2020-B4-4

Author Topic: Opinions on rear gear swap  (Read 902 times)

b_hill_86

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Opinions on rear gear swap
« on: June 07, 2020, 06:11:38 PM »
My 77 T/A has a 2.41 posi in it and I converted the trans from auto  to a 4 speed. The 2.41 makes the trans gears pretty tall, I feel like staring is a little hard on the clutch, and I would like to wake it all up a little. I do like driving on the highway up to 70 (and would consider 2-3 hour trips) so I was shooting for a 3.08.

My question is regarding how to accomplish the swap. It doesn’t seem to be a recommended job for the average backyard mechanic and I’m hesitant to have a local shop do it just cause of being burned in the past on the engine and trans rebuild. Not to mention the shop I had planned on using, which I was fairly comfortable with, claims they can’t get a 3.08 from their supplier. They said they’d allow me to supply my own parts but because they make money on their parts and labor they said labor cost would go up and they wouldn’t offer much warranty. They were figuring $825 for labor and I’m figuring at least $500 for a new 3 series posi and the ring and pinion right? Maybe more?

Another thought is finding a complete 3.08 posi axle and swapping the whole thing. It doesn’t seem like it would be a bad job and I actually found one 2 hrs from me for $850 obo. I don’t know if that’s fair, not fair, a good idea or something I should shy away from.

Any ideas, opinions or tips? Thanks.
-Brian-

1977 Trans Am 400 4 speed

Wallington

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Re: Opinions on rear gear swap
« Reply #1 on: June 08, 2020, 01:01:47 AM »
That's what I did, rebuilt an entire second axle with good used centre. All new brakes and springs, swapped the lot in one go and no downtime with cars left on hoists etc, all work carried out on work benches prior making it very straight forward. But of course, I had all that work to do at once so it made sense, you may only need the gear swap and nothing more.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #2 on: June 08, 2020, 11:53:45 AM »
I know I have at least one bearing making noise. Whether that’s the pinion bearing or one of the axle ones, I don’t know. Axle bearing I could handle. I’m kind of leaning toward taking a chance on the used axle to save some money and do it myself but I know it could end up being trash and I’d be in the same boat minus another $850 or so, so I can’t pull the trigger yet.
-Brian-

1977 Trans Am 400 4 speed

TammyTA

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Re: Opinions on rear gear swap
« Reply #3 on: June 08, 2020, 12:37:25 PM »
You are looking at 40 year old parts in that rear end. ( sounds like a broken record)   If like mine, the splines are shot & all bearings have seen better days.  The used one with lower gears I bought was worse yet.  I took a shot at it & rebuilt my own with all new stuff.  No haven't got them on the road, so may have a mess, but it was something I always wanted to try.

Long story short, make sure you replace all bearings, seals & axles.   Was going to recommend an auto with overdrive, but it sounds like you want to row that bird.  lol 

Let us know what you end up with.   :D

b_hill_86

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Re: Opinions on rear gear swap
« Reply #4 on: June 08, 2020, 02:35:57 PM »
What is the likelihood of a posi unit being worn out after 40 years? I’m just thinking, if I got a decent deal on the used axle, and it didn’t need a new posi carrier but maybe needed new bearings and rings and pinion, maybe a bench build would be easier and still less expensive than paying someone to fully rebuild my current rear.
-Brian-

1977 Trans Am 400 4 speed

JJ 109

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Re: Opinions on rear gear swap
« Reply #5 on: June 08, 2020, 08:02:18 PM »
Swapped mine up to a 3:73.

You don’t have to change the to a series 3, you can use a spacer.

New rebuild kit was only ~$60.

I think I did it all for ~200.
JJ
76 pro tour project
Twin Turbo LS1
Detroit

b_hill_86

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Re: Opinions on rear gear swap
« Reply #6 on: June 08, 2020, 08:43:26 PM »
I want to avoid a spacer. Too many horror stories. You still would’ve had to do the whole paint the gears, assemble, check, repeat process right?
-Brian-

1977 Trans Am 400 4 speed

JJ 109

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Re: Opinions on rear gear swap
« Reply #7 on: June 09, 2020, 07:48:10 AM »
Yes process is the same.

