toomanyhobbies

So I am getting pretty far along in my build of my '72 Supreme his/hers. My engine and trans are about done. The engine is the original 350 a mild stock-appearing 350 that will make 350-375 and the trans is the original TH350 rebuilt with a mild shift kit and a 2600 stall.

The rear in the car now is a 2.73 LSD that I am told is clean and in good shape. It had been my hope and intention of changing out the ring and pinion to a 3.23 or a 3.42 (I never could really decide but I want a car that I can jump in and take a 2 or 3 hour drive in on the highway without it being a screamer). The trans guy that I am using, who is knowledgeable and honest has told me that I really need to replace the entire unit and not just the gears to "do it right" and this was not in my budget. I was hoping that I would just change the gears to change the ratio, and replacing the LSD unit was not in my budget.

Here is his scope of work:

Tear down factory complete rear differential. Remove axles, remove internal carrier assembly and pinion shaft/gear. Clean internal components. Clean surface area, case deck.
Replace original 273 gears. Install carrier assembly with new internal spider gear set with limited slip differential. Convert to 342 gear ratio. Replace ring and pinion. Replace and install new carrier/ pinion bearings. Replace pinion seal. Assemble. Set end play/backlash. Reseal and reinstall rear differential cover.


So his price is $1500 to replace the parts.

My questions are:

1. 3.23 or 3.42 and will I notice a difference in performance off the line?

2. Do I need to do the carrier unit and what do you think of this quote? I was really hoping to avoid replacing it.

Thanks

toomanyhobbies

55 views and not a reply. Somebody here must have a point on view on this one...

rcorrigan5

I'm having my '69 done right now and the price seems consistent with what I will be paying here in Olympia.


3.42:1 will clearly get the jump on a 3.23:1 but, in my opinion, unless you are somewhat hard core in getting that jump all the time, a 3.23:1 will be much better off on gas for that 2-3 hour drive.


My '68 has its original 3.08 posi and I like it just fine, but I'm not a racer by any stretch of the imagination. My '69 has a 3.23 open differential and it is just slightly quicker than the '68, but not enough to impress ol' light-footed me!


Bottom line...if it were me and I were deciding between a 3.23 and a 3.42, I'd go with the 3.23.


Randy C.

Run to Rund

I think you should decide what you will want in the future before putting that kind of money into your current differential. You can build a stout 9" ready to bolt in, from Quick Performance and others, or you can have Brian Trick (507Olds) or Monzaz build you a GM rear from mild to wild. Many times the heavy duty stuff costs very little more than standard parts.

toomanyhobbies

Thanks for the replies. Run to Rund I am not sure I understand what you mean by "what you will want in the future." I am planning on building the car the way I want it and enjoying it from there the way it is. I am not going to be putting it on a strip on on the track. My goal all along has been consistent in that I am going for "a street cruiser with *****" and a stock appearance. Everything on the car is stock original and a one owner car, so there is a real limit on my personal willingness to mess with it (at least in terms of appearance). Therefore I really do not want to change out the rear case from stock, I just want it to have quality guts inside that are reliable.

So to phrase the question a different way: do I need to replace the carrier when I change the gearing for risk of failure?

Thanks

don71

Edited....

I was in a similar situation a few years back. After crunching the numbers, it was about the same cost to have my rear re-geared or just by a rebuilt complete rear.

I bought the rebuilt rear, but I didn't care about numbers matching. Also, I don't have the proper tools to set up the gears correctly.

As a car guy, it only takes a few hours to swap out the whole thing. That was the easy decision for me. I fretted over 3.23 or 3.42. I chose the 3.23

Fun71

Don71,
That is correct for the 1967-1970 O-Type 12 bolt rears. His 72 should have an 8.5" corporate 10 bolt rear, though. The break on that rear is 2.73 so he can use the existing carrier with 3.23 or 3.42 gears.

My opinion is replace the ring and pinion gears, seals, and bearings and use the existing carrier. I have no idea why your mechanic is saying to replace it. Mine is an original '68 so it's 47 years old now and I have no concerns that it will somehow break.

monzaz

God 1500.00 for just parts replacement is STEEP... I can build a whole rear for that price...LOL

If the car is original the posi unit is probably fine with a 350 car. and 2.73 gears. Worse case you need to ad a .005 thous shim on each side of the clutch pack and call it a day.
Moving on to the gear set. 3.42 with 3 speed trans is going to get pretty old after a bout 2 long runs. stay at the 3.08 3.23 range.

What size tires are on the car and are your intentions to stay with the same diameter tire? MORE diameter will KILL ratio and LESS diameter will INCREASE ratio.

Bearing kit for the center and axles set of gears and someone to set them up which is where most shops will kill you as they do not do gears all the time and pad the bill to make up for goofs that may occur...

I would say 750-850 is max for the deal I just mentioned
rebuild kit (I can supply)
Axle kit (I can supply)
Gear set (I can supply) 3.08 3.23 3.42
Set it up in the rear.

NO MORE than 1000.00 to do this seem unacceptable to me.

toomanyhobbies

Monzaz I just sent you a PM.

Thanks

chadman

Price seems way high and the posi replacement is unnecessary IMO. Truthfully for 2-3 hour trips I would stick with what you have or go with 3.08 gears.

Fun71

I just re-read and saw I missed this question.
Yes, it will be noticeable. Back in the 80s I switched from a 2.56 rear to a 3.08 rear and it felt like having a more powerful engine. Then later I switched from the 3.08 to a 3.55 and again it felt like I had a more powerful engine.