Spicer 6853-B Alternative to the 6453

[MyothersanM1]

I have mulled over the alternatives for replacement transmissions, i.e. 6, 7, 8, 9 or even 10 speed, for my M818, but have run into issues including over length, over width and PTO incompatibility. Some of these problem may be cured, but only after major modification to the drive train. So, I have largely given up on that front.
After some searching, I may have a line on a Spicer 6853-B which is in the same series with the OEM 6453. This series shares many characteristics and parts. The prospect transmission has a different length input shaft, clutch housing and cover/shifting assembly. My plan is to just swap the main shafts and counter shafts from the 6853-B inot my 6453 case retaining the 6453 drive gear.

I am no expert when it comes gear ratios. However, I do see there is difference with the 6853-B having closer on the average ratios, especially at the 3rd to 4th "hole", than the 6453.

Any expert thoughts and opinions are greatly appreciated on this.

6853-B Ratios: 1st...5.08 2nd...3.05 3rd...1.49 4th...1.00 5th... .695 Rev...5.10

6453 Ratios 1st...6.07 2nd...3.40 3rd...1.79 4th...1.00 5th... .78 Rev...6.09

 

[msgjd]

I'm no mathematician but yes you likely know it will increase the gap between 4th and 5th, getting a top speed around 58mph if your stock is 52mph, and you are correct it will reduce the notorious 3rd-4th jump. What I believe you really want to know is if the NHC250 has the power to handle the new 5th high ratio on the flat and whether anyone has done this before and what the performance results are. I'm sure you and the rest of us know it's not going to hold 5th on hills as far as it already does.

Also, if you change the tranny to the 6853-B, you're gonna have a hard time starting out in 2nd high, if it will even do it. But the B's 1st gear ratio should give you a bit more speed/momentum to make the knuckle-buster shift quick enough.

Of course if you're hauling a loaded trailer, the old-school option is starting that B-tranny in the appropriate gear for the situation in low range and move up the rest of them and then switch to high and hit 4th-5th again. The sequence might be a little different with the B-tranny.

Empty, your truck should be able to reach its new top speed on the flat but it's probably gonna take time and plenty of road to get up there, like the old gas 5-tons. You are gonna see your extra speed falling off pretty quick on slight upgrades. In this situation you will probably have to downshift into 4th where you might not have ever had to if you kept the original tranny. Is losing 5th gear torque a good trade for improving the dreaded hole? It's gonna depend on your situation and terrain.

Remember, horsepower is proportionate to the speed you expect to maintain. Whatever you decide to do, it's like specking out a new class-8 CMV for your business or department. Your gearing and HP should match the terrain, load, and speed you expect to operate.

I'm no expert but that is my 2-cents. I think all of us here who still haul or have hauled heavy loads would agree the 3rd-4th hole is the worst thing ever. When in low range with loads on hills, that hole is still almost as bad and just as stupid. It would not surprise me if we learned the 6453 was the cheapest 5sp-OD on military bid, its ratios engineered to be used with an Eaton electric 2-speed rear axle, and the top brass accepted them anyway for use in a driveline they were not originally designed for. This would not surprise me a bit, much like the early AR15's/M16's being fed ammo of a type they were not originally designed to use.

Your idea is solid and certainly would require less-extensive work than modifying the truck to accept a roadranger and keeping the transfer case etc. I am very familiar with 5T roadranger mods (they are not worth the extensive & expensive trouble, imo) but I'm not familiar with your B-tranny swap idea . Am also unfamiliar with your B-tranny, unless it was in some of the 1950's-70's single-axle Loadstars/Fleetstars and Louisvilles I ran long ago

 

[MyothersanM1]

I'm no mathematician but yes you likely know it will increase the gap between 4th and 5th, getting a top speed around 58mph if your stock is 52mph, and you are correct it will reduce the notorious 3rd-4th jump. What I believe you really want to know is if the NHC250 has the power to handle the new 5th high ratio on the flat and whether anyone has done this before and what the performance results are. I'm sure you and the rest of us know it's not going to hold 5th on hills as far as it already does.

Also, if you change the tranny to the 6853-B, you're gonna have a hard time starting out in 2nd high, if it will even do it. But the B's 1st gear ratio should give you a bit more speed/momentum to make the knuckle-buster shift quick enough.

Of course if you're hauling a loaded trailer, the old-school option is starting that B-tranny in the appropriate gear for the situation in low range and move up the rest of them and then switch to high and hit 4th-5th again. The sequence might be a little different with the B-tranny.

Empty, your truck should be able to reach its new top speed on the flat but it's probably gonna take time and plenty of road to get up there, like the old gas 5-tons. You are gonna see your extra speed falling off pretty quick on slight upgrades. In this situation you will probably have to downshift into 4th where you might not have ever had to if you kept the original tranny. Is losing 5th gear torque a good trade for improving the dreaded hole? It's gonna depend on your situation and terrain.