The key is you have to use the longer hardened bolts. I think many stories are just that.

I am putting ~800 HP through mine. Granted with street tires they just go up smoke.
JJ
76 pro tour project
Twin Turbo LS1
Detroit

b_hill_86

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Re: Opinions on rear gear swap
« Reply #8 on: June 09, 2020, 08:26:32 PM »
Still not sure if I’m comfortable with it but I do appreciate the suggestion.

So the shop I was going to use can get 3.08 now apparently. And the used axle I was interested in sold already so I guess I’ll just have the new parts installed. That won’t likely be until early July though now. I guess on the plus side there seem to be almost no car shows still scheduled in the area so I won’t be missing out on much.
-Brian-

1977 Trans Am 400 4 speed

b_hill_86

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Re: Opinions on rear gear swap
« Reply #9 on: June 19, 2020, 12:44:14 PM »
I’m starting to reconsider and maybe leaning toward a used axle swap again. How likely is it for a posi rear end to be trash? I think the limited slip can be rebuilt right? Is that a backyard mechanic doable thing or specialized. I’m concerned about buying a used axle and finding out it’s trash
-Brian-

1977 Trans Am 400 4 speed

Grand73Am

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Re: Opinions on rear gear swap
« Reply #10 on: June 20, 2020, 07:21:47 AM »
I've had good luck over the years, buying used complete rearends, cleaning and painting them up, and just installing new axle bearings and seals and fill with gear oil and posi additive and they've all worked fine. I done it at least 6 times. Before buying, I just pulled the covers and inspected the gears to make sure there's no obvious damage, rust, or extreme wear. I also used to buy them for $150-$200. So, I wasn't risking a lot of money. It's been a long time, and I'd expect prices to be a little higher now. But I wouldn't take a chance on paying $850 for a used rearend. I think $400 would be my limit, and if I couldn't find one for that, I'd put the money toward new parts.

I haven't had to change clutches in any of the rearends I've had, but it's probably not too difficult if you had to do it. There are instructions on the factory service manual for your car, and there are probably videos on youtube on how to do it too.
Steve F.

Wallington

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Re: Opinions on rear gear swap
« Reply #11 on: June 20, 2020, 07:58:25 AM »
If you get a good deal on a used rear, give it a go. If it whines and carries on then you are still at the same point of replacing with new, worth trying.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #12 on: June 20, 2020, 09:38:01 AM »
Finding one reasonable has been the issue so far. They seem to be hovering around the $800 mark. I did find a company 2 hrs from me that sells rebuilds in the $1100 range. That is actually a little cheaper than it was going to cost me to have mine rebuilt and that way I could still keep my original rear end. Time will tell I guess. I’ll continue to keep an eye out for a reasonably priced used.
-Brian-

1977 Trans Am 400 4 speed

hakitup

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Re: Opinions on rear gear swap
« Reply #13 on: June 20, 2020, 09:38:25 AM »
"I don't know what you call it, I only know the sound it makes when it lies!!"

b_hill_86

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Re: Opinions on rear gear swap
« Reply #14 on: June 20, 2020, 06:16:24 PM »
Ok good to know. Thanks.
« Last Edit: June 23, 2020, 05:39:19 AM by b_hill_86 »
-Brian-

1977 Trans Am 400 4 speed

Re: Opinions on rear gear swap
« Reply #14 on: June 20, 2020, 06:16:24 PM »


