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Old 06-16-2015, 11:46 AM   #1
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Allison MT643 Bellhousing Cracks

Hello,
I have a 1991 Dynasty 36ft class A pusher powered by a 6CTA8.3 250hp thru a MT643 which I purchased in 2009. No power mods and it runs, drives & shifts great. Last year I noticed some cracks around some of the bellhousing bolts. I hate to admit this but I completely forgot about them until I was lubing the driveshaft this spring. Nothing has changed and we have used it several times even towing our toad. There is no trans support, only two side engine mounts and one front (rear) engine mount. Seems like alot of unsupported weight hanging there. The lower bellhousing bolts are fine, for all I know these cracks might have been there since I bought it!
Anybody heard of this before?
Thanks, Ken
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Old 06-17-2015, 08:38 AM   #2
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Not being a transmission guy, no I have not heard this before but, I certainly would get it looked at by a Allison shop. I can't see it being overly expensive to have a replacement bellhousing installed.
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Old 06-20-2015, 10:48 PM   #3
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Thanks for the reply Martin, I have a feeling these are nothing new and the only reason I noticed them was because I replaced the intake hose which exposed the top of the trans & engine block as my air filer housing is at the rear of the Cummins mounted on the front firewall next to the hydraulic reservoir. Nothing surprises me anymore!
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Old 06-22-2015, 11:29 AM   #4
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The MT643 did not require a rear mount. It's acceptable to be overhung as you described. The only time I've heard of cracking around the converter housing to engine flywheel housing mounting bolts is if they were left loose at some time in the past. I would pull one out and see if the hole is egg-shaped. That's a sure sign the bolts were loose at some time.
Keep in mind that housing is a cast housing and they often show casting lines that are mistaken for cracks.
One thing that can possibly cause cracks in the housing is driveline irregularities. Make sure the u-joints are tight and lubricated properly and the shaft is properly phased. Just a suggestion.
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Old 07-01-2015, 06:36 PM   #5
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I agree with dbarton291

I would check for loose bolts and see if there is any deformation in the holes. I would check the torque on the bolts, if all of them are tight then you should be OK.

I would think that if there was a problem with the drive shaft or U-joints you would know unless it was with the previous owner.

One thing you could try is to take some JB Weld and put it on the outside of the holes that are cracked and then monitor it to see if it cracks, this would at least tell you if the bell housing is flexing.

I worked in the mining industry, we did have some luck in a case like this of welding the holes up and retapping.
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Old 07-01-2015, 06:40 PM   #6
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Quote:
Originally Posted by dbarton291 View Post
Keep in mind that housing is a cast housing and they often show casting lines that are mistaken for cracks.
That's what I was thinking too.
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Old 07-02-2015, 10:20 PM   #7
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Quote:
Originally Posted by jacwjames View Post
One thing you could try is to take some JB Weld and put it on the outside of the holes that are cracked and then monitor it to see if it cracks, this would at least tell you if the bell housing is flexing.
I really appreciate all of the input, especially the JB Weld idea. I removed some bolts, checked & found no "egg shaped" holes. Went on a very short trip last weekend and had no trouble. Next week we are going on another semi-short trip so I will try the JB Weld trick and check it when return to see if there is any movement. This time we will be towing my wifes PT Cruiser and even though we can't even feel it back there it will put additional stress on the bellhousing bolts and give me the JB Weld a good test. I guess I'll know more by the 13th of July!

Have a Safe & Happy 4th of July Holiday!
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Old 07-05-2015, 01:02 PM   #8
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Bellhousing cracks

I'm sure you've closely examined these cracks, but is it possible that they are "wrinkles" in the bell housing casting?
It's not uncommon to see there wrinkles in larger aluminum castings. They're caused but the liquid aluminum rapidly cooling on the surface as the foundry pour solidifies.
There can be surface wrinkling "inies" or 'outies"
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Old 08-11-2015, 07:37 PM   #9
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Quote:
Originally Posted by dbarton291 View Post
The MT643 did not require a rear mount. It's acceptable to be overhung as you described. The only time I've heard of cracking around the converter housing to engine flywheel housing mounting bolts is if they were left loose at some time in the past. I would pull one out and see if the hole is egg-shaped. That's a sure sign the bolts were loose at some time.
Keep in mind that housing is a cast housing and they often show casting lines that are mistaken for cracks.
One thing that can possibly cause cracks in the housing is driveline irregularities. Make sure the u-joints are tight and lubricated properly and the shaft is properly phased. Just a suggestion.
Yes im a trans tech for 30 years many people mistake casting marks for cracks,ive even been mistaken.See if something thin will go in the crack.If it was cracked on several bolts i think it would keep breaking (getting worse) since there is no mount. Just a thought.
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Old 08-12-2015, 11:39 PM   #10
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Quote:
Yes im a trans tech for 30 years many people mistake casting marks for cracks,ive even been mistaken.See if something thin will go in the crack.If it was cracked on several bolts i think it would keep breaking (getting worse) since there is no mount. Just a thought
Since I posted this we have taken my coach on several weekend trips which includes many very steep Massachusetts hills while towing my wife's 3500lb PT Cruiser and had no trouble! Had these been true "cracks" I'm sure that trans would have suffered a catastrophic failure! So I'm hoping I can put this to rest! (and keep my fingers crossed)
Thanks to all who helped,
Ken
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