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

[sweatybetty]

seems the easy fix would be to drill out the end holes just a bit. might allow enough room for expansion without shearing off the stud. do these studs go into the water jacket?

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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?

[Patcon]

I suspect that it is the head that grows more than the manifold. If I remember right aluminum grows more with heat than steel. So it may be the head that really moves not the exhaust...

[Zed Head]

I have several exhaust manifolds, and three different heads and I've never seen a wear mark on the outer exhaust studs from the manifold touching it.  The three center bolts locate the manifold and there's a pretty big gap around the outer stud holes when everything's clamped down.  For the 280Z's and ZX's anyway.  The stud is clamped at the top and the length of the stud is free to act as a lever on the base of the stud.  I think that it just moves back and forth with every heat/cool cycle.  The differential is greatest at the ends since growth is away from the clamped center section.  Add in the tensile forces from clamping acting on the narrower root of the thread and it seems reasonable that the outer ones break most often.  Like breaking a piece off of a coat hanger.  That's my fatigue theory in a big nutshell.

 

Someone could probably design a clamping mechanism that allows the manifold to slide under the clamping nut and washer to remove the side load.  It would have to be super durable though and would probably cost a lot of money.

[Namerow]

I like your theory, Zed Head.  The outermost studs, at 8mm, are probably just a bit too small to withstand the bending cycles.  The factory's field fix (re-tap for a 9mm stud) suggests that they were thinking along these lines too (note that the 9mm replacement stud would have reduced the longitudinal clearance for the stud within the manifold mounting hole -- which suggests that it's not an issue of actual stud/manifold contact). 

 

This kind of fatigue failure is based on number of cycles (assuming that the bending load falls slightly short of causing instant fracture), so a lot of L24's may have the original front and rear studs still intact but well on their way.   It may be that the rear stud is found sheared off more frequently than the front one because the rear stud also mounts not only the engine slinger plate but also the coolant rear transfer pipe.  It's the latter item that probably got messed with the most (during attempts to replace perished coolant hoses), leading to the 'uh-oh' moment when just a bit of wrench force on the nut snaps off the already-weakened stud.

 

Interesting that Nissan never up-sized these studs from 8mm to 9mm over the full 240-260-280 production series.  Maybe the engines had to get up over the typical ownership period (say, 60 to 70 thousand miles) before the studs saw enough thermal-load cycles to start breaking off.

[Chickenman]

The field fix from Nissan specifically stated that the manifold holes had to be enlarged when the 9 mm studs were used.

[Zed Head]

Is there a TSB out there?  I love reading those old documents, they're educational.  The exhaust manifold  would be a tough material to work with, for enlarging that hole.  Probably need some sort of grinding media, like a Dremel tool.

[Chickenman]

Drilling  cast Iron is very easy. It's a fairly soft metal.  Any decent shop will have a Drill press or they can send it to a local Machine shop. 5 minutes tops to enlarge a manifold hole with a Drill press. 

 

 

 

[Zed Head]

I'd still like to see the factory documents.  Sometimes in a local area one guy will get the reputation as the guy that knows things.  But he's really just another guy making his best guess.  And I could easily see going up a size after screwing up the threads drilling out a broken stud.  Seems like if the small stud was ID'ed as a problem that there would have been a recall or an if-the-manifold-is-removed fix TSB to upgrade the studs before they broke.

[Chickenman]

I clearly remember a ( Canadian ) TSB on this... but I haven't saved the old paperwork from the 1980's

The number of cars affected is fairly small, and most were out of warranty when the studs started breaking. And it was a relatively trivial item ( in those days ). Manufacturers don't issue recalls unless they really, really REALLY, have to. Just look at Audi and Toyota throttle issues, exploding Firestone tires, flaming Pinto's etc. 

Recall is a whole different ball game from a TSB. Recall's cost manufacturers MILLIONS of dollars.. The 70's and 80's didn't have the Litigation that we have these days.

 

 

 

Edited December 31, 2015 by Chickenman

[Zed Head]

I've never seen a "field-fix" document.  So don't really know what that means.  Did Nissan send something out or is it just word-of-mouth?  

The hole enlargement to avoid the issue, your December 20 post (what happened to the post numbers?), sounds interesting.  Just never heard of it until this thread.  Seems like it would be a well-known while-I'm-at-it for anyone that removed an exhaust manifold.

[Chickenman]

No it was Factory Nissan Service Documents. TSB's. I worked as a Dealer Partsman and Assistant Manager for close to 18 years in Canada. Both the Parts and Service departments got copies of the TSB's from Nissan Canada. I assume Nissan USA worked the same way. Edit: " Field Fix " is a fairly common term up here in the Automotive Warranty Business. " Field Fix " ( in our neck of the woods ) usually refers to a " Dealership modification " that does not require a complete redesign of a component. Maybe the addition of a auxiliary harness or relay. Or bending a bracket to provide clearance for a hose or line.

In this particular case the factory is not going to pay for a brand new cylinder head and all the associated time just for a broken stud... so they come up with a " Field Fix " to put in a larger stud... if the threads in the head get damaged removing the old broken part. Very common practice. That was a time when the factory  expected Techs to fix and repair things... not just replace parts.

As for, " I may as well fix this before it breaks". Well, warranty repairs didn't ( and still don't )  work that way. The Dealership only get paid when parts are broken or defective. "  The factory won't pay the dealership X hours Re and Re time for replacing an unbroken stud on a manifold or any other part. Even if you know that 6 months down the road it will break. Pre-emptive fixes and Initiative are not rewarded in the Automotive Warranty business. Still true to this day in most cases.

Edit: Again Recalls are a different matter and are usually safety related. The occasional broken manifold stud is not something that the NHTSA is going to force a manufacturer to Recall unless it is a proven safety hazard and may cause a fire etc. And don't forget that most of these broken stud cases occurred well after the warranty had expired on the cars.

 

Edited December 31, 2015 by Chickenman

[ZCurves]

On 12/20/2015 at 11:07 AM, Namerow said:

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. 

...

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?

 

Ok, if you are still needing some practical input..  I wrote on this exact topic several years ago - Unfortunately, I have had to do this a couple of times on several Z's over the years.  Both times what I did was remove the exhaust manifold to make room.  I then used a torch to heat the area on the Head near the broken stud - not the stud itself, just around it.  Now for the 'fun' part - Using a small, very sharp chisel and a ball peen hammer - you begin tapping (with a corner of the chisel) counter clockwise until the stud begins to back out.  Re-heat every once in a while.  The stud will back out - You have to be real patient and take your time.  Be careful not to nick the Head, let the chisel bite into the stud as you work it backwards.  When you are done, be sure to blow out the bore with compressed air and use Anti-Seize on replacement stud.