You don’t need to be around WW2 jeeps very long to encounter a broken engine manifold. The mounting “ears” on the intake half often crack when the manifold is torqued to the engine block. Design defect? Poor manufacturing?…or user error? Let’s take a look.
The manifold on the L134 consists of two separate castings - an intake manifold and an exhaust manifold. The two components are held together by four 3/8” bolts with the joint protected by a heat resistant gasket. If you are dealing with an original, working manifold, there is no sporting reason to disassemble it! I guarantee the bolts will be rusted tight and you will turn a 20 minute project into a several hour one when you break one or more of those bolts.
Fixing the very important heat control valve will require disassembly - a topic worthy of a future blog. Hide your impact gun, use loads of heat (an induction heater is awesome, (I use oxy-acetylene) and plenty of spray penetrant or you WILL break a bolt or two or even three or four and then be running down the drill/tap/EZOut rabbit hole/purchasing a new manifold. Moral of this story, be patient Padawan and use the heat wrench until the bolts come out with modest force!
There is sufficient slop between the two components of the manifold assembly that that a mechanic can misalign the faces that mate up to the engine during reassembly. That means that those fragile little cast iron intake manifold ears are going to be put under a lot of strain when you torque the manifold to the engine block. (Make sure you use the CORRECT two conical washers!)
Remember that cast iron is strong but not flexible (the bow-tied professor says "low ductility coefficient") so you will absolutely death and taxes 100% break misaligned ears when you torque them down. Repairing cast iron is a rapidly dying art so you may have just made yourself a $200 vintage paper weight.
Careful alignment helps but is no guarantee that everything is going to work out swimmingly. The technical term for the flat that we seek is “coplanar”…meaning that all 5 of the manifold mounting faces touch the gasket/engine block at the same time. No magic, just flat - "co" "planar" - sharing the same plane. Having the faces flat means that the torque from tightening down the nuts on the manifold studs is only putting compression pressure on that cast iron manifold and its fragile little ears. Cast iron is plenty strong in compression so torque away. What we must avoid is creating a bending - technically “flexural” stress of the cast iron.
Back when Rosie was building this jeep, the manifold was likely assembled at the factory as follows. Rosie installed the heat control valve, grabbed an asbestos gasket and then bolted the two halves of the manifold together being attentive but not super careful with alignment. Rosie then walked to a running belt sander and touched those magic five manifold faces to the whirling and flat but not crazy perfect sanding belt. Viola! Rosie just make a coplanar manifold that did not break upon installation and performed as designed. I doubt there was much of a jig other than probably laying the cast (unfinished) carburetor mounting flange against a known flat base to act as a guide. She probably then rotated the manifold and put the now coplanar manifold face against that same belt sander base to "kiss" the carburetor flange and make it nice and flat.
"Let's go Rosie, keep it moving!"A couple of things worth noting. Both the manifold/engine and manifold/carburetor surfaces receive thick and compressible gaskets. The gaskets will make up for a good amount of surface imperfection so there was and is no need to machine these surfaces to a perfect finish with .000001 tolerance. Look at an original manifold - you can see scratches on both surfaces…sure looks like from sandpaper to my eyes. Willys/Ford needed a simple, cost effective and fast finishing process for this part. Cast iron is great exhaust material for lots of reasons and also happens to be easy to sand. Belt sander? Check.
One of the many benefits of quality mass production is that you can take parts from any two of the same product and they fit on another without adjust - this is called Interchangeability of Parts. This concept helped the Allies win the war. I surmise that not so much concern was given during WW2 to the knowledge and experience that by flattening on a belt sander, “human factors” would introduce alignment differences between components on different manifolds. In practice, a broken manifold would have been replaced with a new one. Maybe some manifolds would have been reworked in the field due to necessity, but I doubt reworking was a huge priority and the field mechanics would have quickly figured this stuff out anyway and grabbed a file or some sandpaper.
For restorers today, this variability in Rosie’s machine work means you see manifolds that sit at slightly different angles on the engine block. One might have the exhaust angling toward the engine just a smidge. Another is noticeably tilted from the front of the engine to the back. Rosie was probably chatting with Annie and put more pressure on her left hand than her right. The resulting manifold is completely serviceable, but if I happened to need to swap out an old with a new exhaust manifold, I am setting myself to break the original intake manifold ears if I am not paying close attention.
The online WW2 jeep gurus will tell you to keep those 4 manifold assembly bolts loose to make sure everything goes “coplanar” before you torque the connecting bolts together. The problem with this “wisdom” is that if there is a manufacturing related misalignment between the two castings (likely), this “procedure” does nothing but relocate the misalignment to another location that you are not going to see unless you look very carefully.
With the manifold assembly bolts loose and the manifold torqued to the engine correctly, there will be an angled gap between the intake and exhaust manifold where the high temp gasket is installed.
Torquing down the four bolts creates forces trying to close that gap and clamp the gasket between the faces of the exhaust and the intake manifold. Something has to give. With any amount of misalignment, the mounting “ears” on the intake are going to be put under the same bending pressures as if you eyeball aligned and there were different angles on the face. I promise you the same broken paper weight result.
My process goes as follows. I have a piece of busted countertop granite - plenty flat for this application. I use the granite as my mounting surface (like it was the engine) and get the manifold put back together as one piece. Often, you start the tightening process flat, but you find that the intake manifold flanges touch the granite on only the top or the bottom once the four bolts are fully tightened. Expect this result.
I grab 100/120 grit wet/dry sandpaper. I use water to stick the paper to the granite and to lubricate and clean the sandpaper. Cast iron manifolds sand like “butter” (with a Brooklyn accent). Depending on how much material needs to be removed, you may want to sand a bit, observe the sanding marks and then loosen/readjust/tighten the two halves to save time. Get the scratches consistent across all 5 mounting flanges, install confidently and get back to winning the war.
Coming Soon: Part II: What to Do with the Broken Manifold: DIY Guide to Welding Cast Iron
Here is a "short" video on this topic:
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