M939 series diffs

charlesmann

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To leave the M920 diff thread alone. Has anyone swapped out the R&P on the 5 ton axles to a more OTR ratio? I would think ring & pinions for Mack trucks could be swapped into the rockwell housings. Are my assumptions wrong?
 

simp5782

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To leave the M920 diff thread alone. Has anyone swapped out the R&P on the 5 ton axles to a more OTR ratio? I would think ring & pinions for Mack trucks could be swapped into the rockwell housings. Are my assumptions wrong?
They are out there. They aren't as common as they once were. And not all mack top loaders can swap over. I had a set of 4.88s at one time.

I think @74M35A2 has the firetruck set which may be different

You wouldn't want to go much below a 5.81 being that the transfer case on a 939 is a high OD

Would probably be cheaper to go with a 7spd MD3060 2wd setup takeout/etc and only use 6th as the top end. Etc. They were found behind 8.3s


I'd rather swap an Allison for an Allison than doing the diffs. The 3rd members on the rear are a pain to swap even thru the top on a cargo truck and tractor. Ive done one on the side of the road in about 3hrs for a front tandem

 

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74M35A2

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I do have a set of all three air locking Rockwell 5 ton top loader axles. They have larger axle shafts so are rated at 7 tons. Ratio is 5.8x. Center one has removable power divider. Will sell for decent price. Considering to keep front for self unless a lot of money comes my way. PM if anybody wants them.
 

charlesmann

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@simp5782 it would be much more difficult doing it in a 936 and would consider removing the boggie from the truck to do that kind of maintenance. I would like more speed, but the 936 wt. is already a killer and is a bit gutless for my likings. To make the gears taller, would, i think, make matters worse on being gutless. A manual or even a 10 spd auto would help me out, esp if i had 20,000 #s of load/trailer behind it, compared to now. Once i get a few of my irons in the fire hammered out soon. Im gonna take my spare 936, yard it apart, and see about starting a manual xmsn swap, or maybe a 10spd auto that doesn't require tons of electronic monitoring crap. I HATE electrical, and the less things that can go wrong, the better for me and my wallet once my temper causes more broken parts. Plus do some body work and repaint most of the truck. If i could get the paperwork on it, i would just put a knuckle boom, or a national crane boom on it, build a flat bed for it, and put i ton the road. But when i bought it in a paired sale with the running 936, i was told it was not able to be titled, and was a salvage/parts truck. I wouldn't even know where to start to get the appropriate paperwork to be able to title it as a salvage vehicle.
 

charlesmann

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You wouldn't want to go much below a 5.81 being that the transfer case on a 939 is a high OD
Why would i not want to go much below that? Would the T-case cause the ratio to become even taller, causing the RPMs to drop drastically once in 6th gear?
 

simp5782

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Why would i not want to go much below that? Would the T-case cause the ratio to become even taller, causing the RPMs to drop drastically once in 6th gear?
You wouldn't want to go below 5.81s with 395s tires and stock transmission and transfer case. It won't like low end off the start. You will be split shifting your transfer case to not be soo slow


T1138 Transfer case OD is .73. Which is the same as most 10 and 13spds double od. The first Od is generally a .83/86. So basically you would have a double OD and it wouldn't like running in the 2nd OD on the transmission. Maybe the first.

My 923 with a .83 trans od and t1138 Did well and could run 90mph on 395s. It did well with load on the trailer but the 16speed it was down shifting alot on any hills with the NHC250.

If you decide to go 10/13spd. Stick with stock gears. Swapping ratios would be the last resort being that transmissions are cheaper and easily found. Plus it would give you more power to the ground over the automatic.
 

charlesmann

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You wouldn't want to go below 5.81s with 395s tires and stock transmission and transfer case. It won't like low end off the start. You will be split shifting your transfer case to not be soo slow


T1138 Transfer case OD is .73. Which is the same as most 10 and 13spds double od. The first Od is generally a .83/86. So basically you would have a double OD and it wouldn't like running in the 2nd OD on the transmission. Maybe the first.

My 923 with a .83 trans od and t1138 Did well and could run 90mph on 395s. It did well with load on the trailer but the 16speed it was down shifting alot on any hills with the NHC250.

If you decide to go 10/13spd. Stick with stock gears. Swapping ratios would be the last resort being that transmissions are cheaper and easily found. Plus it would give you more power to the ground over the automatic.
With 14R/20s, that thing is a turd off the start. I havent put my 395s under it. Thanks for the reminder. I need to order 14 orings from that website you linked me to for the rims.
I was planning on breaking all 7 395s down, putting them on my current 5tn rims on the tck. But work decided i need to be ndt level 1 certified, so after a day hm from 3 wks in chile, sa, i have leave for commifornia for a wk, then off for 2 wks, instead of my 3. Hopefully i get to the tire swap this time home, and get it registered and by lay, in their eyes, road legal.
 

simp5782

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With 14R/20s, that thing is a turd off the start. I havent put my 395s under it. Thanks for the reminder. I need to order 14 orings from that website you linked me to for the rims.
I was planning on breaking all 7 395s down, putting them on my current 5tn rims on the tck. But work decided i need to be ndt level 1 certified, so after a day hm from 3 wks in chile, sa, i have leave for commifornia for a wk, then off for 2 wks, instead of my 3. Hopefully i get to the tire swap this time home, and get it registered and by lay, in their eyes, road legal.
It's an A2 truck correct? Do you have the aneroid valve closed? Fuel turned up?
 

charlesmann

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It's an A2 truck correct? Do you have the aneroid valve closed? Fuel turned up?
Yes sir, A2. What the heck is the aneroid? Negative, i have not messed with the fuel.
I need to do an elec/air temp switch to fix the fan from staying on all the time, now that iv got cooling system, according to a temp gun and the gauge, running at or below 200, while driving it on the back roads, or operating the crane.
I just cant find the thread for the horton switch-ology to replace the oem temp plunger thing.
 

simp5782

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Yes sir, A2. What the heck is the aneroid? Negative, i have not messed with the fuel.
I need to do an elec/air temp switch to fix the fan from staying on all the time, now that iv got cooling system, according to a temp gun and the gauge, running at or below 200, while driving it on the back roads, or operating the crane.
I just cant find the thread for the horton switch-ology to replace the oem temp plunger thing.
Solenoid

Depending on which fan you have I can tell you which thermal switch you need. If your fan uses air to engage its a Normally closed type switch. If your fan defaults to on with no air pressure the. Its a Normally open switch.



