The Stovebolt Site Forums The Shop Area Driveline Rear end swap question Update

Helping a friend with a 51 1/2 ton. He wants to take out the 3 speed and put a T5 in it which means changing the rear end. We're trying to keep this as simple as possible and he wants to keep his original 6 lug wheels. We have access to a late 55 - 59 rear end. Would it work to put early 55 springs in the truck and bolt this rear end in? I'm a TriFive guy so I'm turning to you guys for input so we don't make any mistakes. Thanks.

Gordy

If you keep the original springs you can use the adapter spring Pearch .not sure if the newer rear end has the same spot we’re the spring goes .the ones pictured are offset to put the axle in the center of the wheel well when using the original springs .

Hi Gordy,
A few questions...
Does your friend already have the T5?
If the answer is yes, do you know the ratio chain? (it might start with 1st gear as 4.03 or some other number)
Do you all know what the ratio of the late 55-59 rear end is? The reason I am curious is because some combinations work very well and some don't work as well.

There is a good chance the late 55-59 rear end will work and the early 55 springs probably will work...might need a little adjustment fore/aft but no big deal. Please let us know!

One other thing...I used a 1969 rear end and I used my 1952 shock absorber mount pieces. I was able to use the 1969 rear brake setup and alter the parking brake setup just a wee bit to make it work just fine.

Gordy - Here's a really long ago torque tube conversion thread on the topic that should give you some good basic insights on the rear axle.

Others will chime in on the springs and adapting the later TF axle housing soon I'm sure.

Thanks for the input. The transmission is being rebuilt as we speak. It is an S-10 trans with either the 4.03 or 3.93 first gear. The shop is closed for Thanksgiving so I won't be able to find out which until Monday. The measurement of the 55-59 rear from locating pin hole to locating pin hole appears to be the same as the stock rear. It has a 3.90 gear. We were going to buy the early 55 springs so we didn't have to use the locating adapters. Didn't realize the later carrier would bolt into the early housing. That would certainly make things easy. Has anyone had any issues after welding up the pivots?

Gordy

A 3.90 rear gear ratio with 28" stock wheels would get along nicely with a .73 overdrive T-5.

A 4.03 first gear is perfect with a 3.07 or 3.08 ratio rear end. It puts you about exactly at the 12.08 overall ratio the GM engineers wanted first gear to be in these 1/2 ton trucks. Good luck!

I just last month switched out my rear end that is behind my T5, I had a 3.0 and switched to a 4.11.( 3.0? don’t ask, came to me w/ a Currie rear housing).
I would suggest focus also be on the overdrive ratio/cruising speed in addition to 1st gear.

My experience in my 46 1/2 ton was that with the 3.0 rear gear is that for 5 years I never ever used 5th gear, my truck would highway cruise perfect in 4th, about as fast as I ever want to go in a truck this old, even though the suspension/steering is 100% redone and aligned, and newer bendix brakes and master fitted and a fresh full pressure 235. I felt that aerodynamics kept 5th gear for only if I were to drive off a cliff.
On the first gear side yes it is slightly low (and I now shift at the other side of the intersection to 2nd) but it is noticeably easier on the clutch and as important reverse is now a lower ratio making backing into the tight garage a whole lot more controllable.

I used one of the many online rear ratio calculators and input rpm, tire height(29.3” for mine), trans gear ratio, and rear ratio to show the mph.
I’m glad I went thru that process, thought I wanted a 3.90 but for mine the numbers didn’t say that, I am pleased with the result, and I can use 5th on the highway now…
chuck

Thanks Hank. I agree with you on the 3.0's. We're hoping to use 3.90's.

Gordy

I recently sent a '60 Apache carrier with a 3.90 ratio to ndkid275. He is in the process of swapping a T5 into his 3100. It is currently a 3 speed with a torque tube.

The '55-'60 carrier is a direct bolt in and allows you to use your original axle. You would still need to add perch mounts but at least you wouldn't need to worry about width or bolt pattern.

Send ndkid275 (Chris) a PM, he should be able to give you all the details.

Thanks for the tip. Has anyone had any issues with just welding the pivots?

Gordy

What pivots are you asking about?

The closed driveshaft rears are mounted to the springs with a pivot to allow for suspension travel. The 55 - 59 rears are securely mounted to the springs with U-bolts.

Gordy

The pivots have a big rubber bushing incorporated into them, similar to a control arm bushing. It's extremely difficult to get a good weld, especially since the pivots are cast, not machined steel, and the rubber contaminates the weld beads.
Jerry

I considered using those adapter perches when I did mine. Ended up just buying a set from the farm store and drilling my own holes to move the axle back. Super cheap way to do it. I think they were $10-$20.

believe getting the axle centered in wheel well takes something like 1.75” offset in the perch. My truck had been previously converted to open driveline and I didn’t want to screw with a new driveshaft. So I opted for less when drilling the hole….i think just an inch forward.

