Grinding issues after rebuild.

[MADMODDER]

Rebuild thread found here.

Post in thread 'Rebuilding a M50D-HD'

Aug 28, 2025

Okay yeah, I'm an idiot, I see it now. IIRC the big outer race on the new 30207X is what I've always seen. I don't know what's going on with the outer race on the original input shaft bearing.
I don't understand how it's possible both those races are supposed to spec the same dimensions when the tapered bearings are exactly the same and the races are so completely different looking.

So is the wider-looking outer race from the last pictures, -X4/80EW, overall larger OD than the skinnier-looking outer race from the first pictures, -X3EW? And the original race is too small for the...

• MADMODDER

During my rebuild I inspected all of my components and got new bearings, seals, shift bushings and shims all from a highly recommended shop in the PSW. Transmission Exchange, I believe it was. The gentleman that helped was amazing. My syncros, gear teeth, forks, dogs, ect, all looked great.

About a month ago, my pedal dropped to the floor and would not return. Master cylinder went bad, no big deal. Repalced it with a new, bench bled, OEM piece and everything was good as gold. For about 2 weeks. Then my trans started grinding going into 3rd. Roughly 10k on the rebuild and I'm not leaking any fluids and no loose hardware.

I did replace my trans mount while I was in there but have yet to replace my engine mounts. I know unevenly worn mounts can cause shifting issues in European cars (I'm a euro tech) but I have not found this to be the same with domestic models. Admittedly, I do have a lack of domestic experience.

I've also found a few online sources saying when they encountered this, they would drain the ATF and replace it with a 50/50 split of Redline lightweight Shockproof gear oil and MTL or MT90.
Is there any truth to this?

I do have a brake flush machine that pushes and pulls fluid. I will be doing that just to see if it helps.

Thanks in advance.

 

[85_Ranger4x4]

They are pretty particular about atf or similar.

 

[MADMODDER]

They are pretty particular about atf or similar.

Thats all I've been told and found through my research on here.

So I thought it was totally backwards. But results are results.

 

[scotts90ranger]

Aftermarket synchros aren't the same as factory no matter where you get them, my friend that does transmissions says he tries to avoid changing them if they aren't a huge problem beforehand... chances are that synchro is a little looser than it should be and isn't speeding the collar up fast enough... if it's just a click and not a hard grind I don't know what you could do about it unless you want to get in there and replace one synchro... The only other thing is if there's some air in the system and it's not disengaging completely...

The shifting isn't tied to any bushings other than the ones at the pivot in the transmission, as long as the shifter is in the middle of the hole in the floor you should be fine there.

 

[MADMODDER]

Its definitely a grinding. But only 3rd.

If it was grinding in all gears, I'd suspect air in my clutch line.

If I have shift ever so gentleman into 3rd it wont grind.
When it does grind. Ill come to a stop and go back and forth between 3rd and 4th, then when I take off again, it wont grind.

If I do syncros, it'll be OEM. But they looks great when I had it all apart.

 

[Shran]

I rebuilt the one in my old '88 and got about 40k out of it before it started grinding into 2nd and 3rd. I did use new synchros. I lived with it for a while and noticed a pattern that it happened more on really hot days. I swapped out the ATF for 10w40 engine oil and the problem disappeared for quite a while, although shifting in really cold weather was pretty stiff. My kid has the truck now and he says it grinds going into reverse if you're not really gentle with it but forward gears are fine. I think the aftermarket synchros wore really fast and the thicker oil has just been a bandaid.

At least you're not dealing with one that pops out of gear and destroys itself in a couple thousand miles. The last one I rebuilt did that several times. Pretty frustrating. I followed the book to a T. The FM145's and 146's that I built around the same time have lived a very happy life, weird!

 

[MADMODDER]

It pretty normal for reverse to grind if you're not gentle with it. I dont recall them having syncros. So you you need to be at a complete stop snd go easy.

But you've had luck with with heavy oil in a manual trans that calls for ATF?
Interesting.
I'm still skeptical so im going to do more digging.

