Broken L24 rear cylinder head stud - Tips wanted for drilling & tapping in situ

[Namerow]

This topic came up here about two months ago.  In fact, I even commented at the time (without much optimism).  However, now I have to deal with the reality of my own situation and I'm looking for 'been there / done that' tips from anyone who's managed to accomplish the task successfully without pulling the head and manifold. 

 

Issue:  the rear-most manifold stud on my L24 engine has been snapped off (not by me!) flush with the face of the cylinder head. The exhaust manifold is still in place, and I'd prefer to leave it there if I can.  That means drilling out the stud with a hand drill and working within the confines of the hole in the manifold flange (which is about 0.56" dia. and 0.60" deep). 

 

The consensus seems to be that this particular stud was undersized in the first place, meaning that there's not much point in worrying about preserving the original M8 x 1.25 thread.  Next size up would be M10 x 1.25. 

 

• Max OD of the existing M8 x 1.25 internal thread appears to be 8.34mm (dimension was taken from a thread chart)
   
• Recommended ID for the pilot hole for an M10 x 1.25 tap is 8.8mm
  (which is only .003" different from 11/32", so that's the drill I'd plan on using)

 

That nominally leaves 0.016" difference between max ID. of old of 8mm thread vs. OD of the new pilot hole.  It suggests that the pilot hole would need to be centered accurate to within 8 thou of true center for a 'clean' pilot hole.  Not much room for centering error when starting the drill, then.  Fortunately, though, the thread pitch won't change from old to new, so that (perhaps) provides some forgiveness for starting the pilot hole a bit off centre.

 

There's still, of course, the matter of aligning the drill 'tilt' so that the pilot hole runs parallel to the the centreline of the existing hole.  If I've done my trigonometry right, a 5 degree error in drill tilt will generate about 0.09" (one-and-a-half sixteenths) in top-to-bottom misalignment for a 1"-deep hole.  Cut that down to just 2.5 degrees of drill tilt error and you've still got 0.04" of misalignment at the bottom of the 1" pilot hole.  Acceptable for an exhaust stud hole, I suppose, but only just.

 

Now it gets interesting.  The L engine is, of course, not mounted vertically so using my hand drill's bubble-float aligner won't work (and that would only assist with alignment in the vertical plane anyway).  So how to gauge the right 'tip' angle for the drill, up-and-down and side-to-side?

 

In addition, there's the matter of centre-punching the top of a sheared-off 8mm stud while looking down through a 0.6" dia x 0.6" deep hole in the exhaust manifold flange (while leaning over the fender of the car).  And BTW, that hole in the manifold flange isn't likely to be centred all that accurately relative to the stud.

 

Which leads to my question:  Has anyone accomplished this job successfully with engine and exhaust manifold still in place?  If so, what tips can you offer for: a) centering the drill bit's starting point on the stud, and; aligning the drill so that the pilot hole runs true?

 

Or is this just a bad idea?

[Pop's Z]

FWIW, I had a similar issue with the head bolt on an Onan generator down inside the block. A friend who was a tool distributor got me some reverse drill bits and surprise, surprise the head bolt just backed right out. I had to work in a very confined space in a boat so was unable to get it lined up perfectly, but it didn't matter. I was very relieved at the time. I have used the same bits on my Zed occasionally (not a cylinder head, though) and they worked their same magic. Good luck.

 

Cheers, Mike

[madkaw]

You might have to just bite the bullet of reality and pull off the exhaust. Why take the chance of

possibly boogering up a bad situation by making it worse. I know it's a PITA for one bolt, but being able to see it better might open up more options. With it off you could penetrant in there better and maybe pay someone with a welder to weld a bolt to it and just back it out. You drill it and then you've committed to that option

[Mark Maras]

If the hole in the manifold was centered over the stud a centering punch and a left hand drill bit would be my first choice. That not being the case, I'm with madkaw. Pull the manifold. It will probably save time and remove a lot of guesswork.

[Chickenman]

You need some space to work. Like it or not you're going to have to take the manifolds off. Get some REALLY good penetrating spray. PB -Blaster or Kano Kroil is very good. ( WD-40 is Meh ) Interestingly a home brew of 50% ATF and 50% Acetone keeps coming up to the top of the penetrating list preferred by Machinists. Let it soak in for at least 24 hours.

 

Another very good bolt removing spray is CRC Shock Spray. This cools the bolt down to -30c and shrinks it. This breaks the rust crystals and allows the penetrating fluid to wick in between the threads. Used a lot in heavy industry, Turbo applications and Marine applications. I'd use a combo of the two.

 

Then use a brand new left hand drill bit ( Cobalt steel )  to remove the old stud. Once you drill out the center, that will relieve the wall tension and it should come right out. Pilot hole with 3/32" right hand drill bit. Then switch to LH 1/8" drill bit and move up to LH 3/16" as necessary. You don't have to drill all the way through. Just enough to release the wall tension. You can also use a center punch to " tap " the stud out by turning it.

 

Edit: After a good penetrating oil soak and maybe the CRC Shock Spray, try the old " center punch " tapping technique. Sometimes you get lucky and don't have to drill.

 

Nissan did come up with a field fix with 9mm studs... not 10mm. The important thing is that the hole in the manifold should be at least 1 to 2 mm greater in diameter than the actual stud. This is to allow for expansion of the long 6 cylinder exhaust manifold. Later 260z and 280z  exhaust manifolds used larger " clearance" holes than the 240z, at the ends of the manifold. That's because the ends move more than the center of the manifold. Use 260Z or 280Z 9mm studs if you have to enlarge the thread holes.

