M5OD technical question - 30 vs 36 tooth 5th/reverse cluster


Shran

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I have four M5ODs torn down right now - an '88 4 cylinder, two 93/94 4.0's, and one mid 90's 3.0. As you can probably guess, I am doing the lower gear mod on the two 4.0 cases.

I have run into a slight hangup with the '88 trans. It has a 36 tooth 5th/reverse synchro ring. The synchro itself is junk, along with the slider, and other parts... just very worn overall. The other three transmissions have 30 tooth synchro rings and at least two 5th/reverse clusters are in good shape.

Can I simply replace the whole 36t cluster with a newer 30t cluster? This basically just determines which bearing/synchro kit I buy, plus I would rather use good parts on hand if I can instead of replacing some.

It appears that it will work fine, tooth count is the same on everything else, I'm just hoping someone else has run into this and can confirm it for sure.
 


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I did that on my rebuild/low gear 4.0 trans build. The only issue I seem to have, and I don't think it's from that, is that it pops out of Reverse and I have to hold the shifter in place.
 

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From what I remember that was a '92 change when they changed the slave cylinder, but I have heard the parts are interchangeable. It's been a decade since I rebuilt my trans though...
 

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Cool. Think I'm gonna go ahead and try it. The 30 tooth cluster looks like it's actually kind of an upgrade... there are fewer teeth but they're all much larger. I remember that from the last one of these I rebuilt, I used the 36t cluster and had to replace most of it for the same problem.

These things are remarkably durable - carnage is as follows for those curious:

2.3 trans - leaked a lot but worked fine, input shaft teeth are very worn, chunks missing out of every piece of 5th/reverse cluster
4.0 #1 - no teeth left on input shaft and large countershaft gear was pretty shredded, large chunks throughout, still drove in 4th gear
4.0 #2 - ball bearings missing from rear countershaft bearing, had almost no fluid in it, guy said it drove fine in every gear except it would get stuck in 4th. Chunks missing from 5th gear
4.0 #3 - not using this one, but input shaft snapped off inside its bearing. Made no noise while running, but didn't move obviously
3.0 trans seems to be in good shape, low quality rebuild, bearing retainer nuts have been reused more than once
 

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I just had my M50D transmission rebuilt. It was working very well but with 245K miles on it, I wanted to have everything looked at. All the bearings and seals were replaced. They said everything but the 5th/reverse cluster were in very good condition. They asked if it was popping out of 5th, but I was having zero problems with it. Of course, I authorized replacing the 5th/reverse cluster. Is the 5th/reverse cluster a weakness of the M50D transmission?

I asked for all the replaced parts and the gears that were replaced look good to me, but I don't know what to look for to determine if they are bad. The transmission shop that did the rebuild has a good reputation, so I didn't question what they did. The transmission still works very well like it did before. I am hoping the work done will extend its life considerably.
 

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I don't think it's really a weak spot, it just happens to be one of two points of failure I've seen more often... the other being the input shaft. I'd attribute it to abuse/neglect or just simple lack of maintenance. They are fairly strong if you keep the oil full, change it once in a while, don't beat on it, replace shifter bushings as needed, and don't rest your hand on the shifter while driving (goes for just about any manual.)
 

scotts90ranger

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Yeah, I had to replace something in the 5/R area when I rebuilt mine a decade ago, I was on a budget so I didn't upgrade to the lower tooth count stuff... The aftermarket synchro's aren't as nice as factory and it's making some noise now in 5th on the highway... I need to get a spare to build up and keep on hand...
 

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My '91 4.0 trans ate the input shaft cluster also & I wouldn't be surprised if my family old '91 Explorer when it blew its m5od back in the mid 2000 also blew its input shaft cluster lol. They both failed under the same circumstances; moderate (31's with 3.73s extended cab ranger & probably 28s with 3.27s base 4dr explorer) load up a hill doing ~70mph on the highway in 5th. However the factory m5ods behind my 2.3 turbos making ~275hp never ever had an issue & I beat the crap out of them lmao.
 

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I wish I knew what caused the impending input failure on my 2.3 trans. It looks like the teeth have cracks in them that run the width of each tooth. I suspect lack of oil. It was leaking like a 6" puddle every day, I was filling it once a month, and it got hot enough on several occasions that it was hard to shift.
 

Shran

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After comparing the clusters and related parts, I went ahead and attempted to use the 30t cluster. It appears to work perfectly. I will note that the 5th/rev shift fork is also different so you must use the one from the 30t cluster, the 36t shift fork's pads are thinner by roughly 1/8".

I pulled the trans out of my Explorer and was shocked to find that I have a lot of slop in the input bearing. I was hoping to have a usable trans for a spare but it appears that I'll be going through that one too. It wasn't making any noise or anything, shifted nice, I would have left it in there indefinitely had I not wanted to do the lower 1-2-3 gear swap (and it was leaking enough to irritate me.) Fortunately I would imagine a bearing/seal kit should be all I need, guts are most likely in good shape.
 


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