1979 Differential

[Glenn28132]

What is the tolerance on the end play of the side yoke going to your wheel
the yoke that is coming out of the side of the differential.

[Joe L.]

Re: 1979 Differential

Glenn------

There is no published maximum end-play for the stub axles. However, they should be pretty tight. I recommend about 1/16", maximum. Two things cause the clearance to increase. The first is stub axle end wear and the second is posi clutch pack wear. The stub axle end wear is the bigger problem. That's because the stub axles are case hardened. So, for a long while there is very little wear. However, once the thin case hardening is worn through, wear accelerates very rapidly. Unfortunately, some of the stub axles which were manufactured in the mid-to-late 70s seem to have thinner case hardening than their 60s counterparts. So, they don't live as long.

[Duke W.]

Re: 1979 Differential

I disagree, Joe. I'm quite certain there is a yoke end play spec for positraction axles in the Chassis OVERHAUL manual (not the Chassis Service Manual). I have seen it and distinctly remember that is was there, but I don't remember which year issue it was, though I imagine the data carried through the years.

I went searching for a spec because one is not listed in my '63 Shop Manual, so I borrowed a later overhaul manual (early seventies as I recall) and found the spec and then researched the parts catalog to find the shims.

Group 5.511 lists three shims for Eaton positraction units that fit between the outer ends of the case and the clutch pack. They are illustratred on page 5-3 of the IPB (9-76 revision in my IPB), item 13. The three shims are five, seven, and nine thou thick, and there is no reason such a shim assortment would be available unless they were there to dial in a very narrow end play spec.

This yoke end play specification may only apply to positraction axles, not open versions, but I'm certain it is listed.

I know we've been through this one before. If anyone out there with a mid-sixties or up Chassis Overhaul Manual ( and I emphasize, it's in the OVERHAUL manual in the positraction rear axle section, not the Service Manual), you can help resolve this issue by looking for the spec and posting it.

We will normally see significant end play on old axles because as the positraction clutches wear, yoke end play increases, but a fresh positraction case assembly with unworn yoke grooves and snap rings should yield only a few thousanths end play after assembly.

Changing these shims is not easy, because you have to essentially assemble the whole diff to measure the play, and if it is not in spec, you have to pull the case out and disassembly the clutch pack to change the shims. Wanna bet that most mechanics just slap in the new case and forget about it? It takes time, but as with the rear wheel bearings, if you take the time to get it right, you will be rewarded with a nice tight drivetrain and suspension and long service life.

Duke

[Joe L.]

Re: 1979 Differential

Duke------

I've looked for this specification in both Chassis Service Manuals and Chassis Overhaul Manuals. I've never been able to find any specification for yoke end play. There are specifications provided for various aspects of setting up and installing the posi unit, including shimming. But, I've never found any spec for yoke end play. I've looked through several 68+ manuals. So, I suppose it's possible that it was present in earlier manuals. But, if it is, why did Chevrolet drop it in later manuals?

[Wayne W.]

Re: 1979 Differential

I am not sure those 78-79 rear axles had any hardning judging from the evidence I have seen over the years. They were terrible especially the 3:70 jobs.

[Duke W.]

Re: 1979 Differential

It may be in a later Overhaul manual, Joe. I think the one where I found the spec was early seventies - maybe '71.

Again, to any of you out there who have sixties or, in particular, seventies version Overhauls manuals please check the positraction differential overhaul and specifications section to see if the yoke end play shows up. I'm sure it is listed in one of those seventies version Overhaul manuals.

We need to get to the bottom of this, and I swear I'm not crazy.

Duue

[Chris D.]

Re: 1979 Differential

There is a reference in the Corvette Chassis section of the old Chevrolet Power book. It recommends end play to be reduced to less than .005" using hardened steel shims which must be fabricated.

For us mere mortals on the street, my bet is the group 5.511 shims set the backlash on the spider gears and the yoke end play was considered close enough if you started with new clutches and yokes.

[Duke W.]

Still need reference from '70's Overhaul manual

I've never seen a backlash spec on the differential gears, just the yoke end play, but as you reduce yoke end play, differential gear backlash is reduced, too.

Anybody out there have a seventies vintage Overhaul manual? Somebody must have one!

Duke

[John Liner]

Ring Gear Backlash on 1979 Differential

Duke,
Ring Gear to pinion Backlash...........005" - .008" as per specs in 1979 Chevrolet Corvette Shop Manual. Bought mine new, with the car. But it doesn't show any specs that I can find under 3C Rear Suspension or 4B Rear Axle regarding end play on side gear yokes.
I belive Wayne was correct regarding soft yokes, mine was worn badly at 32,000 miles. So bad that the U-Bolts had cut a groove into the differential side case.
Wish I could help more.

John Liner

[Duke W.]

Clarification

Begining in 1966 rather than a separate "Corvette Shop Manual" Chevrolet published two manuals that covered the entire Passenger car line. One was the "Chassis Service Manual" and the other was the "Chassis Overhaul Manual". The former covers service and R & I procedures for major components. The latter, the "Chassis Overhaul Manual" covers ovehaul of major components such as the differential. It is a Chassis Overhaul Manual that I found the yoke end play spec in the positraction section or spec section of the axle overhaul informantion.

The chassis service manuals are fairly common, but it appears next to nobody has a chassis overhaul manual. That's the one that has the spec. Anybody out there have an overhaul manual from the early mid seventies who can look up the yoke end play spec in the rear axle overhaul section?

Duke

[Joe L.]

Re: Clarification

Duke------

I believe that the seperate "Chassis Service" and "Chassis Overhaul" manuals were discontinued about 1974. The 1974 Manual was a "giant" manual that covered both areas and for a few subsequent years there were "supplements" for each model year. A few years later the Corvette got a seperate manual whereas the earlier ones had been for all Chevrolet models. After the Corvette got the seperate manual, everything was included in just the one manual.

Anyway, I have a 1970 Chassis Overhaul Manual (which includes 1969 Corvette). I have "scoured" this in the past and, again, today for a reference to an end-play spec for the side yokes. I can find none, though.

[Duke W.]

Re: Clarification

Thanks for the info on the later shop manual organization. If you say there is not spec in the '70 O/H manual, I have no doubt you are correct. My best recollection is that the one I looked at was 1971, but I can't remember if the differential side yoke endplay spec was in the specifications section, or in the text - like the distributor endplay spec in the '63 Corvette Shop Manual.

Duke

[Pete Whelan]

Re: Still need reference from '70's Overhaul manua

Duke,
I found a differential gear backlash specification of 0.001-0.006" in the '66 Chassis Overhaul Manual (pg. 4-22, reassembly step #10). This specification applies to the Eaton positraction unit; I didn't find one for the Dana unit. Send an email if you want a scanned copy of the pages.

Pete

[John H.]

Re: Clarification

Duke -

I have several "Overhaul Manuals", but the most recent is 1969, and none of them make any reference at all to yoke end play specs, anywhere; the only mention they have is for shims/thrust washers to set backlash for the spider gears.

A few months ago, as the result of several posts on other boards on distributor shaft end-play, I spent a few hours searching my entire tech library to see how many times and in what places the .002"-.007" spec was mentioned, as several posters said they couldn't find it, and they understood it to be .030"-.060". At least in my collection, I found it clearly stated in eleven different Chevrolet Shop/Service manuals from 1955-1969, always in the text - never in the distributor "specs" charts. Gotta know where to look.

[Duke W.]

Interesting

If one were to measure yoke end play and differential gear backlash, they would probably come out about the same. (Everyone please note that differential gear backlash is not the same a ring and pinion backlash - totally different measurement and they do not affect each other.)

Duke