Here I go again, only bigger this time. (M51A1 resurrection)

[92ramaro]

After about a decade or so of sitting in my friend's side yard, we finally worked out a deal that landed this M51 in my driveway.

I've got one project to wrap up before I can, but I'm eager to dive into this thing using lessons learned during my first go-around with Military 6x6s.

Main issues:
-Power steering fights against the operator, and the steering wheel only spins maybe 3 turns counter clockwise compared to maybe 1 turn clockwise. It is "bypassed" at the moment but the wheel still only turns as described. (Main reason the truck was parked and eventually traded to me.)

-Air leak in the fuel system that prevents engine from running.
I'm going to start by rebuilding the fuel tank selector valve, replacing the filters, and replacing the supply line to the injection pump.

-No brakes.
Not surprising at all to anyone who has been on this forum for at least 5 minutes.

-Sprag transfer case.
After going through what I did with my Deuce, I plan to install an included 2.5 ton air shift transfer with some adapter plates into the dump truck. I really dont want to waste my time the same way I previously did when I was trying to get a sprag transfer to behave.
(I do still have to do some reading to confirm PTO/gear ratio compatibility beforehand.)

Beyond those, some basic maintenance and a barrel of Kroil should do this truck some good.

 

[92ramaro]

 

[G744]

Working that ruck will just eat up that little 3053 trans.

Why not use one from a 900-series?

Air shift, overdrive high range, same series U-joints.

Somewhat smaller than your original, easier mounting as well.

Really tho, I've given my old M54A1 a lot of hard jobs and many miles on the original transfer without any problems. It operates a world differently than the deuce in reality.

 

[Mrgior31513]

First thing I had to do was change out the two soft lines on mine. I wound up leaving the line that routes behind the injection pump in place til later because it was so stiff I'd have had to cut it out.

 

[92ramaro]

Working that ruck will just eat up that little 3053 trans.

Why not use one from a 900-series?

Air shift, overdrive high range, same series U-joints.

Somewhat smaller than your original, easier mounting as well.

Really tho, I've given my old M54A1 a lot of hard jobs and many miles on the original transfer without any problems. It operates a world differently than the deuce in reality.

Working the Spicer in my M35 will eat it up?
Or did I make it seem like I was installing a 3053 in my 5 ton?
That wasn't my plan if I made it appear so.

Admittedly, I was basing my decision to swap the sprag transfer case of the 5 ton almost entirely on the woes I endured with the sprag that my Deuce came with. That, and a 2.5-ton air shift transfer case was included with the M51.

Installing an air shift transfer in the M35 turned my frustrating liability into a cherished offroad asset literally overnight. So, I was hoping to skip the frustration if I could on my second go-around.

If the 5-ton sprag is leaps and bounds above what the sprag in my 2.5 was, I'll give it a single chance to prove it to me.

I will also look into 900 series transfers in the background for certain.

I was led to initially believe that there weren't any 5-ton air shift transfers that would fit this M51 for some reason.

 

[msgjd]

View attachment 926948

You lucky duck, there's a Mack in your truck!

(had a M52A1 back in the day)

 

[BKubu]

Working the Spicer in my M35 will eat it up?
Or did I make it seem like I was installing a 3053 in my 5 ton?
That wasn't my plan if I made it appear so.

Admittedly, I was basing my decision to swap the sprag transfer case of the 5 ton almost entirely on the woes I endured with the sprag that my Deuce came with. That, and a 2.5-ton air shift transfer case was included with the M51.

Installing an air shift transfer in the M35 turned my frustrating liability into a cherished offroad asset literally overnight. So, I was hoping to skip the frustration if I could on my second go-around.

If the 5-ton sprag is leaps and bounds above what the sprag in my 2.5 was, I'll give it a single chance to prove it to me.

I will also look into 900 series transfers in the background for certain.

I was led to initially believe that there weren't any 5-ton air shift transfers that would fit this M51 for some reason.

I have quite a few M39 and M809 series trucks with sprag t-cases, and I never had a problem. Never allow the truck to roll backwards while in a forward gear, or forward while in reverse. That causes that sprag to bind up and that is how they become damaged. A lot of people don't know how to drive these particular trucks with the sprags (usually civilian agencies who may have owned them) and they destroy them. Then, we get them and the sprags no longer function.

 

[msgjd]

If the 5-ton sprag is leaps and bounds above what the sprag in my 2.5 was, I'll give it a single chance to prove it to me.
I was led to initially believe that there weren't any 5-ton air shift transfers that would fit this M51 for some reason.

1: The sprag that is in that M51 is very tough.. Its transfer case, when maintained and operated properly, can handle more than it can be given... The spline on the driveshaft feeding it will break before you overload / over-torque the sprag.. Out of a half-dozen M39-series owned during the last 25 years in heavy construction and farm service, I've never had a sprag break.. And I have regularly loaded 12 to 14 tons on the M51's on private roads and at sites.. I did lose one transfer case due to years of continuous low-speed operation (upper shaft bearing failure)..

