Why no love for the Mile Marker/Nash OD?

[Triumph215]

I see lots of 'I hate that thing' on here so I'm wondering, why is it so bad?
On paper the 16% OD seems an easy choice. I just thought I'd like to have a few less rpm's on the highway but you guys have creeped me out about the OD unit.
My Jeep came with the 'original' Quadatrac in a box and a replacement in the truck. I believe them because I have lots of old papers about it that came along with the truck. I replaced the worn chain and renewed the seals and am about ready to put it in there and some of you have helped me with my questions about it, thank you. But I don't want to go through the trouble of switching it out just to find I hate this thing too.
(The rest of the story is the full time unit in there binds badly on dry pavement and I need to figure out why that's happening. I don't see a way around opening it up to look inside, if you have an idea please let me know.)
Just wondered...

[tgreese]

What's the problem? Does the case you're putting in have the MM OD kit? If you have a 1:1 full-time QT ready to go in, I'd say bolt it up and run it.

Coming from the CJ world, these Jeeps already have very tall axle gears. If you are only using your Jeep on the highway, making your final drive ratio 16% higher may seem appealing. I kinda doubt you'll see much difference in fuel economy with the overdrive sprocket and chain installed. You will have lower engine noise due to the taller gearing. If the engine noise has been affecting your average speed downward, it seems likely you will drive faster with the overdrive sprocket in place. Since the major determinant of fuel economy is wind resistance, and since wind resistance goes up like the square of velocity, I'd predict you'll get worse economy with the overdrive gears.

Additionally, it's a part-time kit that requires a different replacement chain than the factory sprocket. Ramsey may be supplying quality chains now, but even with a quality chain, you'll need a new chain every 50K-60K-75K miles? The converted cases are a smaller fraction of the market than not, and I'd expect market conditions to affect availability more.

Other than as a low-miles-per-annum hobby car, I don't know of any perfect solutions for these cars. If you are going to drive it a lot, a NP241C swap might make sense, but the rear offset is wrong for your existing axle. When these cars were a decade old, there was a continuing gas crisis, and the part-time kit was sold as a more fuel-efficient upgrade. Plus there was a segment of owners that let their worn-out chain go so long that they had catastrophic failures. Bad press supported the part-time conversion. Given all this, to me, the OD kit seems like an offering of highly questionable merit, even in the day. You can't squeeze blood from a stone.

[fulsizjeep]

I am not a big fan of the part time kit, either one.

The contact surface on the slider/coupler when in 4wd is a small area that takes wear. It is the weakest link.


The 16% OD. I have no use at all for that. Most of the 74-79 FSJs either had 3.07 or 3.54 axle gears. I'd say a majority of them are 3.07. I tried an OD QT on a Wagoneer with 3.07 gears and had to use second gear a lot on the mountain highways in Colorado. There was no measurable difference in gas mileage. Maybe there is a better experience with 3.54 gears but I'm not going to try it.

I don't hate the PT kit. I just have no use for it. Experience varies.

[Stuka]

The 16% OD is 'ok' if you run stock small tires and drive primarily on the HWY/FWY. But soon as you put larger tires on it, you will wish you had that 16% back because the 16% effects ALL gears, including low range. Its not like having an over drive transmission that just has an extra gear.

Plus the strength issues that Flint mentioned above.

[letank]

When I had the 16% OD installed on the 74, I never saw any mpg improvements... going from 3.07 to 2.6x (calculated) was not good for spirited driving in the sierras. The 85 has the 3.31 gearing -not QT of course- but same tires, and I can almost avoid the low range in some cases. I drove a FSJ with the 3.54 a few times... and it is really the best combo.

As Flint's diagram laid out the coupler pictures, I remember Frank's solution was to weld extensions, where the red dots are, because the coupler does not engage fully to the QT shaft -I cannot find the pict, Frank's site is 404 now-

[coolhand75]

Hi
I’m going to chime in. 75 j10 with mm kit be in there for about 30 years. Part of me wishes it was still full time the other part no(wear and tear on front end parts) or so I’ve been told. My truck came with 373 gears someone along time be fore me blew up the rear gears and put 354 in them. I put 411 gears in never been happier. With the 33/1250 tires and the small overdrive I can go 65 and my rpms are only 2000. By my calculations I’m close to the factory acceleration. You can figure it out for your self on many websites oem tire size 29 with 373. I will say after I installed the new gear set I put that skinny pedal to the floor and the old truck barked both rear tires(33/1250) when it shifted in to second gear. It’s never done that before. I’m pleased with my choice. But that’s it. My choice. I’ve asked a lot of people before I did it and about made my self sick with worry trying to make the right choice.


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