2020-B1-8


Fbird

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Re: Opinions on rear gear swap
« Reply #15 on: June 21, 2020, 07:40:31 AM »
rebuilding a rear end is actually pretty straightforward..... it does require a FEW special tools but nothing too big or bad.
for reference i would recommend this...
https://nastyz28.com/threads/8-5-rebuild-and-upgrade-advice-and-guidance-needed.330423/page-7
67 Firebird
1981 Turbo TA (injected 400 now :) )

b_hill_86

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Re: Opinions on rear gear swap
« Reply #16 on: June 21, 2020, 07:44:45 AM »
From what I can tell, a press and maybe a dial indicator or two? I actually found a harbor freight shop press I might pick up to assist.
-Brian-

1977 Trans Am 400 4 speed

Fbird

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Re: Opinions on rear gear swap
« Reply #17 on: June 22, 2020, 05:47:30 AM »
no press needed......to install new bearings go to grocery store and get 2 lbs of DRY ICE and 1 gallon of rubbing alcohol ;)
put in a PLASTIC (or styrofoam) cooler...... put your bearing in a ziplock bag....drop into cooler....PUT LID ON (helps to keep it from boiling out so fast) ....that will get you down to -150 deg F. then i little tap with a brass hammer to "seat" ...it's all good.
BTW you are NOT going to want to try and save the old inner pinion bearing anyway (i have NEVER had one survive removal) ...a .034 shim under the "new assembly" should get you close enough for a good pattern. I have rarely needed to vary from that to get a pattern on a 10 bolt.

i hate FORCING things together....just seems WRONG..lol

another "tip" when getting ready to install new ring gear.... take carrier put in trash bag... (tied shut due to smell) in leave it in the freezer over night....clean ring gear with BRAKE cleaner...then soap n water wash......set on cookie sheet...put on your gas grill LID OPEN as your only trying to WARM it up to about 200-300 degrees......get carrier out of bag...add a little GREASE....simply DROP ring gear onto carrier ad 2-3 bolts (for alignment) ...snug them up...give it 1 "tap" with your BRASS hammer......its all good.
67 Firebird
1981 Turbo TA (injected 400 now :) )

b_hill_86

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Re: Opinions on rear gear swap
« Reply #18 on: June 22, 2020, 07:17:16 AM »
Hmmm. Good tips. I didn’t know the ring gear had a pressed fit.
-Brian-

1977 Trans Am 400 4 speed

b_hill_86

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Re: Opinions on rear gear swap
« Reply #19 on: June 22, 2020, 07:21:50 PM »
I may have found a used axle to look at Thursday. Any tips on determining its condition? Been sitting for 20 years allegedly with oil in it still.
-Brian-

1977 Trans Am 400 4 speed

Fbird

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Re: Opinions on rear gear swap
« Reply #20 on: June 23, 2020, 05:52:44 AM »
clean and remove the cover!!!! yeah codes are stamped on it but ....so what.....remove the cover count teeth, verify it actually has clutches. Inspect axle tube->center section looking for stress cracks (drag racing with slicks ;))

gears should look "shiny"....lol with NO sharp edges (knife sharp would indicate it's HIGHHHHH miles)
make sure it rotates BOTH directions fairly free

While the ring gear is NOT a press fit...the slight temp changes allow very easy install. The idea of using bolts to "run the gear up" allows you to screw up if there is the slightest burr or cock. Some may have .002-.003 clearance!! but i have also had some that would only go on (by hand) by heating the ring gear. At 250 degrees the ring gear will "expand" around .003" allowing you to drop in on and rotate for alignment...also allows you to FEEL that it is sitting FLUSH. I like to FEEL things at assembly. With 20+ years of building and maintaining high speed stamping dies and molds...its a feel thing.
« Last Edit: June 23, 2020, 06:01:59 AM by Fbird »
67 Firebird
1981 Turbo TA (injected 400 now :) )

b_hill_86

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Re: Opinions on rear gear swap
« Reply #21 on: June 23, 2020, 07:24:57 AM »
Great info! Thanks
-Brian-

