I wonder whether the end of this stud (broken-off well below the surface) could have been successfully built up with a MIG to the point where a nut could have been tacked onto the end?  Anyone care to comment on whether this example was within the limit of the MIG strategy?

I wouldn't think that there's going to be an issue with the HeliCoil insert backing out because of heat cycling.  But you never know.  Did you consider using Loctite on the outer threads of the insert?  Did the HeliCoil instructions have anything to say about this?

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7 hours ago, Namerow said:

Did you consider using Loctite on the outer threads of the insert?  Did the HeliCoil instructions have anything to say about this?

No mention of it.  I saw mixed info online using LocTite with Helicoils.   Maybe for a frequently used bolt, but once the stud is there, it ain't moving. 

3 hours ago, 7tooZ said:

I have never tried the helicoil system I have always drilled out the stud and tapped the hole. Is there a benefit to the insert?

No benefit if you can save your threads the way you suggest.   I didn't try running a M8 1.25 tap down it, I guess I should've tried first.

If I were to do it again I'd probably use a Time-Sert instead.  Looks like a better solution, but pricier.

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