Aneroid is on the rear of the pump. Under the Allenhead plug. Has a star gear inside. Turn it towards the motor till it stops.

The Aneroid prevents full fuel to the injectors until the turbo boost picks up.

A turbo swap would always help. The h1E is what comes on a 12 and 24v 6bt
 

charlesmann

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Solenoid

Depending on which fan you have I can tell you which thermal switch you need. If your fan uses air to engage its a Normally closed type switch. If your fan defaults to on with no air pressure the. Its a Normally open switch.



Aneroid is on the rear of the pump. Under the Allenhead plug. Has a star gear inside. Turn it towards the motor till it stops.

The Aneroid prevents full fuel to the injectors until the turbo boost picks up.

A turbo swap would always help. The h1E is what comes on a 12 and 24v 6bt
I have a normally closed. 30-45 sec, maybe a min, after the eng is running, the fan kicks on.

Well, i do so happen to have my stock turbo off my 06 dodge 24v cummins after i upgrade to a 80mm turbo.

Ill mess with the fuel, but first drill and tap the manifold for a pyro probe to get a base line, then if not done already, turn the fuel up.
 

simp5782

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I have a normally closed. 30-45 sec, maybe a min, after the eng is running, the fan kicks on.

Well, i do so happen to have my stock turbo off my 06 dodge 24v cummins after i upgrade to a 80mm turbo.

Ill mess with the fuel, but first drill and tap the manifold for a pyro probe to get a base line, then if not done already, turn the fuel up.
Aneroid doesn't mess with fuel. It just allows fuel all the time rather than above 16lbs. I always adjust it first.


You want thermal switch 993607. 205 on. 198 reset

 

charlesmann

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@simp5782 i am striking out on find a solenoid. i found them once, had the link open and accidentally closed the all the web pages, and my computer was acting stupid, and couldn't open last closed pages. i hadn't thought about it till now. you got a link for the air solenoid?
 

simp5782

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@simp5782 i am striking out on find a solenoid. i found them once, had the link open and accidentally closed the all the web pages, and my computer was acting stupid, and couldn't open last closed pages. i hadn't thought about it till now. you got a link for the air solenoid?
I posted it above

 

charlesmann

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If you decide to go 10/13spd. Stick with stock gears. Swapping ratios would be the last resort being that transmissions are cheaper and easily found. Plus it would give you more power to the ground over the automatic.
Im gonna copy some of the manual swap that someone did on an A0/A1 and go with either of those spd ranges. Im fin to pour a slab between my conexs and wall it in, so with the cab, wrecker bed and hood off, i should be able to roll the spare truck in/out with minimal effort, even moving with my farm tractor.

[/QUOTE]
I posted it above

yes, i saw that AFTER, i sent the reply. sorry. i was referencing to a horton brand i had found. I'm trying to order the solenoid now. sorry for having you repeat yourself. i completely brain spaced.

Thanks for all your help sir
 

simp5782

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Im gonna copy some of the manual swap that someone did on an A0/A1 and go with either of those spd ranges. Im fin to pour a slab between my conexs and wall it in, so with the cab, wrecker bed and hood off, i should be able to roll the spare truck in/out with minimal effort, even moving with my farm tractor.
yes, i saw that AFTER, i sent the reply. sorry. i was referencing to a horton brand i had found. I'm trying to order the solenoid now. sorry for having you repeat yourself. i completely brain spaced.

Thanks for all your help sir
Remember A2 trucks the transmission sits almost a foot forward in the frame as an nhc250 truck does. Shifter placement could be fun

Or a really bent shifter.
 

charlesmann

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True, and i think there something to do with a spacer too ad possibly the bellhousing itself.

I wont know till i try it. I did find a 13spd for $2500 locally, which im sure its gone by now. But we have a big truck salvage yard up the road, i could maybe “rent” it, bolt it up with no flywheel/clutch pack and set the cab and see where the shifter will sit.
I think even in that manual xmsn swap thread, someone posted about an adapter plate or something that moved the stick location a lil further to the rear of the xmsn. Maybe between that and slightly bent stick, it might work. But as the says goes, mites are on a chicken’s ass.
I can only try and if the most i get out of it is a new paint job and some much needed body work, ill have a nicer looking truck to swap the data tag in to.
 

simp5782

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True, and i think there something to do with a spacer too ad possibly the bellhousing itself.

I wont know till i try it. I did find a 13spd for $2500 locally, which im sure its gone by now. But we have a big truck salvage yard up the road, i could maybe “rent” it, bolt it up with no flywheel/clutch pack and set the cab and see where the shifter will sit.
I think even in that manual xmsn swap thread, someone posted about an adapter plate or something that moved the stick location a lil further to the rear of the xmsn. Maybe between that and slightly bent stick, it might work. But as the says goes, mites are on a chicken’s ass.
I can only try and if the most i get out of it is a new paint job and some much needed body work, ill have a nicer looking truck to swap the data tag in to.
The allison uses a spacer between it and the bellhousing.
 
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