Gordy, I may have missed something in the thread, if you are changing the rear axle out with an open driveline 55.5 and up rear-end the pivots will not be an issue as they should be removed. If you just swapping out differentials then you will need to secure the pivots. On my 54 I added an axle perch to the back side of the axle and another bottom plate.

OK. We won't mess with the original pivots. We'll use the later housing and u-bolts. Thanks for all the info.

Gordy

On my 54 Canadian gmc I used a 69 chev 4 by 4 rear end, 3:73 ratio with a v8 camaro t5. the 261 loves this combination.

Gordy, would your friend consider changing the 3.90 rear end to a 3.38 ratio? My thinking is 3.90 x 4.03 is 15.7. That is a really strong first gear ratio. 4.03 x 3.38 is 13.6 and at that point we're getting somewhat closer to the 12.08 the GM engineers designed the truck for. 3.08 would be a perfect first gear ratio, however I'm not sure a ring/pinion set for the rear end is available in that ratio and there are some 3.38 gears available. I mention this because 15.7 is going to have your friend wanting to shift to second pretty quickly (maybe not even completely through a city intersection) and a ratio of 12 will allow him to drive around a lot more comfortably.

I mention this because (a) he is spending the money to have a T5 rebuilt and (b) I suspect he wants to enjoy driving this truck and (c) I've had friends who have ended up with a truck which had nearly a 16 overall first gear ratio and they grew to hate it because it is irritating unless you're on a farm pulling a trailer around or something. Here's the thing: in the T5s with a first gear of 4.03 the second gear is 2.37. 2.37 x 3.90 = 9.24. Taking off in second gear with a ratio of 9.24 is fine if you have a big V8 but if you have a 235 it might not be the best choice...especially in city traffic like Anaheim or LA. You need the right overall gear for the most enjoyment is what I'm saying and the GM engineers knew it.

Just a thought, Gordy.

Just talked to the trans shop. They are building it with the 3.76 first gear. OD is .72. We think 3.08s would be too tall for a stock 235 when it's in OD. I had a 56 Chevy 210 several years ago with the stock 235 and factory 3 speed OD. I took the 4.11s out and put a 3.70 in it and it was darn near perfect for all around driving.

Gordy

Hi Gordy,
3.76 is better. That first gear in the 56 210 would have been 2.94. With a 3.70 rear first gear overall ratio would have been 10.9. That's super in a vehicle like the 210 where you won't be hauling weight. However your friend's 3.76 with a 3.90 rear is still going to be 14.66 first gear overall. All considered, I really like 3.38. 3.76 x 3.38 = 12.70 and that is almost ideal. I have 4.03 x 3.07 = 12.37 and you truly can not tell it from the stock 3 speed. When you get to fifth (with a 3.38 rear and .72 fifth) you'd have 2.43. If he is using 15 inch wheels and 205/75R tires, divide that by .97 to get 2.51. A stock 3 speed overdrive in a 1960 El Camino (came with a 3.70 rear end if you had the 235) will get you .70 (overdrive reduction) x 3.70 = 2.59 final ratio. 2.59 with the 235 was okay with the GM engineers in the El Camino (which actually weighed about 500 pounds more than the 1/2 ton pickup and also the station wagon.

With a 3.90 rear you'd have 3.90 x .72 or 2.80 and that will be good in fifth gear. In fifth I have .86 x 3.07 = 2.64 and it works fine. A tad over 2.59 is better in my experience. Your friend at 2.80 will have a bit of an advantage in more hilly areas. He can expect to be a shade over 2000 rpm at 60 mph with 28 inch diameter tires by my calculation. That's about where his maximum torque will be, also. Aside from first gear all should be fine.

Did the trans shop have any thoughts on the 3.90 rear gears?

They thought the 3.90s would be fine. Thanks for all the calculations.


Gordy

Please keep us posted on how it goes, Gordy. Thanks for the question.

Will do. Thanks.

Gordy

Update. The T5 trans is installed. With help from forum members I picked up a '55 1st series rear end with 3.90 gears. I'm in the process of rebuilding it and waiting for a new pair of '55 1st series rear springs from Eaton Detroit Spring.

Gordy

Update II.
Got my buddy's truck finished. The trans with the 3.76 first gear works great with the 3.90 rear gear. Thanks to all for the advice and suggestions.

Gordy

Looks and sounds great Gordy. I bet you're buddy is enjoying the new gearing!