Rear gear change

[JOZW30]

Am planning to change the rear gears in my 72 W30 4 speed this spring and would like some advice from the experienced.

The car originally came with 3.73:1 posi but somewhere in the past was changed to 2.73:1 (it's really pretty sad.....).

I usally go to several shows where I drive 45 minutes to an hour on the freeway. That's really abou the worst of it. The rest is 35 - 50 MPH to the local cruises and shows.

I want to have fun off the line when I feel like shredding some rubber. Should I go all the way to the 3.73's or would I get about the same rush from a change to 3.42?

Also, I have never done this before but am certainly mechanically inclined. Is this a do-it-yourself project or better left to professionals? I'd love to do it myself if it not too risky.

Also, what parts do I need to get? Gears, bearings, clutches, axle seals? Any other tips or advice?

I can't wait for Spring this year!

[beater68427]

I put 3:42's in my olds, It had 4:11's for a while I started to get worried with the oiling system at the higher sustained engine rpm. The 3:42 work fine with the 455 a blast to drive and good all around gear. Your car is a 72 so it should have a corp 10 bolt with 28 spline axles, parts are very available at a decent price, you might just post a wanted ad, but you will need your 5 stars

[GM-26]

My Chevelle has a LS-6 454, M-22 tranny and 3:73's. This is a really good overall gear ratio. I drive all over Long Island to my friend's houses with no problems. With this gear ratio the car is really responsive and isn't screaming on the parkway. I didn't tackle the rearend rebuilt. I brought it to a local race shop.

[markjohnson]

I think it would be wise to assemble a complete new rearend and just swap the entire unit out. 2.73's will turn ANYTHING into a slug! On the other hand, I think 3.73's are always a good choice for a fun street car with great highway manners. A second rear end would also mean very little downtime when it comes time for the swap. You could be done by Noontime if you start on it early on a Saturday morning.

[JOZW30]

Thanks guys,

I found a calculator for RPM with a specific gear ratio and tire size. With my BFG's at 65MPH with the 3.73's, I'll be running around 3100RPM. Again, it's usually less than a couple hours total between 3 or 4 trips.

I don't think I have the space for a second rear to hang around just for highway trips but the idea makes sense.

I sense from the responses so far that having this done proffesionally is the best route.

Thanks again for the input.

Joe

[Salvatore]

373's!

[Jerry@CHP]

Joe,

The 3.73 rear ratio is a good choice. You probably have an M20 which is the better trans to have for your application.

As to setting up rears, I've been doing them now for almost 40 years and I had one yesterday that kicked my as......Had the pinion out four times setting the depth, changed the back lash about six times trying to get a good pattern on the gears. The aftermarket gears are much harder to set up in getting a good pattern. They give you a tolernace on the pinion to check depth but most of the time after you check the pattern when setting the depth, it's not good at the depth that they recommend. You also need a good supply of shims for the pinion, side carrier shims, clutch shims, etc. The aftermarket shim kits are a joke! GM has always been the best for these items but a lot is being discontinued now. As to tools, you'll need a depth gage, inch pound torque wrench, .001" dial indicator, a torque bar attaching to the yoke to help in crushing the crush sleeve and more. After all these years, I still hate doing rears!

Jerry

[JOZW30]

Jerry, good info, I will have it done professionally. The place I usually get my trans work done can do this. They did suggest I get the parts myself as "this stuff is getting harder to find" (I think they aren't very internet savvy).

I found a place that sells GM (OEM) ring and pinions. Is this best option?

I want to use the best parts. What else should I buy?

Oh, and 3.73 it is!

Joe

[Jerry@CHP]

Yes, GM gears are the best for your application.

In the world of 12 bolt Chevy, typically, the pinion shim is always a .029 shim and might be the same case with your rear axle. Gear patterns always come out nice with GM gears using the .029 shim and about .008" back lash in the world of 12 bolts.

Most people who have been doing rears for many years usually have a big supply of GM shims and such on hand. I do. But it's a 40 year accumulation.

Get them to shim your posi unit clutches while you have it apart.

Going through my parts yesterday, I found a pair of NOS BE posi carrier spring plates. Still have the parts stickers on them. Is anyone getting aroused yet? I used to have tons of BE rear stuff back in the 1970s as that was what would hold up in drag racing!

Jerry

[Jerry@CHP]

Joe,

Being you're an Olds guy, you would love my buddy Sam Murray's cars. Has the Brainbeau 67 Olds 442. Car can run 10.90's in good air, E/Stock Eliminator 400 inch engine, 4-speed.

I turned Sam on to a buddy of mine's car a few years ago. 1971 Olds Cutlass 350 4-speed that Wayne Shipley built from the ground up. Wayne lost interest in the car, sold it to Sam and Sam just set the NHRA national record with that car, think it's 11.37 with a low compression 350 in a 3500 lb car! It's a rocketship! Tom Boucher does all of his machine work up in the New England area. Sam is one of my best buddys and has even stooped low enough to help work on the new 302 mutt that I just built.

Jerry