Ya, it doesnt pop out or anything like that.
Today I noticed a slight noise in 4th gear. Not when I shifted into it 4th but while in 4th and accelerating. Tiny chattering noise.
Not clutch chatter. Id move the stick ever so slightly, and it would go away.

Maybe I got bad bushings or didnt seat them properly?

 

[pjtoledo]

in the mean time double clutching 3rd should keep you going for a while.
A.K.A. speed matching

 

[lowspeedpursuit]

I think the aftermarket synchros wore really fast and the thicker oil has just been a bandaid.

I thought this was the general consensus on gear oil/engine oil in an ATF manual. You get less noise because the thicker oil is muting it, but ultimately, the thicker oil can't get everywhere it needs to get and lube everything it needs to lube, so you have more failures over time.

There's MTF with roughly the correct viscosity, but I've never seen any of them on a shelf.

Does someone have the manual for an HD truck? The old 3rd synchros were unique, but the aftermarket shipped all synchros the same. The old manual specifically explains the unique synchro was to prevent griding on upshift. I originally replaced mine, then ultimately had a bit of grinding and put the used one back.

 

[Shran]

It pretty normal for reverse to grind if you're not gentle with it. I dont recall them having syncros. So you you need to be at a complete stop snd go easy.

But you've had luck with with heavy oil in a manual trans that calls for ATF?
Interesting.
I'm still skeptical so im going to do more digging.

Ya, it doesnt pop out or anything like that.
Today I noticed a slight noise in 4th gear. Not when I shifted into it 4th but while in 4th and accelerating. Tiny chattering noise.
Not clutch chatter. Id move the stick ever so slightly, and it would go away.

Maybe I got bad bushings or didnt seat them properly?

All the gears in an M5OD are synchronized including reverse. I have had a couple that you have to be a bit gentle with getting into reverse. Most have no issue. The one I rebuilt just started getting really bad. Seems like no coincidence that I had problems with other gears too.

I have experimented with several oils in these. Sometimes 5w-30 synthetic has worked better than ATF. Sometimes it made no difference. 10w-40 dino oil worked better in this one particular one. None of those are what I'd consider "heavy" oils though.

I thought this was the general consensus on gear oil/engine oil in an ATF manual. You get less noise because the thicker oil is muting it, but ultimately, the thicker oil can't get everywhere it needs to get and lube everything it needs to lube, so you have more failures over time.

There's MTF with roughly the correct viscosity, but I've never seen any of them on a shelf.

Does someone have the manual for an HD truck? The old 3rd synchros were unique, but the aftermarket shipped all synchros the same. The old manual specifically explains the unique synchro was to prevent griding on upshift. I originally replaced mine, then ultimately had a bit of grinding and put the used one back.

O'Reilly has Valvoline Synchromesh on the shelf. $10/quart, it's good stuff. I use that in FM-145/146/132 transmissions and in the AX15 in my Jeep. I have not used it in a M5OD (yet) but there's no reason not to, it's on the thin side and safe for yellow metals. I expect it would be a good oil in a high mileage/worn M5OD.

Main thing is NOT to use GEAR lube - 80w90, etc. Some of it is not safe for yellow metals and is often way too thick to flow like you said. I have had no problem with engine oil.

I remember that part about the 3rd gear synchro - IIRC it even said something like "assemble it with a normal one, drive it, and see if there are problems." Well that's a lot of work. I started reusing that part on my rebuilds if I had a good one on hand just to prevent the problem.

 

[lowspeedpursuit]

Interesting. Everything I remember reading about motor oil was back in Jeepland. The AX-15 wanted GL4, and it could be expensive/hard to find, so people were substituting 10w-30 because the viscosity was supposed to be comparable.

I used to have a spreadsheet of MTF viscosities, but I can't put hands on it now. Pennzoil Synchromesh and Royal Purple are the only ones I ever see around, and both were different from Mercon, but I can't remember by how much and in which direction.

Ravenol was dead-nuts the same as Mercon, but IIRC it's Euro and nobody sells it.