exhausted from manifolds

[butch atkins]

have had to put 3 exhaust manifolds due to cracks on my m211,spent last saturday at junkyard pulled 3 int/exh mans to get 1 good one,6 hours labor,busted knuckles,125.00, is any one else having this problem,do it by the manual each time ,thinking about machining manifold flat before installing,the one i have has a bow to it ,you can slide 2 quarters under the inside flanges with it laying flat on the floor,am tool and die maker by trade so machining isnt problem,anyone else have any ideas about how to solve this problem? i dont think double stacking gaskets is a good idea,but am open to new ideas ,last one cracked in 2 places,and im tired of redoing this job

 

[hippiedude]

I feel for man!!!!!!!!!!! Mine is cracked too !!! They are like gas tanks , can`t find a good one any were......Tim

 

[badgmc56]

After installing the manifold , do you retighten when the manifold is hot? That has worked for me on many strait-sixes .The manifold has to be true of course first. I'm looking into performance exaust for my truck. I may even try to make a set of headers for it.

 

[nattieleather]

I had that on my 211 could never get that thing to seal up. I had the manifold machined and it made it worse. What you need is to pull the head and have the intake exhust and head all machined together. I've been told that the "o-rings" are very important to the operation of sealing of the manifold.

 

[M543A2]

This manifolds problem is typical of straight six engines. You might notice that on some engines, especially diesels, there are expansion joints in the manifolds to prevent this problem. The problem results form the high coefficient of expansion of cast iron. The manifold lengthens and shortens by an amazing amount every time it heats and cools. Providing it the ability to move prevents the cracking. Locking it down tighter with more torque only makes the problem worse. It tries to move, can't, and then cracks from the stress.
Part of the answer is to not tighten the bolts very tight. Also, all sealing surfaces on the head and manifold need to be straight. I did this, then the movement of the manifold destroyed the stock gasket. I then used a new gasket set as a pattern to cut out a sheet copper gasket. The copper sheet I used was about .060 thick. This gasket allowed the manifold to slide on the bolts and not wipe out the gasket. I ran this setup for years with no problems. I checked tightness of the manifold bolts frequently to be sure they had not loosened too much, allowing leakage. I wish I could tell you a specific torque I used on the bolts, but I cannot. It was one of those "feel" things from many years of mechanic and machining experience. I was a little past what I would term "snug" on the tightness. I used hardened washers under the manifold bolts to allow movement without binding.
Regards Marti

 

[acetomatoco]

be sure to put the little sleeves in the block and install the exhaust and intake parts separately, loose on one another..and use the correct OEM fasteners... after you have installed them and run them in and retorqued the fasteners, then you can tighten the bolts which hold the two together...and check them every month or so as described by Marti..and others...

 

[badgmc56]

Never had a manifold crack from a retorque. Many tech manuals even recomend it. Most manifolds crack because of vibration and EXCESS HEAT. Poor air-fuel mixture can contribute to manifold cracking because of the fuel igniting in the manifold. Not using the proper exaust hangers can also be a cause. The exaust system MUST be flexible.

 

[DDoyle]

Like ACE said, be sure the little sleeves are in place. Someone mentioned headers. I know the 270 was popular with racers back in the 50s, and I'd be surprised if the 302 wasn't as well. Various aftermarket intake and exhaust set ups were made for the 270, including multiple carb arrangements. Check with a vendor who specializes in the growing retro racing hobby.

DD

 

[acetomatoco]

The 302s in the racers at Westboro MA speedway in the 1960s were small block killers...boy would they go.. and they had a sound all their own....we sure had fun... even had a beer stand in the pits... and 2000 burnouts Eastbound on Route 9 at the end of the night...with the State Police just watching with their fingers crossed....

 

[nattieleather]

Sleeves that's what I meant by "o-rings".

I was told often when I had my leaky fill the cab with smoke M211 that the 302 was popular with the race crowed...but I could never find any headers for it...but then again the internet was just a bunch of bbs's hooked together back then....

 

[badgmc56]

The manifold on my truck is in perfect shape so far. No leaks or discoloration but if it gives me any trouble I won't try to put some used , warped piece of junk on it. I have a friend who owns a performance shop and he is working on finding headers for the 302. If they make them, he will find them.That motor can only run better with a tuned exhaust.I will post part numbers and company name when I know.

 

[JasonS]

I have purchased from this company with good results: http://www.patricksantiquecars.com/

 

[majortom]

I have over 100,000 miles on my M135 i have several other GMCs that we have used for many many years we have no manifold failures the part was under alot of stress in the orignal disign there are several factors that promote the falure todays fuels burn much hotter ether increase the main jet size in carb by drilling out jets one numbered bit larger, find bigger jets (good luck), or use an m35 carb (it has bigger jets), advance ignition timing by at least 6 to 8 degrees. while you are messing with your distributor check that the advance mechanasim is functioning use a timing light rev the engine to 2500 RPMs or higher, a nonfunctioning advance is the most common reason for excessive heat cracked manifolds and rediculously slow trucks

 

[JasonS]

The carbs use standard Holley jets. My M35 gas engines had smaller jets than my M135??