1977 Trans Am 400 4 speed

b_hill_86

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Re: Opinions on rear gear swap
« Reply #22 on: July 06, 2020, 07:09:23 AM »
I’m going a different route than the used axle. I found a shop that will rebuild a core with all new parts for cheaper than my local shop was going to charge me to rebuild my own. This way I can stash my original setup. That leads me to my question:

Is swapping a rear axle assembly as straight forward as it seems? Remove driveshaft, brake line, leaf spring u-bolts, remove and replace? Or am I missing something? Do I need to drop one end of both springs or can I get away with man handling it over top of them? Thanks.
-Brian-

1977 Trans Am 400 4 speed

JJ 109

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Re: Opinions on rear gear swap
« Reply #23 on: July 06, 2020, 02:20:11 PM »
Yes just that straight forward. Add shocks and sway bar to that list as well.

You do not need to drop springs.

JJ
76 pro tour project
Twin Turbo LS1
Detroit

b_hill_86

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Re: Opinions on rear gear swap
« Reply #24 on: July 06, 2020, 05:27:26 PM »
O yea duh. Forgot about shocks and sway bar. Good to know though. Thanks.
-Brian-

1977 Trans Am 400 4 speed

RamblerRacer

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Re: Opinions on rear gear swap
« Reply #25 on: July 09, 2020, 08:22:54 PM »
I’m not sure how close to pulling the trigger on this but consider this. Cost of the rear gear work and money from selling your 4sp another 1500 or so and swap in a tko 600 with a .82 5th. Same set up as mine but for a 3.08 gear. Just a thought

b_hill_86

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Re: Opinions on rear gear swap
« Reply #26 on: July 10, 2020, 12:36:30 AM »
I have considered it and it would be the best result but I’m getting to the limit of my budget. Wife wants her kitchen redone which is happening in a few months so I’m just happy she’s letting my spend the money on the rear lol. I wish gear vendors ODs weren’t so much too
-Brian-

1977 Trans Am 400 4 speed

cyber104

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Re: Opinions on rear gear swap
« Reply #27 on: July 10, 2020, 05:52:16 AM »
I'll be very interested to hear how you like the 3.08.  I've got my original 3.42 and highway cruising at 70 (or even 63) can get a bit tiring.

I considered a 3.08 but I'm not sure even that will drop my RPM's low enough at 70.  I'd like to see around 2500 RPM at 70 MPH.
1978 TA 4-Speed W72

b_hill_86

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Re: Opinions on rear gear swap
« Reply #28 on: July 10, 2020, 08:11:06 AM »
I can let you know. I did a lot of number crunching for the same reason. I do drive e way with the car and want to retain the ability. The 2.41 is obviously great for that but not well suited for street fun nor the 4 speed I swapped in.

If my numbers are right, with my 27” tire, 3.08 should have me around 26xx rpm at 70.
-Brian-

1977 Trans Am 400 4 speed

cyber104

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Re: Opinions on rear gear swap
« Reply #29 on: July 10, 2020, 09:59:01 AM »
If my numbers are right, with my 27” tire, 3.08 should have me around 26xx rpm at 70.

I'd be VERY pleased with that - please keep us up to date - thx!
1978 TA 4-Speed W72

b_hill_86

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Re: Opinions on rear gear swap
« Reply #30 on: July 12, 2020, 03:55:47 PM »
Soooo..... I thought I had a 2.41 posi in my current rear but now I’m not so sure. Lifted the rear a while back, spun the wheel one revolution and counted, what I’m pretty sure was, just under 2 and a half revolutions. However, using rear gear calculators online, the numbers more closely resemble a 2.56. I have 255/60/15 tires which should be 27 in tall. So according to the calculators, with a 2.41, 27in tall tire and 70mph, I should be at 2100 rpm. But when driving and using a gps for speed, I’m closer to 2250 RPM which is closer to the numbers for a 2.56.

