Gear upgrade advice on 7.25 axle with 2:76 gears

[OBannion71]

Hi all, newby post here. I have a 71 duster and live in the uk so getting parts is very difficult over here, or very costly! I have a 7.25 axle with 2:76 gearing but want to upgrade to 3:55 gears. I have found somewhere I can get a 3:55 ring and pinion but wondered if this would be a straight swap or if I need to change anything else at the same time? Any advice appreciated but I'm new to this so go easy!

Thanks all.

 

[65 Dartman]

Welcome to FABO. Wondering why you want to swap to 3.55s?

 

[OBannion71]

Was looking for quicker acceleration as generally drive smaller country roads rather than cruising down highways.

 

[Garrett Ellison]

What vehicle and engine do you have? Asking because of the inherent limitations of the 7-1/4 AEM axle.

 

[OBannion71]

It's a 318 in a 71 duster. I'm aware the axle is the weak point but near impossible to get an 8:25 or 8:75 over here as they are very desirable and too costly to ship. Just need to work with what I've got. If upgrading gears is going to be an issue, and folks think the axle can't take it, I will leave it as it is. Cheers.

 

[65 Dartman]

I have found somewhere I can get a 3:55 ring and pinion but wondered if this would be a straight swap or if I need to change anything else at the same time? Any advice appreciated but I'm new to this so go easy!

Thanks all.

As long as the 3.55s are for a 7.25, it should be pretty much a straight swap - as long as whoever is doing the work knows what they are doing.

 

[Garrett Ellison]

If you have a sure grip 7-1/4 then it would be worth doing. The sure grip is much stronger in a 7-1/4 than the standard open rear.

 

[AJ/FormS]

If you currently have a stock torque convertor, and a 2bbl; then
Just upgrade to a 2800 and a 4bbl; which will allow your secondaries to pop open, and about double your take off torque. Plus
at 30 mph in second gear, you are now probably taching around say 2000 at WOT; so yes, she's a bit of a dog.
3.55s will get you about 28% more rpm (at 30 mph) whereas
a 2800 & 4bbl, will get you about 47% more.
If your 318 manages 200ftlbs at 2000, that is 76hp.
If she manages 280 at 2800, that will be 149hp; not quite double, but close. The torque difference at 30mph will be astounding.

As for the 3.55s being a straight swap, yes they will,
BUT the pattern adjustment might NOT be;
AND now, you will be stuffing more torque thru the tiny planetaries, more of the time.
AND don't forget, those planetaries are probably 54 years old.
AND so now, you really need a Limited Slip, to keep her from peg-legging, which is how they usually seize the crosspin, and fail. But sometimes the planetaries shatter; and sometimes the crosspins attempt to vacate the housing and if the crosspins bust thru the cover, but don't come out, that then locks the rear-end up, and usually the car spins out. Could be a real bad day for you, depending on where it happens..
So install a bolt-in Limited Slip, and change the gearoil to a top quality oil, and keep her topped up.
AND yes, the 2800 is really-really gunna stuff the torque to her.
But it will be so much fun!

 

[OBannion71]

Thanks for the replies, very helpful. I do have some upgrades from standard such as Edelbrock performer intake and Holley four barrel and headman headers. No upgrade on cam or heads though.
If I increase the torque converter will that increase the likelihood of damage to the axle, as that appears the weak link.

 

[AJ/FormS]

If I increase the torque converter will that increase the likelihood of damage to the axle, as that appears the weak link.

