The GMC CCKW

With regard to the problem of exhaust manifold cracking, I am
wondering whether the inclusion of a flexible downpipe section,
just below the manifold flange may be a worthwhile idea.

I spoke to my exhaust specialist breifly, and he said it would be
easy enough, as long as a support bracket is made to support the weight of the exhaust muffler system.

I know that many people feel that mods away from original design are
unneccesary, but when u observe the fact that the muffler system/exhaust has no flexible mounts and that the engine does visibly move under torque, u do wonder where those forces resolve, whilst bouncing down the road.

A 63 yr old cast iron piece at its weakest point seems the likeley answer!
If I could introduce enough flex just below it, might greatly reduce the stress ( I am thinking 4 to 6 inch flex piece, from near to the flange)

There was a previous thread along these lines I believe.
Has anyone actually had it done?

Now would probably be the right time for me, seeing as I will be needing to drop the exhaust.

Getting the design considerations right would be great, the exhaust bloke I use is very clever, but these vehicles are not normally in his experience, and of course he is always a little rushed for time.



regards tony

Sounds like a good idea the military did this on the m274 mule.
But, you will more then likely not have this problem again.

Awg, I not sure that fitting a flexy section would be effort well spent.

In my opinion the manifold cracking is more likely to be a problem caused by temperature. This is especially true if the manifold heat flap is set or worse siezed, in the winter "cold" position. Putting the engine underload by driving away too quickly before things have warmed up, makes the problem worse.

Remember the Jimmy has a "manual" 3 position setting that needs seasonal adjustment, unlike the good old jeep, which is automatic.

Just my 2p's worth.

my manifold heat flap was seized in the cold position while the manifold itself is in great shape. We tried to fee it up without any luck so we compared it to one on a bad manifold and bent it to a permanent "summer" position and it was this or finding another maifold. I dont plan on driving the truck in cold weather. I just wonder if I did drive in cold weather then how important is it to set the flap in the COLD position. I understand that sitting it in the COLD position helps to warm the carburator...

What do you guys normally do with your heat flap?

Hi Mark, I've experienced problems with these flaps before. Here in the UK I run my flap at its middle position, all year round. But then we dont get quite the extremes of temps that you guys do.

It takes a little while for the carb to warm up in the winter months, but thats a small price to pay against cracking the manifold, if accidentaly left in the COLD position on a hot day.

Re your question there will be no problem in running the heat flap COLD providing the intale air stays cold, but should you forget and then drive in HOT conditions, the manifold will surely overheat then warp and/or crack !

I started this consideration a while back. This spring, I will add a flex section to my engine pipe. It costs about $70 for the flex unit.

As I pointed out before, the engine pipe is a long lever prying against the manifold with the pivot point on the centerline of the manifold studs. The lever works both ways potantially lifting the manifold above or below the canterline of the studs. If the muffler was not tied down so hard, it might work better. You do however have a lot of mass there that can swing back and forth then even if it were not hard mounted.

One of the reasons I firmly believe this situation exists is that the Chevrolets I own do not exhibit this problem. They have a donut between the engine pipe and the manifold which is held clamped by 2 bolts. What you effectively have is a ball joint.

Every few years, (and many miles) I have to replace the donut on the Chevy. Every few years (and many miles) I have to replace the manifold to head gasket set. If I am lucky, that is all I have to replace....

If you put on 2,000-3,000 miles per year, year after year, you learn a lot!

Hi awg good call to modify the exhaust, i will be doing the same to mine as i ve had 2 manifolds so far. i was told this could help to prolong the life span of my new one.