Rear gear change

[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

[SmallHurst]

Lets see, '72 Olds having the 8.5" GM 10 bolt...lots of choices indeed.

For the torque that you have in that Olds a 3.42 or a 3.55 would make for a real runner. Mine had the 3.23s in it and on the drag strip, they ran within .02 of the 3.91's. If I could go back, I would try to find a set of 3.42's but, Richmond does not make that for the 'O' axle. 3.73's will make it a little buzzy, but you can still run down the highway.

[beater68427]

[ QUOTE ]
Lets see, '72 Olds having the 8.5" GM 10 bolt...lots of choices indeed.

For the torque that you have in that Olds a 3.42 or a 3.55 would make for a real runner. Mine had the 3.23s in it and on the drag strip, they ran within .02 of the 3.91's. If I could go back, I would try to find a set of 3.42's but, Richmond does not make that for the 'O' axle. 3.73's will make it a little buzzy, but you can still run down the highway.

[/ QUOTE ]

I agree with you, what the original poster should do is figure the cruise speed he likes on the freeway, and figure that with 3:73's what RPM he would be at with tire size lets say in the area of 3000rpm then take his car in current form put it in 3rd gear and drive on the freeway for 10 miles or so at 3000rpm see if he cars for the running rpm of the car for a time period, then do the same with a 3:42 calculation, and see what typical cruise rpm he is comfortable with. also a typical Olds especially if it has alot of miles on it likes to fill the valvecovers up with oil at a higher sustained RPM a typical problem of them, it is worse if they are a older engine the two return holes in the back of the head are quite small and sluge up at the base leaving only .125 or so of oil return. I would recommend restrictor push rods with plans of such a rpm increase, If the engine is rebuilt with a HV oil pump and restrictors have not been installed in the rebuild I would highly recommend them as well. If you need info on stuff like that you need to talk to Mondello. http://www.mondellotwister.com/ I have had quite a few Oldsmobiles, and driving characteristics are quite different than a BBC on cruising RPM. This is my current 72 442 under a long rebuild... Good Luck with the change over!

[JOZW30]

Engine has about 3k miles since rebuild with NOS W31 cam and higher than stock compression pistons (not flat top, but lower cc).

M20 was rebuilt last spring.

I think I'm going with the 3.73's and worse case, I can travel country roads at 50 MPH if it's that bad on the freeway.

Calculators say I'll be between 3100 & 3200 RPM @ 65 MPH with my tires. 3.42's would lower it by roughly 200 RPM.

Appreciate all of the input!

Joe

[m22mike]

Joe
If you can cruise @ those RPM'S you will will be OK I think.
My L78 Camaro, M22 with 4.10's and F70 14 tires is at 3800 R's at 62 MPH.
Now I sure would not take off on a power tour, but I sure as heck ain't given up those 4.10's
You need to do this at times

I do make some 20 to 40 mile trips on the I state's and 60 seems to be my comfort zone. Good luck.

Mike