Only if you abuse it by doing one wheel-peels, neutral-drops, and or if you take off on pavement with the the throttle wide open, every stinking time, lol.
If yur driving a lot of gravel, using a lot of throttle, you absolutely need to tie the two wheels together with some type of Limited-Slip.
In any differential, when one wheel spins, it rotates at double the other, and all the torque is going thru those little planetaries. But worse is that the planetaries are now spinning on the Cross-pin, which has no bushings nor bearings; it's metal on metal, with a thin film of oil that nobody can guarantee you is gunna stay there. When it boils, that's when chit happens.
Also, every time you hammer the gas, then back off, Engine-braking is slamming the planetaries back and forth on the cross-pin.
And, those little rear-ends hate shock-loads. like spinning on gravel/dirt, onto pavement. If the wheels come to a sudden halt when they hit traction, BANG!, the planetaries get a wallop! If just one is spinning at double roadspeed, there's no predicting what will happen.
The point is this, if you abuse that little rearend, it ain't gunna last long and the convertor is NOT the badguy

 

[OBannion71]

Only if you abuse it by doing one wheel-peels, neutral-drops, and or if you take off on pavement with the the throttle wide open, every stinking time, lol.
If yur driving a lot of gravel, using a lot of throttle, you absolutely need to tie the two wheels together with some type of Limited-Slip.
In any differential, when one wheel spins, it rotates at double the other, and all the torque is going thru those little planetaries. But worse is that the planetaries are now spinning on the Cross-pin, which has no bushings nor bearings; it's metal on metal, with a thin film of oil that nobody can guarantee you is gunna stay there. When it boils, that's when chit happens.
Also, every time you hammer the gas, then back off, Engine-braking is slamming the planetaries back and forth on the cross-pin.
And, those little rear-ends hate shock-loads. like spinning on gravel/dirt, onto pavement. If the wheels come to a sudden halt when they hit traction, BANG!, the planetaries get a wallop! If just one is spinning at double roadspeed, there's no predicting what will happen.
The point is this, if you abuse that little rearend, it ain't gunna last long and the convertor is NOT the badguy

Fair point and great advice. Thanks.

 

[Garrett Ellison]

I've been fortunate with my 7-1/4 experience as I've split the spider gears before blowing the cross pin out the carrier. It probably doesn't help anything that the early nine bolt axle cross pin is retained by a roll pin instead of a bolt! I've looked at one before, and while it wouldn't be much of an upgrade, a nineties 7-1/2 ford ranger axle (also 5 on 4-1/2) looks like it would be a suitable replacement for a 7-1/4 on the cheap if nothing else is available. Here's a link that covers those axles and possible cross fitment in some detail.
Ford 7.5-Inch Rear Axle History & Specs - The Ranger Station

 

[clementine]

Call me crazy but is it possible to investigate other cars in the UK that have a limited slip differential with 3:55’s?

If it can be had for cheap sometimes it justifies the cost of buying tools to make the swap……like a welder n stuff.

Any jeeps or fords in those wrecking yards?

 

[clementine]

What part of the UK are you?

 

[OBannion71]

Thanks all. I think it will be something to think about in the future but for now I'll just stick to what I got.

 

[timk225]

Way back in the late 90s and early 2000s, I blew 6 7-1/4" axles with Slant Sixes. Now my Slants were tuned up a bit, lots of hard accelerations, and my 904's had the Mopar magazine shift improver mods, like a 3.8 kickdown lever, higher line pressure, a few valve body separator plate holes drilled out, etc. On the average, they lasted 5000 miles, but one hung in there under my lead foot for 41,000 miles! They failed in different ways. Broken gear teeth, stripped side gears, failed wheel bearings, etc. I never rebuilt one, I'd just swap in another axle assembly.

My suggestion is to pay the cost of an 8-1/4", rebuild it how you want it, and never have an axle problem again. If (and probably WHEN) I blow my 7-1/4", I'll assemble an 8-1/4" with a sure grip I will rebuild, and 2.94 gears.

 

[TF360]

Hi all, newby post here. I have a 71 duster and live in the uk so getting parts is very difficult over here, or very costly! I have a 7.25 axle with 2:76 gearing but want to upgrade to 3:55 gears. I have found somewhere I can get a 3:55 ring and pinion but wondered if this would be a straight swap or if I need to change anything else at the same time? Any advice appreciated but I'm new to this so go easy!

Thanks allI had a good time there today. Besides catching up with Al and T, I met @TF360 for the first time and his way cool red Duster.

 

[TF360]

Where did you find the 3:55 gears for the 7 1/4?