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March 10, 2025

Exhaust Manifold

The exhaust manifold on the MGA is not remarkable in any way.  It is a pretty standard cast iron piece with three runners--the middle one serving the siamesed ports from cylinders 2 and 3.  It's a relatively sturdy items, and mine was intact, but appropriately rusty. The "wet" area was from spraying penetrating oil onto the studs.




Speaking of studs, that was my first order of business.  These items are notoriously often difficult to remove, living with extreme temperature cycles in a location prone to getting wet.

My first attempt was naïve, with a stud puller on a 1/2 inch drive ratchet.  The stud sacrificed itself almost immediately.  Manifold--1, Me--0.




I soldiered on, but with a little more finesse this time.  With heat, penetrants, and patience, I finally got the second stud to budge a little.  It would move back and forth slightly, but wouldn't go beyond hard limits.  I was pretty sure it was galling the threads, and didn't press it any more.  More sophisticated measures were clearly called for.

But before going further, I decided to clean up the manifold, if for no other reason than to make it more pleasant to handle.  This really just meant de-rusting. 

I first experimented with media blasting.  This worked fine, but was slow.  I regrouped and switched to an even slower method, but one that didn't require me to stand over the part for the entire process.  I submerged the casting in my favorite de-rusting potion, which is just a solution of phosphoric acid.




Over a couple of hours, this loosens the rust to form a black smut, which is easily brushed off.




Now, back to those studs.  They clearly had to be drilled out.




In a feat of heroic work-holding, I mounted the manifold on my mill.  Drilling works best if the drill can be accurately aligned with the axis of the stud.



I cutoff the remaining studs to a more workable length.  This allowed a little alignment tube to be fit over the stub, so that the drill could be centered.




Then a 1/4" drill, followed by a 5/16", which is the tap drill size for the 3/8" studs.  The remains of the stud--pretty much just the threads--sheepishly fell out in utter defeat.




The other long stud went similarly, but the broken one was a little more problematic.  Alignment was a little more approximate, so the drills were slightly off center.  This meant that the final remnants had to be picked out.

At the beginning of this exercise, I put the odds of saving all three threaded holes at about 20%, and had Helicoils ready.




Since the manifold flange was mounted so flat and secure, I didn't see any reason not to face it off.  The realization that it isn't actually a sealing surface came later.




The surfaces that would really benefit more form facing are the ones that mate with the cylinder head.  I was able to check for coplanarity, and found that the middle runner was out of plane by about 0.012".  I don't think this by itself would have been a problem, but the faces were also pretty pitted.

In an even greater feat of heroic work holding, I got the manifold fastened to the mill table with the runner faces flat and level.




The setup had marginal rigidity, so I limited cuts to 0.005".  It took about 0.018" total to get everything to the same plane, with the center runner coming in last.




Throughout this process there was one little thing nagging at me.  This little bulge in the runner ports for the two end cylinders.  Neither the head nor the gasket had matching features.  It seemed like a bald insult to the free and natural flow of exhaust gasses.




I know why they are there.  There is a recess in the outside surface of the runner to make room for a socket or wrench to remove the manifold nut.  This bulge maintains wall thickness in that area.  But wall thickness is pretty generous in this part, so I didn't think losing some would be a big problem.  Besides, most of the material removal would be from the edge of the mounting flange.  I opted for surgery.




I am not under the illusion that this will make any noticeable difference to normal or even spirited driving, but it makes my aesthetic sense happy.

 


Last up: new studs to replace the fallen ones.  I couldn't really justify ordering a set and waiting for them, so took an hour or two to make a set.  Unlike the originals, these are zinc plated, so I guess we'd have to call them "uprated".




Before making the studs, I had painted the manifold with a high temp paint, and cured it in my oven, following the manufacturer's schedule.

Popped the studs in, and we have a manifold!




This was another of the "cold weather" projects, marking time until we see better temps for garage work. 

It took a few days, but the cost was only about $15--mainly for the paint.

Comments to Ed at elhollin1@yahoo.com

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