 

[stephenfeldmeier]

I hate to be bearer of bad news but the majority of the problems with the manifolds are just due to the cycle rate and age of the manifolds.
First cycle rate – the number of times the manifold has been heated and cooled. Every time a manifold heats and cools some portions of the manifold changes molecular structure when the manifold reaches operationing conditions then back again when the manifold cools down to static conditions. Basically goes from a molecule that looks like a box then changes to one that looks like a 3-d snowflake back to a box again when cooled. Since I do not have my metallurgy book handy, I can’t go into detail about it now but ALL manifolds will develop molecular cracks between their structures due to uneven heating and cooling and high carbon content of the manifolds.

Now age – over time these cracks will develop to where oxygen can get to them. Oxygen will start breaking down the iron forming barriers and once these barriers connect to one another, a crack forms. So any viberation, heat condition, undue stress such a torque on a nut, will split these manifolds. Most manifolds will crack or break in two just forward of the carburetor just behind # two exhaust.

Also if you look at these manifolds, I feel they were cast too thin. If the manifolds were cast 1/16 of an inch thicker on the body, they would not have the problem of cracking.
Again these trucks were not in service with the military long enough to work out all the bugs. (just like the weak reverse gear in the transmission, we have a crappy exhaust manifold.)

So is there a golden cure for the problem? Yea, if you don’t want your manifold to crack, just don’t start your truck anymore.
but 70% of the manifolds out there will crack, if you have one thats not cracked, you're lucky. I have my 4th manifold on now and been running it for about 5,000 miles with no problems. my fingers are crossed everytime I open the hood to check the oil

 

[majortom]

i am sure m35 jets are larger than m135 i agree heat cycle is relavent, but manifold temp is critical. if you have cracked several in a short time you are probably running too hot. i have 12 units in service for 30 plus years lowering exhast temp always extends manifold life

 

[Westech]

Well son of a gun... I did not see you slip in here Tom. Its Richard from Madison.

so anyway on this exhaust manifold problem, i would richen her up a tad there Butch. Also what has worked well for me in the past is always let the truck Idle before you head out and before you shut her down. This will cool or heat up the manifold at a lower rate thus lessing the chance of cracking due to fast heat cycles.

 

[JasonS]

My M135 carb has #62 jets; my Reos have #58. Not that either of ours couldn't have been switched at some point. They are standard holley jets.

 

[butch atkins]

exhaust manifolds

lots of good ideas floating around here,manifold castings are way to thin,compared to anything else ive owned,looked at the site jason s posted ,neat looking headers,might go that route someday,heating and cooling is also major problem,will continue and install used manifold ,hope for the best,33k miles,3 manifolds,my m211 will run 60 mph on flat out governed run,gets 10 mpg,carb "smells" good ,exhaust pipe is gray colored,keep engine tuned,will try going to larger jet size,run regular unleaded gas,,am planning on external fuel filter and regulator,ther are 2 m211s left in junkyard ,told the owner to go ahead and get the ex mans off ,keep them for spares,took carb apart over weekend,little bit of sediment,painted intake and put together the least worn of all the linkage componets together,hunting friday/sat.,then dont go back to work until jan 2,so much to do to truck ,thanks to all who replied

 

[pleasantpeasant_01]

RE: exhaust manifolds

A friend of mine who worked in the motor pool when he was in the Army told me the biggest problem with broken manifolds was when a truck was turned in it had to be washed. Cold water was sprayed on the still hot manifolds. He said you could hear them pop when they broke. This happened almost every time a truck was used. A lot of G.I.'s thought this was a big joke.
I have had and used civilian GMCs for 50 years, the trouble started with age. A manifold heated to it's critical temperature will absorb carbon from the exhaust which changes the composition of the metal. More carbon makes metal more brittle.. When I was in the Air Force, we had to recondition manifolds (called headers) from recip. aircraft engines. One of the things we had to do was heat them in a heat treating oven at 1860 degrees for one hour to "precipitate" the carbon out of the metal. It would flake off in sheets. This process was called "annealing" for you welders. There are few used manifolds that are not cracked. I have used up all that I had. Some times they can be patched by welding. A warped one can be fixed if it's not too bad by taking a 2x6 about 4 ft. long and stapling a sanding belt, if you can find one long enough, to it and "plane", the manifold (both intake and exhaust) unit back and forth till it's all shiney on it's mating surface. This is a very labor intensive job but it works and is much cheaper than taking it to a machine shop. Might try buying one at "oldGMCtrucks.com" or on E-bay. Good luck.
Frank