To complicate things further I double checked my rear end code which is PJ which should be a 2.41 open in 77. Well, it’s definitely a posi rear so with a little more research and looking at the next character stamped on the rear it’s G which indicates a Chevy rear so I guess it’s not original to my car.  I guess it won’t matter once I get my new 3.08 axle but I’m curious if anyone knows what PJ would indicate on a Chevy rear. 2.41 or 2.56?
-Brian-

1977 Trans Am 400 4 speed

Grand73Am

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Re: Opinions on rear gear swap
« Reply #31 on: July 12, 2020, 04:54:53 PM »
All 2nd gen F-body, including TA, used the G-Chevrolet-Detroit gear and axle, so it doesn't mean it's necessarily from a Camaro, although it could be. But, it could be from a Firebird too. My book also shows 2PJ as 2.41 open. If you're getting less than 2.5 revolutions, it's likely it is 2.41.

Since it has a posi carrier, both wheels should turn forward when both are off the ground, when one wheel is turned forward. If so, it would seem the carrier was changed from open to posi at some point, so it doesn't match the code.

Have you looked inside? You could look at the numbers on the gears to determine the ratio, or count the teeth, and verify the posi carrier.
Steve F.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #32 on: July 12, 2020, 05:55:16 PM »
Ah ok. That makes a little more sense. I assumed it would be stamped with a P instead of the G. Unless I messed it up somehow I’m fairly confident I got just under 2 and a half revolutions of the driveshaft when I spun the wheels once. But the online calculators aren’t matching up with that. I do know it’s a posi. Both tires spun the same direction. I haven’t been inside of it. Maybe once I get my new axle assembly and swap it in I’ll get inside the existing one for the heck of it. Overall I guess it doesn’t matter I’m just curious about it.
-Brian-

1977 Trans Am 400 4 speed

Grand73Am

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Re: Opinions on rear gear swap
« Reply #33 on: July 12, 2020, 06:00:25 PM »
I'd be more inclinded to believe your driveshaft rotation count. I've found that the dash tachometers on these cars aren't real accurate, so may be skewing your calculations a bit.
Steve F.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #34 on: July 12, 2020, 08:52:24 PM »
Could be and I wondered the same. The RPM difference between a 2.41 and 2.56 isn’t a ton. Like 130 rpm. I just assumed it would be more likely for a tach to read low vs read high if it was wrong. Not sure why someone would add posi to a 2.41 but who knows? I’ll post an update once I install the new rear and open up the old one.
-Brian-

1977 Trans Am 400 4 speed

Grand73Am

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Re: Opinions on rear gear swap
« Reply #35 on: July 12, 2020, 09:35:14 PM »
Not sure why someone would add posi to a 2.41 but who knows?

Yes, I was thinking that it would be an unlikely thing to do too, but don't know another reason it doesn't agree with the code, other than I suppose the factory could have made a mistake when they stamped it? It should be stamped PS for a posi 2.41.
Steve F.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #36 on: August 01, 2020, 02:28:57 PM »
Picked up my new rear end today. Looking forward to getting it swapped out.
-Brian-

1977 Trans Am 400 4 speed

428 78T/A

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Re: Opinions on rear gear swap
« Reply #37 on: August 02, 2020, 04:18:09 AM »
Worst decision of my restore was going from 342's to 373's. But that was with a th350 too though.

b_hill_86

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Re: Opinions on rear gear swap
« Reply #38 on: August 02, 2020, 06:41:08 AM »
I’m only going up to 3.08 from 2.41 in my 4 speed swapped car
-Brian-

1977 Trans Am 400 4 speed

b_hill_86

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Re: Opinions on rear gear swap
« Reply #39 on: August 03, 2020, 11:39:37 AM »
Question. The bracket on the rear axle where the distribution block for the brake lines attach, does that have a name? And are they available or should I try to make one/reuse my old one. Current axle has it but the new one doesn’t.
-Brian-

1977 Trans Am 400 4 speed

Re: Opinions on rear gear swap
« Reply #39 on: August 03, 2020, 11:39:37 AM »
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