WARPED/BROKEN MANIFOLDS

[joel gopan]

Are a fact of life with the CCKW, all CCKWs will experience this at some point. NOS and NOS Surplus manifolds are scarce and will not be available forever. General Motors has not manufactured any for years. It will be a wise investment to buy a spare set for the future. There are no active GMC 270 Powered vehicles in the Army Inventory, the last ones to use the Exhaust manifold were the M-135/M-211 GMC, M-50 Ontos tank, M-59 APC, and these were retired by the 70s. (I highly doubt that there is a stash of these stored at a Depot waiting to be surplused) NAPCO was the last of the Gov't Contractors to have these in quantity, and wise dealers snapped them up immediately. The CCKW is 60-65 years old. The manifold is just one of the many rapidly disappearing parts on the CCKWs.

[dr deuce]

The 3 piece M59 type manifold will fit with modifications to the pipe to the booster as I recall. I had one on that I had bought from Joel a number of years back.

The problem as I see it is caused by only clamping the manifolds to the head and having a hard mounting system for the exhaust pipe.

If you look at a modern manifold, most have 2 bolts per exhaust port to hold the manifold on evenly.

The Chevys (all of them civy and mil) have an exhaust pipe that is bolted on with 2 bolts and has a rounded donut as a sealing gasket between the two. The Chevys do not have this problem anywhere near the way the CCKWs do.

I believe that the fact that the engine can move a little on its mounts and that the pipe really cannot because of the mounting of the muffler puts a bending pressure on the manifold. With the vertical length of the pipe, it has a pretty good lever on the manifold.

[joel gopan]

Possibly, but the greatest contributor is heat concentrated at the center of the exhaust manifold, it gets more intense if the control valve is closed and redirects it to the base of the carburetor, that is why it it is best to dump it directly out by leaving the valve in the summer position. As far as stress on the Exhaust Manifold Flange, that Cast Iron is much stronger than the thin steel exhaust pipe and the attaching points of the Exhaust system at the muffler hanger(a thin strap) and tailpipe at the Mudflap. Did you realize that at operating speed there is a blue flame from your engine that heats the exhaust manifold and pipe to a Cherry Red(You can see thru them) and I am sure that the pipe softens and gives a little. I do not know how loose Dr D's Engine is on its mounts, or the gyrations that his truck is put through, but if the Transmission Strut Rod to the Chassis is installed properly, there is no longitudinal motion to the engine, it is mostly limited to torsional motion, all it can do is rock slightly. The Chevrolet and CCKW Exhaust Pipe is securely securely connected to the Manifold with bolts or studs, and nuts, whether 2 or 3 connectors is used is immaterial, they are supposed to be tight(GMC has 3 extra lock nuts) I guess we could get into stress analysis, (That is my old stomping ground as I used to be responsible for structural steel analysis and testing of bolts in the Strength of Materials Lab I was responsible for) but this site does not pay me enough.

If one has the genius, deep pockets and background of Jack Rousch, who is famous for his Nascar Racing Team, Hi Performance Ford Engines, and updates to the metallurgy of Rolls Royce Merlin Engines as used in WWII Fighters, then it would be possible to rejuvinate some life into the rotten, cracked, and generally tired old manifolds we have.

Time, heat, and inattention to the manifold are the greatest culprits, it just self destructs from the moment you install it. The 3 piece M-59 manifolds are hard to find, they are nice, and command the same $275.00-$325.00 average price of NOS conventional GMC Exhaust manifolds. The 3 Piece will be easier on Manifold-to-Head Gaskets, but the sealing rings at the slip joints can be damaged by a do-it-yourselfer during installation and there were never any spares. Just be aware of the fact that in a few short years there ain't going to be any spare NOS GMC Intake or Exhaust manifolds to be found. I am stashing two away for my truck so that the next owner who invests big bucks in my truck will continue to have "happy Motoring".

[dr deuce]

Well we agree that the ex manifolds are a problem and will be more so in the future.

Another contributing factor I think is the new gaskets are not made of asbestos like the originals.

If you think about the length of the down spout portion of the CCKW engine pipe and what kind of a lever that is against the ports on the side of the head, it is quite large.

It is not the front to back motion (assuming the strut rod is set up correctly), it is the tortional (sp) rotation of the motor that I believe it the problem. With that long a lever, the engine does not have to rotate too much.

The Chevy essentially has a balljoint at the ex manifold - engine pipe junction because it only has 2 bolts and a roundish dounut for a seal. A Chevy is much more likely to move the ex pipe on the donut than the ex manifold against the head.

It has been my experience over 33 years with Chev 235 engines and 25+ years with GMC 270-302's that I loose the donut on the Chevy and the ex port on the side of the head on the GMC's. It has happened too many times to be a co-inky-dinky.

The next time I have to do an ex manifold set, I will try cutting the engine pipe and putting in an expensive (you can buy them at the auto parts store for about $70) piece of braid-shielded flex pipe as an experiment to see if I have better longevity by eliminating this lever action.