Remember, horsepower is proportionate to the speed you expect to maintain. Whatever you decide to do, it's like specking out a new class-8 CMV for your business or department. Your gearing and HP should match the terrain, load, and speed you expect to operate.

I'm no expert but that is my 2-cents. I think all of us here who still haul or have hauled heavy loads would agree the 3rd-4th hole is the worst thing ever. When in low range with loads on hills, that hole is still almost as bad and just as stupid. It would not surprise me if we learned the 6453 was the cheapest 5sp-OD and its ratios were engineered to be used with an Eaton electric 2-speed rear axle, but the top brass accepted them anyway for use in a driveline they were not originally designed for. This would not surprise me a bit, much like the early M16's being fed ammo of a type they were not originally designed to use.

Your idea is solid and certainly would require less-extensive work than modifying the truck to accept a roadranger and keeping the transfer case etc. I am very familiar with 5T roadranger mods but not with your B-tranny idea . Am not familiar with the B-tranny unless it was in some of the 1950's-70's single-axle Loadstars/Fleetstars and Louisvilles I ran long ago

Thanks for your reply. The sale I have my eye on said the tranny came from a 1979 International S1900. In what capacity (bobtail box, dumptruck, tractor), its not stated.
We have several decent grades out here and I've had to take them all in 4th high, one in third. So, I should be OK there. DIrect is direct.
I do not haul with the truck. It's a toy mostly. I'm looking not for much more speed, but just to close up the ratios and drop back my cruisng RPM's.
How much RPM loss do you think I'd have 4th to OD with the B?

 

[US6x4]

Are the two trannies similar enough that only the 3rd gear could be exchanged and keep the others stock?

 

[MyothersanM1]

Are the two trannies similar enough that only the 3rd gear could be exchanged and keep the others stock?

Yes, you can. The mainshaft and countershaft are common to both. However, The drive gear ratio is different between the two. So, I did the math and just replacing the third gears widens the hole between 2nd and 3rd to around 78%.

 

[msgjd]

Thanks for your reply. The sale I have my eye on said the tranny came from a 1979 International S1900. In what capacity (bobtail box, dumptruck, tractor), its not stated.
We have several decent grades out here and I've had to take them all in 4th high, one in third. So, I should be OK there. DIrect is direct.
I do not haul with the truck. It's a toy mostly. I'm looking not for much more speed, but just to close up the ratios and drop back my cruisng RPM's.
How much RPM loss do you think I'd have 4th to OD with the B?

That S1900 likely had a 2-speed rear axle mated to that B-tranny, no doubt was a good combo... I can't answer your rpm loss question, and what follows won't help you with your particular situation,. However, what I'm about to share may help others out there who, like you, wish to close up that dreaded 3rd-4th hole but are running back roads, not looking for speed. If that is the case, they should consider the spicer #6352 direct-5th tranny found original in the 5-Ton M39-series gas jobs. The 3rd-4th hole is practically eliminated in the 6352 and is a noticeable improvement when installed in a multifuel or diesel 5-ton. But don't expect to go much over 48mph unless the brakes have failed !

Three of my mulitfuel 5-tons came from the military with the 6352, one was an A2-conversion from gas that retained its original direct-5th, and the other two are original A2's which apparently were "de-speeded" during a depot rebuild in the 90's, their shift pattern tags are new unlike the other tags in their cabs. Can't go over 45 anyway on the roads they run, and yes the close shifts are nice on the hills. Again, I know that is not your situation, but there's likely others following this thread that work their trucks and are looking for closer ratios and don't care if they lose speed

 

[AHRENSJIM]

I have mulled over the alternatives for replacement transmissions, i.e. 6, 7, 8, 9 or even 10 speed, for my M818, but have run into issues including over length, over width and PTO incompatibility. Some of these problem may be cured, but only after major modification to the drive train. So, I have largely given up on that front.
After some searching, I may have a line on a Spicer 6853-B which is in the same series with the OEM 6453. This series shares many characteristics and parts. The prospect transmission has a different length input shaft, clutch housing and cover/shifting assembly. My plan is to just swap the main shafts and counter shafts from the 6853-B inot my 6453 case retaining the 6453 drive gear.

I am no expert when it comes gear ratios. However, I do see there is difference with the 6853-B having closer on the average ratios, especially at the 3rd to 4th "hole", than the 6453.

Any expert thoughts and opinions are greatly appreciated on this.

6853-B Ratios: 1st...5.08 2nd...3.05 3rd...1.49 4th...1.00 5th... .695 Rev...5.10

6453 Ratios 1st...6.07 2nd...3.40 3rd...1.79 4th...1.00 5th... .78 Rev...6.09

Hi, I am looking for a #1 SAE transmission bellhousing, top cover and probably input shaft from a 6853 Spicer. Perhaps we could arrange a purchase of the parts you are not going to use? Or anyone else that has these parts? Please email me at AHRENSJIM@HOTMAIL.COM Thank You.

 

[Iceman3005]

Does anyone have a 6853B for sale? DM me. I like that lower overdrive. That would be nice for highway cruising!