2: However, the above does not mean the M39/M809-series sprag is bombproof.. They do need an operator's ear and feel to notice whether or not it has properly shifted in and out of reverse, the sprag needs to be in the proper position for the direction of travel.. They also can be damaged if air is up and truck rolls backwards with transmission in neutral or any forward gear. . Likewise, same goes for rolling forward with tranny in reverse. The air poppets on top of tranny and the servo cylinder on transfer case have to be in good working order.. Every time you shift in and out of reverse you will need to hear/feel the sprag servo cylinder shifting .. You should hear and feel a "thunk".. When these trucks are being operated often, hear/feel is usually back-burner stuff.. However, when these trucks sit a lot, we all need to closely listen/feel the sprag shift system..

3: Note that I said in #2 above, "...if air is up." This means without air, a properly-functioning servo cylinder will put the sprag in the neutral (unengaged) position.. This is where a front axle air shift valve seen in many 2-1/2T trucks comes in.. One of those in-cab valves can be used in M39/M809-series 5-tons to deactivate the sprag during long-distance on-road operation, or if there's trouble causing the sprag to not shift properly.. However, the sprag system needs exercise, especially that servo cylinder on the transfer case. As G744 related above, the sprags in these 5-tons are a totally different animal than a deuce sprag.. Another difference is they engage/disengage automatically depending on rear wheel slip.. I also concur with him that you can make the M939-series 5T case fit, which is also a different animal of its own

 

[msgjd]

I have quite a few M39 and M809 series trucks with sprag t-cases, and I never had a problem. Never allow the truck to roll backwards while in a forward gear, or forward while in reverse. \

you forgot to include rolling backwards in neutral .. the poppets are activated only by the reverse position and neutral position on the tranny .. By default, neutral affects the forward gears .. (cannot get to any forward gear without going through neutral first)

 

[92ramaro]

1: The sprag that is in that M51 is very tough.. Its transfer case, when maintained and operated properly, can handle more than it can be given... The spline on the driveshaft feeding it will break before you overload / over-torque the sprag.. Out of a half-dozen M39-series owned during the last 25 years in heavy construction and farm service, I've never had a sprag break.. And I have regularly loaded 12 to 14 tons on the M51's on private roads and at sites.. I did lose one transfer case due to years of continuous low-speed operation (upper shaft bearing failure)..

Breaking the transfer case isn't really the concern I have. I am mostly worried about it actually engaging when I need it. It was 50/50 on function with my M35, and it led to some very time-consuming recoveries until I installed the air shift (and a factory winch.)

Since the majority of replies are speaking positively about the 5 ton sprag, I'll leave it be for now.

I still need to get it running properly before I get too wrapped up in a possibly unnecessary major component replacement.

 

[US6x4]

It's often advised to drain the t-case, fill it with diesel, and go for a 10+ minute or so drive with it to clean the sprag & t-case of built up junk, and then drain & replace with gear oil.

 

[jeffhuey1n]

Subscribed!

 

[92ramaro]

I found some time last weekend to get the fuel tank selection valve un-mounted and disassembled.
Typical for my region, a cut-off wheel on a 1/4 grinder ended up being the only reasonable way to get the mounting bolts for the toolbox and step removed.

All of the hoses connected to the valve are in surprisingly good shape. The O-rings inside of the valve itself are pretty fresh as well. Which is a good thing because the ones I had planned to use were incorrect entirely.

Removed O-rings, cleaned out some gunk in the valve that had unseated them, lubed and reinstalled them, and the truck fired right up after I primed the fuel system with an air siphon at the injection pump inlet.

The engine seems very well behaved for the short time I ran it. The dump bed also works as designed as far as I can tell.

Due to the way the power steering gear is bypassed there isn't any fluid in the reservoir, so I only ran the truck long enough to park it out back for the time being.

Once I have the majority of the parts on the checklist in my possession, I'll be pulling the truck into the shop for tear down:

-Need to get the front winch removed for repair or replacement (seriously, stop using bolts in place of shear pins. You know who you are.)

-Repair or replace power steering gear.

-Pulling all of the hubs off for inspection/service/repair of brake and wheel bearing components. Knuckle boots included.

-Drain and flush the ancient diesel from the fuel tanks.

-Replacing dried out intake/coolant/hydraulic hoses etc.

-LOF on entire truck. From the front winch to the rear tailgate. Probably doing the above-mentioned transfer case diesel flush as well.

I'm sure the list will grow once I dig in. Outside of meager endeavors like this tank selection valve, I'm usually good at posting photos to document progress, so stay tuned gentlemen.