Missing Rear Exhaust Manifold Stud

[Real638]

To start, I have a '78 Black Pearl, (Real638), which I've owned since early '81. The car has not been driven since around 1990, & I just started working on bringing it back to life a couple of months ago. The engine had never been apart - it made it 184K & still ran ok when parked. It is missing the rear lifting lug & for all I know it may have been gone when I bought the car.

I know the issue of broken rear exhaust manifold studs has been covered several time before, but I'm not satisfied with the explanations I've seen for this & I can't find an answer on how to solve the problem. From what I've read the hook is gone & the stud broken on nearly all 1st generation Zs. Banzai Motorworks says they reproduced the early lug because nearly all the cars that came into their shop were missing it.

When I pulled the manifolds I found the stud had broken flush with the head. I broke 2 others during removal, as well as the bolt in the front, and one bolt for the radiator hose fitting on the other side, but they all broke with about 1/2" showing. These came out easily with a little heat & vise grips. Yesterday I looked at a '78 in a wrecking yard. Rear stud broken flush with the head. Same with a '74 & early 240 in the same yard. I did find a '77 with the stud, nut & lug still on it! When I put a wrench on it the nut & part of the stud literally fell off - broken flush with the head.

I've seen a couple of theories to explain this. One is warping of the manifold causing the stud to break. The other is over tightening during repairs. Neither of these theories, while possible, satisfy me. I think Nissan used the lugs during assembly & that little 5mm stud bent, at the head, & cracked. I'm thinking about making up a sleeve, with room for manifold heat expansion. Thoughts on this?

Edited May 4, 2014 by Real638
Part of thread cut off

[Jeff G 78]

The rear stud usually breaks after the transmission exhaust hanger is removed during exhaust replacement. The exhaust was designed to be rigid back to the rear hangers so that engine roll doesn't put load into the manifold studs. Once that trans hanger comes loose or is removed, the stud will regularly break. The best bet is to make sure that hanger is in place and install a flex pipe in the system to help reduce stress into the system.

Here is a picture of my flex pipe while I was mocking it up. You can see the trans hanger at the left side of the picture. It was in terrible shape, so I fabricated a new section to fit with my exhaust. I installed the flex pipe to ease the stress on my race motor and mate the MSA exhaust to my freebie header.

[Arne]

My experience seemed to point to the manifold warping issue. One of my 240Zs had a '79 L28 in it, with the broken rear stud. When I removed the manifolds and replaced the stud, I found that the manifold was warped bad enough that I couldn't get it back on without enlarging the rear hole. I was personally convinced that the manifold put enough side pressure on the stud to eventually break it off. I've never seen any other theory that fit my situation as well.

[Real638]

Don't think it's an exhaust hanger issue; trans hanger was intact on my car, and it had 100% original dated exhaust parts on it. Plus I don't see how that much lateral stress could occur; it's a 5mm stud thru a hole the size of a dime. My manifold wasn't badly warped, and I don't see how that would explain why only the rear stud has the problem. Visualize the stress placed on that one small stud when engine weight is placed on it, particularly at the point where it goes into the head. Assembly line stress still seems the most plausible explanation to me.

[Arne]

Properly tightened and torqued, the stress from lifting the engine is not borne by the little stud—it is borne by the friction of the entire contact area of the lifting lug to the mounting surface. The stress on the stud would only be significant if the nut was not properly tightened.

[Zed Head]

The front stud tends to break also. Wrote a whole nerdy thing about materials and expansion/cooling but it probably boils down to you only get so many cycles (metal fatigue) before the studs break. The ones on the ends move the farthest during a heating and cooling cycle. A good reason to replace the studs if you have the manifolds off.

[Real638]

True, the front stud/bolt will break, (see my original post), and that is not just a Z car problem. I've broken bolts in Y-block Fords, small block GMs & Toyotas. However I've never come across a situation in which the bolt head or nut/stud simply fell off; a little twisting is needed to snap it. And again, why does the rear Z stud seem to consistently break flush with the head?

[Pilgrim]

IMO it's just the effect of cooling and heating cycles. I just went through the head on my 280ZX and it took a submerged laser cutting technique to get the rear stud out, after which the hole was heli-coiled. I replaced broken front and rear studs with hardened ones and hopefully won't have a problem with them. My car is a 1983 ZX Turbo with 126K miles.

[Dave WM]

found this thread while searching. Mine (last bolt on exhaust manifold with the engine hoist) just fell off. I was under the hood and noticed the bolt was no longer sticking out straight but bent down and the hanger was at an odd angle. it was completely sheared off.

Next question what do I do about it? leave it be or can I get a professional bolt extraction service that will not require pulling the head? any harm doing nothing about it for now?

[siteunseen]

I drilled mine then retapped it to a larger size, 12x1.75 I think.  It is for the Nissan Titan pickup.

No harm other than a little more noise back there.  Mine was broken for a year or so.  I drilled the broken stud then used one of those Extractor bits.  There is such a difference in the size you can waller it out without any worry.  I used a propane torch too when i got ready to back it out.

[Jeff G 78]

Drill and extract and then insert a Helicoil.

[Dave WM]

I have read up some on this, Seems like using a sleeve that fits the manifold hole (assuming its centered over the stud) snuggly and has a hole in the sleeve that is sized for a small drill bit. Then after a starter hole another sleeve with a larger center hole as a guide for a larger bit, repeat once more with a sleeve and bit that just about consumes the balance of the stud.

I think if I use the sleeves there will be enough control to do this operation without removing the head.

Anyone here use this sleeve method for accurate centering of the drill bit?