HI folks, new member to this forum but I've sourced info from these threads for years now.
I just recently picked up a 1977 f600 pretty sure it's got a 391, standard 4spd with 2 spd Eaton rear.

now I'm a bit of a diesel junky so my longtime goal has been to swap an international DT446 with a 5 or 6 spd behind it.

so as I'm looking into transmission options I'm running into a bit of an issue with the gear ratios.

with diesels running at a far lower RPM I would need a pretty crazy overdrive to hit 65-70 highway speed. With stock rear and tires I would need an overdrive around .5:1
probimatic because most medium duty truck tansmissions only have a overdrive of like .75 at highest.
I'd really like to keep the stock 2spd rear if possible but at 6.33 in high range the ratio is just too low.

my thought and question I'm looking for feedback on is how possible or feasible would it be to simply replace the pinion gear for one with a tooth or two less. I'm going to be gaining plenty of extra torque so I don't anticipate that being an issue, with my current understanding of a 2 speed rear I believe it would just raise both high and low equally which I'm fine with.
just curious how possible this operation would be and/or if anyone has done something similar.

Thanks, Josh.

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Welcome to FTE.
I moved you to Large Truck as someone here might be able to help you with an axle solution.

As to changing the pinion, I don't think that will work. Even in a regular differential, you never change just one gear--they come as a set.

There are two speeds with slightly better ratios but that would be a bit of a chase. Finding a gear set for a '77 is probably just a big a chase.

I'll do some looking and get back to you tomorrow.

 

I don't proclaim to be any kind of expert on the subject matter of things related to medium duty trucks. But I will say a DT466 with just a straight five or six speed manual behind it, will more than sufficiently meet your needs without further gear reduction. But you knew that already. As already mentioned above, ring and pinion sets are ALWAYS replaced as sets (even when replacing old with new of identical ratio). That said, you will be much farther ahead simply swapping out the entire rear axle assembly with just a simple single speed unit of favorable ratio (I'm going to guess a ballpark between 4.88 to 5.13???) to work with whatever transmission you have, to keep engine RPMs in the appropriate range needed to achieve the desired highway speeds you seek.

 

The MPC shows an Eaton model 16244 two speed with 17,500 lbs capacity and ratios of 4.88/6.78 for model year '78, axle code E4.

For model years '73-'79, Eaton 16244 two speed is listed as 17,500 lbs capacity and ratios of 5.57/7.75, axle code E1.

Model years '78-'79 shows Eaton model 17121 single speed axle with 18,500 lbs capacity and ratio 5.57, axle code EQ.

What is your axle code? We can look for the same model but different ratio.

Searching by axle model will get results but usually a phone call or two would be required to confirm what you have found.
https://truckpartsinventory.com/axle...on-17121-parts

Later model axles with "current" brakes or even air brakes would also be a way to go.

 

I did a Cummins diesel swap in my 56F 600 I used a fuller FS 5306 non-overdrive transmission I replaced the two speed with a Dana 80 with a 331 ratio out of a 99 freightliner M2 45 … my 9R 22.5 tires on the six hole bud wheels bolt

dana 80 with 3.31 ratio out of a 99 freightliner mt 45 (step van/tool truck)

Baja Cantina, Carmel Valley Ca.
right up… I’m turning 1800 RPM at 62 mph with no overdrive.

 

I agree with the rest up here. It will be much cheaper and easier for you to get an entire different rear axle than to start swapping pinion and ring gears. It will be harder to find a lower ratio 2 speed rear, but you can always try.

 

Hi Josh,

I see this thread is a couple years old, but I ran across it as I have done a similar swap a couple times... currently doing this again for the third time and looking for a different approach.

As far as I have seen there is only few options. One is hypothetical, the others doable.
1. Replace Rear Axle Assembly
2. Buy an entire Class 8 Day Cab Semi Truck with single rear axle.
3. Have a set of custom made machined Ring & Pinion Gears made for original 2 speed axle. (Hypothetical)

The rear axle assembly you want to use with decent ratio would be a Meritor/Rockwell Rear Axle RS23160 46 Spline 2.50 Ratio Rear Differential, with Housing/Carrier Casting # 3200-N-1704.

Then you will need to convert this rear axle the hydraulic brake option for these axle types... you will also be using the typical updated wheel/tire setup.

If you go with rear axle swap you might as well swap front straight axle as well so you get the latest Tire/Wheels (22.5) and then convert truck to an Air Brake System.

Your looking at 2k for just rear axle, so it may make sense to find an entire semi truck day cab single rear axle setup and you have everything to do job.

For some reason wrecking yards prefer to keep thier parts so they have lots and lots to stack up, collecting dust, and rub with diapers, thousands of rear axle and diff assemblies taking up valuble space, as Im guessing they must sell thousands of housings, and diffs per month at their astronomical prices, cause diffs must break in every truck every month?

You can purchase an entire semi truck, 2012s for around 5k all day long, then you can part it out and pay for the entire cost of your project....You also get the needed radiator for the engine swap.