8.25 gears in a 8.75

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dusterbnb

dusterbnb

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Ok, I know this is an odd ball question, so bare with me. I have a project that I am starting and need as high of a rearend gear as I can get, but an 8.75 is limited to a 2.76, and I need higher than that, like the 2.24 that came in the 8.25.

Ok, the reasoning is that I am putting a 4bt in my Dart, with an NV4500 behind it. But, to keep the RPMs down in the teens as the 4bt is a diesel and does not spin high RPMs and I am doing this for fuel mileage, I need a good overdrive, with out going to a TR6060 that is built strong enough to handle the 4bt and costing $4,500 just for the trans.

The question is, has anyone heard of people putting 8.25 into an 8.75 since I need the 2.24 gear that comes in the 8.25. It has been done and that is what gave me the idea to do it instead of the expensive tranny and would let me be in gear for longer. But, the problem is I can get ahold of the guy that did it and I have no idea what was done for his machinist to get the gears in the 8.75 housing.

If someone has any idea how this is done or how you would go about it, please let me know.

Thanks.
 
Seeing as how the 2 rearends are of completely different designs, I am sure the machinist had to make or modify at least a dozen things. Good luck.
 
I realize the 8.25 is not quite as strong as the 8.75 but why not find an A Body 8.25 with the gears you need? Or am I missing something here?
 
Just use the 8.250 it is plenty strong.
 
Interesting thought, but I can't see it being productive.

At a guess, I would bet he had to find bearings for the 8.25 carrier that made it fit in the 8.75 pumpkin. Then he did the same thing to fit the pinion. Lastly, he had to have custom axles built that had the right splines and length. Just ideas off the top of my head, no clue if it is even possible.

In the end, since it probably used an 8.25 axle spline and diameter, I would find it hard to believe it was a stronger assembly than the original 8.25 would have been. Even if you stepped up to the later 29 spline carrier, pretty sure the axles are smaller than the original 8.75 axles.

I like interesting things like this, but I can't see it being useful. If you are worried about strength, find the later 8.25 housing that had the bigger axles and ring gear and don't tow a 10,000 trailer ( ;-) ). Not sure they came in a 2.24 ratio, though.
 
BlueDream said:
x2.

I say sell the 8 3/4 and use the money to buy an 8 1/4 with the right gears. Then use the leftover money for the new driveshaft.
Click to expand...

Driveshaft and all the rear discs parts for the 8.25 too, and still have cash left over.

The local pick a part quoted me right around 250 for the entire assembly sure grip or not (didn't change the price)
 
Yes that is what I was wonder was how strong the 8.25 was. I know my 8.75 holds up fine to my 408 in my duster on the street, but since the 4bt has a bunch of torque already even with out any modes yet, I was not sure how strong the 8.25 was. Now granted this will no be a track or even a tow car, so other than playing around on the road, it should not see to much abuse.

That was another thing I didn't know about was the hard ware like brakes and what not. If they were readily available and I was wanting disk, but that is not a must.

I do agree that it would be just cheaper to just use the 8.25. I was just not sure of the strength. I guy that guy really wanted it to look like a factory car with the 8.75.

Can you interchange the diff between all the years of those in the 8.25. I ask, because I read the higher gears use a different carrier than the lower gear ones. So, would I have to get a certain year housing to use the 2.24 I'm wanting? Like if I find them in a diplomat and want to use an A-body housing?

Thanks for all your help!
 
I just found out that they do. I'm going to have to do some more research on 8.25s now. Than you all for your help.
 
Yeah I was thinking about the diff cover. I can make it look like a mini Dana :cheers:
 
dusterbnb said:
I do agree that it would be just cheaper to just use the 8.25. I was just not sure of the strength. I guy that guy really wanted it to look like a factory car with the 8.75.
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The gears in a 8-3/4 are 1/2" larger diameter and the differential is beefier than the parts in a 8-1/4. If you put the 8-1/4 parts in an 8-3/4 carrier (which would be extremely difficult and costly as already pointed out) you would weaken it to the level of the 8-1/4 so the ONLY thing you'd gain is it'd look like an 8-3/4 rear
 
fishy68 said:
The gears in a 8-3/4 are 1/2" larger diameter and the differential is beefier than the parts in a 8-1/4. If you put the 8-1/4 parts in an 8-3/4 carrier (which would be extremely difficult and costly as already pointed out) you would weaken it to the level of the 8-1/4 so the ONLY thing you'd gain is it'd look like an 8-3/4 rear
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Well I have no clue how he did it, but I was thinking ( which may be way off ), but that the 8.25 gears were some how matted to the 8.75s diff so that the axles would still be 8.75 axles. All in all, I have never been in a 8.25 of had one for that matter, so I'm only guessing . You are correct though. Even if they machined it the way I just said. It is still only as strong as the 8.25 pinion.
 
dusterbnb said:
Does the 8.25 have 5x4.5 lug pattern?
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It depends! 8.25" rear ends were used in Dakotas and other trucks which would have a different number of lugs and/or pattern.

There may be some other slight differences that may affect some parts interchangability between 8.25 rears, but I don't have the details.
 
mderoy340 said:
Read this for info.
http://www.hemmings.com/mus/stories/2007/01/01/hmn_feature21.html
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Yeah I seen that. It make worry that if it is correct. I will old have between 80-81 to find one with the 2.24 gears I want.

I also found this for anyone that may be interested
http://www.jeepforum.com/forum/f11/...-up-whole-truth-607435/?nocache=1447432372473

It is a great how to convert to disk brakes if no one has seen it, though I'm sure most have, and I'm just new at looking for 8.25 stuff now.

Now, the one thing I have not found a straight answer to is the 2.24 and 2.45 gears use ether a different axle housing or they just use a different diff. Because if it is the axle housing I will really have to pay attention to what I get, not just any housing and then find gears later.
 
mderoy340 said:
2.45 gear x .75 5th gear = 1.8375 final drive.
With a 26 inch that would be 1662 rpm at 70mph.
27" @70 1600 rpm
28" @70 1543 rpm
29" @70 1490 rpm
http://wallaceracing.com/calc-gear-tire-rpm-mph.php
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Unless I'm using it incorrectly, which I don't think I am. It says when I put it in for 2.45 gears at 70 and 26" tires that I would be at 2,216rpms which does not seem correct. I have been using the one from this web site.
http://americanpowertrain.com/p-7584-speed-analyzer.html
 
Yeah I think that Wallace website might be on the fritz at the moment. Because I did it on the American transmission website, and it came out the same way you had stated. The 2.45 doesn't seem bad, but I like to travel 80 mph when I'm going from St. Louis to Kansas City, And that would put me at 1875 rpm's which still is not bad. With the 2.24 gears at 1715 RPMs I would be right at 80 with 26 inch tires. So, either gear would work just fine. The biggest thing is to stay below 2000 RPMs on the highway at 80 mph.
 
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