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Review: Rear Axle Gear Selection for 1972 Big Block

SVO

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Jan 11, 2011
Messages
332
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Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
I have a 72 coupe with the LS-5 motor, M20 wide ratio 4 speed and air conditioning. Currently, the rear gears are 3:08's and is in need of rebuilt. I would like to upgrade to 3:55's. But looking at GM literature, from 1968 to 1974, Chevrolet never offered a M20/3:55/air combination with the 454. For some years, you could order 3:55's with air and the M21 close ratio 4 speed. So would a 454/3:55's/M20/air conditioning be a bad combination? Bad enough to reconsider going with 3:36's instead, which was a combination that Chevrolet did offered. And gas mileage and driving long distances is not a consideration for the new rear gears. :)

One last questions, how is the quality of SKF bearings compared to Timkin bearings?

Opinions?

Bob
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
I'm going to guess there's a durability problem with those old HVAC compressors running at higher rpm for extended periods.

I'm also going to guess that with the M20 and the 3.55 gear GM was confident there might be a traction problem with the tires available back then. In any event, the 2.54 low and the 3.55 axle for a 8.89:1 final, will give you a lot of torque which which to spin the tires.
 

SVO

Well-known member
Joined
Jan 11, 2011
Messages
332
Location
Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
Thank you for the reply. My concerns didn't factor in the AC compressor longevity. It's extra work, but I'm now leaning toward pulling the 3:55 rear axle out of my LT-1 car and trying them in the LS-5 car.

Thanks again.

Bob
 

GTR1999

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1969,1972,1975
Bob the 355 will work with the M20. I have built and run 336-355-373-and 411's with an Muncie M-20 and ST-10 M20. The ST-10 has a 2.64 first gear over the Muncie 2.54 I don't think there will be an affect on the compressor. All depends on how you're going to use the car. IF around town 355 or 373 ( over stock 370) would be stout. For combination driving 336 would be what I pick. 308's with a BB will work because of the torque, not with a SB though. Since you have a 355 I would give it a shot and see how you like it before you start building one. The only usa made gears left are Tom's, which many vendors are reboxing now. SKF bearings are very good but for a differential stick with USA Timken - same for the trailing arms if you get to them in the future.
 

SVO

Well-known member
Joined
Jan 11, 2011
Messages
332
Location
Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
Hi Gary, I was hoping that you might drop in & offer an opinion. Based on what I'm reading, 3:36's may be a better deal, as the car is used on weekends for what I refer to as town & country driving. Still going to try the 3:55's out of my LT-1 car. I really like the 3:55's with the LT-1 but it is a different critter than the LS-5. Whichever gear set I go with, am planning on ordering them from Tom's. I was surprised to see that Tom's was no longer located in California. Will go with the Timkin bearings also. Appreciate the advise.

Thanks again,

Bob
 

GTR1999

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Messages
1,146
Location
New Haven CT
Corvette
1969,1972,1975
Hi Bob
Tom moved to ID about 10 years ago. Sounds like the 336's would be a good setup for you driving style. I run them in my 72 with TH400 and like them. If you are planning on buying gears I would get them while you still can.
 

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