How can I keep the front axle from engaging on my M818?

[Blythewoodjoe]

I have looked and can't find the info I need. First off, I am broke. If I could afford to buy nice new tires for my truck I wouldn't be worried about this.

I have old tires and can't find tires that are not 20 years old in surplus tires. At least not enough for all the axles. So here's the questions:

First off, how much tolerance is there in the sprag unit that engages the front axle on the M818? In other words, how much difference can there be in the diameter of the tires between the front and rear axles? I looked at 4 395/85's today and two looked pretty good and two were worn. I bet there is at least 1.5 inches difference in the diameter. Now imagine if I end up with two for the front that are good and safe but are a little more than that out of size. Is there a standard for this?

Second, is there any way to prevent the axle from engaging? I am under the impression it takes air pressure to engage the transfer case. Not sure. At my age I have so much info in my head it just all runs together. Can a switch be put on the air source to block the air and prevent the sprag from working with out damaging it?

In a nut shell I need to use the truck to move some equipment to make money but I don't have the money to buy all new tires. I have researched commercial tires and I think I can get two bud wheels and used decent tires for less than I can get two 14.00 x 20's for the front axle. I found a source for decent used truck tires about 60 miles away so that has provided a few more options. I am considering getting all 22.5" tires for the truck for road use. This also would be nice if I have a problem out on the road. Having one of these 14.00's go down on the road would be bad. Having something a commercial tire repair shop can fix is good. I would hate to buy a $700 tire on the road, but finding a surplus tire in a bind would be worse.

As a side note, the used tire place I found today had a stack of Michelin XL's that looked nice. They where all over 20 years old. I have cheated death long enough and I don't plan to run tires over 10 years old anymore. We'll see how that works out. If I crash the truck that would be bad, but throwing a $40,000 excavator in the woods would be worse.


Anyway, thanks for the help,
Joe

 

[gimpyrobb]

Why not just pull the front DS or put some de-splined hubs on?

If memory serves, the rears have to spin 7% faster than the fronts to have the spraqe "do its thing".

 

[Blythewoodjoe]

Why not just pull the front DS or put some de-splined hubs on?

Oh man, that would suck if I got in some mud on the job site. That's the last option gimp.

 

[deuceman51]

You can put in the air switch to cut off the air supply to the transmission. This was an MWO performed by some units. It was covered once on here before with pictures, but I cannot remember what the post is titled.

 

[jasonjc]

My understanding is that the air on the 5 ton shifts the sprag from forward to revses. But I also seem to remember talk on here about putting an air switch to lock it out. Can't remember how that works thought.

 

[clinto]

I believe there is a military MWO that installs an air shift switch in the dash, similar to a deuce. It cuts the air to the transmission air solenoids and this more or less puts the transfer in neutral. I think I'm describing this correctly.

A search in the 5 ton section for something like "MWO transfer" will probably yield some good results.

 

[gimpyrobb]

My understanding is that the air on the 5 ton shifts the sprag from forward to revses. But I also seem to remember talk on here about putting an air switch to lock it out. Can't remember how that works thought.

In other posts, I heard this was not right, its either forward or reverse, no in-between.

I guess someone will have to pull the air cyl off their T-case and see if there is a "neutral" position to verify.

 

[Floridianson]

Yea I never like the mod but owell. Yes you go under the truck and find the air line that feeds the poppet valves on the top side of the Transmission. Where the hard line turns into rubber by the frame you disconnect from there and just splice in you line that you will run to where ever you place you valve/switch to cut off the air. Then reconnect to poppets line.

 

[Blythewoodjoe]

OK, I spent several hours reading up on other threads about how this works. Did not find a thread with pictures and so forth, but I have drawn some conclusions, correct me if I am wrong.

First off, it appears if you have smaller tires on the front, it will not be a problem anyway. If the sprag is engaged when the rear drive shaft turns faster than the front, having smaller tires on the front will not make it engage more than normal. So tires that are an inch or so shorter on the front should work, right?

Second, killing the air on the rubber hose on the transmission line and making sure the dead part is vented would keep the sprag in "neutral", right?

If these two assumptions are correct I should be able to put smaller diameter tire on the front if I can't find a complete set of matching tires. I am trying to find some 22.5 x 12.25 bud wheels to mount 425/65r22.5 tires on. If I can't get enough of the wide bud wheels I will go with a set of 12R22.5 or 11R22.5. This would let me put on the larger, wider wheels and tires on the back and keep the 12R22.5 or 11R22.5 on the front until I can find a complete set of wide wheels. This truck is bobbed so I only have to find 4 anyway. I had thought it would be nice to put all 4 of the narrow tires on the rear and the floatation tire on the front but that will not work with my conclusions unless I kill the air line. I will just have to see how my wheel hunt goes.

The guy in Monroe, NC has any size truck tire available for around $200 each. Not as cheap as surplus sometimes but these tires are not old, which is my biggest issue. They can be fixed on the side of the road a lot easier too.

One last thought, has anyone ever drilled our there hubs and put in 1" studs and mounted up hub piloted wheels on one of these trucks? Aluminum wheels sure would look nice and you can find the hub piloted wheels everywhere.

Joe

 

[Floridianson]

[QUOTE=Blythewoodjoe
Second, killing the air on the rubber hose on the transmission line and making sure the dead part is vented would keep the sprag in "neutral", right?

You can remove the floor pan and disconnect right at the poppets or at the frame. I said the frame so there is no need to fight with the floor pan. You are just makeing a loop in the line and adding the valve so you can cut the air. You can place the valve where ever you want dash or floor. Also since you will be working there check all eight bolts on the poppet valve for tight.

 

[Blythewoodjoe]

One more question on this subject. If the sprag is engaged while moving the truck around, when will it disengage?
Another question: Does the sprag stay engaged in all forward gears? I often spin my rear tires going up my driveway with a light rear ended truck like a bobbed deuce or 5 ton. If the sprag engages in my driveway will it stay engaged after I pull out on to the road?

I plan to put in a air line to relieve the pressure on the transmission at some point. I plan to put tires about an inch taller on the front this weekend and according to my research that is not enough to engage the sprag, but I really don't know.

Joe

 

[73m819]

One more question on this subject. If the sprag is engaged while moving the truck around, when will it disengage?
Another question: Does the sprag stay engaged in all forward gears? I often spin my rear tires going up my driveway with a light rear ended truck like a bobbed deuce or 5 ton. If the sprag engages in my driveway will it stay engaged after I pull out on to the road?

I plan to put in a air line to relieve the pressure on the transmission at some point. I plan to put tires about an inch taller on the front this weekend and according to my research that is not enough to engage the sprag, but I really don't know.

Joe

The front will stay engaged till the load is taken off it, if you spin out and the front engages, just depress the clutch peddle once you are on good ground or just shift to the next gear, this will disengage the sprag.