 

To tight of a stud clearance hole on  240Z exhaust manifolds is what caused the original bolt shearing. It's not a fastener strength issue, but rather a problem with the cast iron manifold getting longer when it gets really hot. The manifold grows in length, puts a sideways load on the stud and shears it off. Some Big Block Chevies have the same problem.

 

If you can get the 8mm stud out with the threads in the head intact, you can still use 8mm studs. Buy the factory Nissan studs or high grade 10.9 or 12.9 grade Metric studs. Next enlarge the clearance holes in the cast iron 240z exhaust manifold by 1 to 2 mm on cylinders #1,#2, #5 and #6. That should solve the problem permanently.

 

BTW,headers usually do not have an issue with the 8mm studs getting sheared off, as the header flange does not grow as much with heat.

Edited December 20, 2015 by Chickenman

[sweatybetty]

my question would be: why mess with it? unless there was an exhaust leak (you really dont state what the problem is) i would just leave it alone. i have an old dodge power wagon with a couple of the exhaust manifold studs broken off and everything works fine. why take the chance of making things worse if there is no real problem (other than a broken stud) to start with?

Edited December 21, 2015 by sweatybetty

[Zed Head]

Seems like there were a lot of good suggestions in that thread from two months ago.  A summary of what you saw there might help focus.  I know I made a suggestion or two.

 

With "focus" in mind, today's camera and laser technology would seem to give some big advantages over what was available in the past.  A drilling fixture bolted to the head or manifold, a bore scope and/or laser to assure that the fixture is centered, and start drilling.  With the right camera you'll get a better look at what's happening than with your bare eye.

 

As far as leaving the broken stud, the gasket will leak and can erode the head material.  Then you'll have to remove the head to have it resurfaced.

 

My theory on why they break is fatigue.   The studs get bent back and forth with every heating/cooling cycle.  The ones on the ends move the farthest.

[Martzedcars]

One stud on my just bought L24 was also broken.  I will weld a nut on it to take it out.. weld it and after a few seconds start to unscrew, it will come out.. (also soak it in a penatrating oil first..)

[Patcon]

Martzedcars

If the stud is broken off flush with the head, how do you plan on welding to it? Lay the nut on the head and weld down through the center to the stud?

[Zed Head]

Issue:  the rear-most manifold stud on my L24 engine has been snapped off (not by me!) flush with the face of the cylinder head. The exhaust manifold is still in place, and I'd prefer to leave it there if I can.  

 

 

Or is this just a bad idea?

I have a slight leak at the front of my manifold and the stud is intact.  I thought everything was fine when I put it together but apparently not.

 

If the engine has been run with no stud it's almost guaranteed that exhaust gases have blown through the seal and done some damage.  Odds of a good seal after the stud is fixed are low.  Something to consider.  It was fun to brainstorm some ideas but you'll probably be removing the manifold after whatever you try anyway.  Plus the risk of doing damage to the head.

[Martzedcars]

Martzedcars

If the stud is broken off flush with the head, how do you plan on welding to it? Lay the nut on the head and weld down through the center to the stud?

 

Yes.. when its flush you still can weld a nut on it!

[Namerow]

Thanks, guys.  Some very useful thoughts (and reminders) have surfaced here.  I agree (sigh) that the manifolds are going to need to come off.  Bore scope or not, trying to accurately mark and center-punch the top of a sheared-off, rusty 8mm stud that's sitting at the bottom of a .5" x .5" hole sounds like a high-odds proposition.  And I'm still not sure how one would align the drill so as to end up with a pilot hole that runs true down the centre of that itty-bitty stud. 

 

The left-hand drill bit technique sounds tempting, but not tempting enough.  The root issue here is galvanic corrosion between the threaded surfaces of a steel stud and an aluminum head.  I'm not convinced that drill-generated heat and bite is going to be enough to overcome that -- although I'm intrigued by the notion that the resulting hole might relieve the 'pressure' between the stud threads and the head.  I think I'll experiment with another alloy casting I've got lying around that also has a snapped-off stud.

 

Note that I've never had the engine running since I bought the car, so I don't know whether there's observable leakage at the back of the manifold or not.  A small voice in my head says, 'Listen to Sweatybetty' and just leave it alone.  But then Zed Head's came along and spoiled that with his comments about potential damage to the head.

 

I've looked at a number of Youtube videos that deal with removing broken studs.  My sense is that the weld-a-nut-to-the-top-of-the-stud strategy is the right one.  The views of the localized heat generated in the stud by the MIG is pretty compelling...

 

 

According to the video narrator, the technique becomes a little more challenging when the stud size gets down in the range of 1/4" dia. -- which, of course, is exactly what I'm dealing with on the Z.  I suppose that if the welded-nut approach doesn't work, it should still be possible to use the drill-out strategy as a fallback.  The extreme heat cycles created by the MIG would certainly assist chances for success with the reverse-drill.

 

BTW, the Nissan-sanctioned explanation that the end studs break off because the manifold expands as it gets up to operating temperature is interesting.  The hole in the manifold flange for the stud measures 0.56 diameter, while the max diameter of the 8mm stud is 0.33" (8.34mm).  If the hole is centred over the stud when the manifold is dropped in place (maybe a bad assumption), that would allow about 0.1" of radial clearance before wall of the manifold hole comes into contact with stud.  That suggests that the exhaust manifold must be expanding lengthwise to the tune of 0.08" - 0.12" at each end (or by up to 0.24" from front to rear!).  That seems hard to believe. Maybe it's more a question of manifold fore-aft misalignment during assembly?