Axle ratio clues

[Patrick N.]

In a prior thread I learned my differential is a 3:08 from a 70's car. I have 68 L68 however.

The last thing I need to do before I drop my engine back in is rebuild my differential. If I wanted to rebuild it as it was delivered with the original ratio I have limited info to track down what came on the car.

Looking at what is the sad remains of my tank sticker, I come to the conclusion that the car came with a 3:36 Ratio (numbers / letter look like "336 R..." in upper right corner of build sheet). The car has M20 trans.

Does this conclusion make sense before I start this part of the restoration?

Thanks again!

[Duke W.]

Re: Axle ratio clues

3.36 was the standard axle ratio with M20, so most likely that's what your car originally had, but 3.08 was optional.

It's very difficult, if not impossible, to read the axle data with the car on the ground during Flight judging, so even your seventies carrier will probably not result in a deduction if you have it judged, and it would not be an issue in PV.

The 3.08 ratio is likely more suitable for the type of driving we do nowadays, so my recommendation is to retain it. Finding a good set of used OE gears and getting them setup properly can be a crapshoot (and expensive), and aftermarket gears are sometimes noisy.

Rebuilding the axle with a new Eaton Positraction case makes sense, but if the ring and pinion are in good shape, keep them. Be sure you know what you are doing in the rebuild or select a vendor who has experience and postive references with these axles. Attention to detail is critical to good operation and longevity.

A few years ago there was an article in the Corvette Restorer on the nuances of rebuilding the '63 to '79 axle.

Duke

[Patrick N.]

Re: Axle ratio clues

Thanks Duke,

I did learn in my prior thread that the dif is not a concern for judging so I'm ok with that.

I'm guessing that you are saying 3:08 is a bit more civilized, less torque? Is 3:08 a good match with the M20 to still deliver a bit of that "throw you in the back of your seat" feel? Is there any noticeable difference with a 3:36 over 3:08 for regular driving?

Thanks
Pat

[Edward J.]

Re: Axle ratio clues

Pat, 336 gearing more low end torque, 308 gearing better cruising at highway speeds.also a little better on gas. I like the 336 engine whines up a little quicker.somewhat more noticeable.

[Jim T.]

Re: Axle ratio clues

I have always liked the 3:08. My 64 convertible with 4-speed 327/300 had this ratio and I liked it. My 70 turbo 400 has a 3:08 and in almost 40 years I have put just over 160,000 miles on it. Really like seeing the lower engine RPM's on interstate trips.

[Duke W.]

Re: Axle ratio clues

A 3.36 will yield a little better off the line performace, especially with a small block, but it becomes a moot point with a L36/68 because you have more than enough torque to light the tire off the line and up to 15-20 MPH in first gear with either ratio.

Unless you plan to do some serious drag racing, you're better off with the 3.08.

If you want "more power" in the mid/upper range where it is more useable, massage the heads and retard the OE cam five degrees. That will more than make up for the 3.08 axle from 4000 up.

Duke

[Patrick N.]

Re: Axle ratio clues

fantastic feedback! I will let sleeping dogs lie and keep the 3:08 and move on.

Hope to eventaually show pics of the rolling chassis.Thanks once again.

Pat

[Gary R.]

Re: Axle ratio clues

Pat
If you have the diff out I would recommend pulling the cover and taking a look to see what you have. Yes it may be a 308 but the things I look for are if the gears are original or replacements. If original GM they will have a date code on the side of the ring gear along with the tooth count per ring & pinion. Next look at the shims in the housing by the caps, the orignals were cast iron but replacement are steel shims which I like better. Shoot some brake cleaner on the posi case by the radius openings and side hole, blow it dry and if you see any wet lines those are cracks and you need a new posi case.

With respect to Duke I would not use a new Eaton as the best choice for a replacement,nor would I use an Auburn cone type. If that posi is good I would rebuild it using solid steel clutches. Pollish & tune it. The 71-79's had the "snowflake" clutches which are not the best, also I don't buy into the fiber coated clutches.

The difference between a 308 & 336 isn't much and isn't worth the cost if everything else is otherwise ok. If you want more off line seat of the pants feel, then I'd use 373's. They are probably the most common ratio I setup, good for most 1:1 trans and overdrives.

Addressing these and other issues now will result in long life of the differntial and a job far superior to most common rebuilds.

Here is a link to the threads I wrote that will help you with yours. Good luck, if you decide to do this yourself it will be rewarding, if you have it done you will at least know what questions to ask someone. If I can help just PM your questions.

http://www.digitalcorvettes.com/forums/showthread.php?t=111107

[Patrick N.]

Re: Axle ratio clues

Gary,
just checked out the link-great step-by-step article. Thanks for the info on how to check out the internals. I like the test for checking if there are cracks in the case. My transmission was full of shrapnel (case was fine amazingly) so I will want the rear checked out and healthy before it goes back in.

I will open the case this weekend and reference your other link to begin the process and post if things do not look right.

Thanks!

PS-pics of my transmision

[Gary R.]

Re: Axle ratio clues

Glad to help, that Muncie looks wild! I have a M-20 & M-22 to do soon. The M-22 came to me in many boxes, the M-20 is right out of the car.

Whenever I get some time I'll write up those threads in format for the restorer too.

[Brandon T.]

Re: Axle ratio clues

Here is my op on this..My 68 is a l36 with the 3.08 and the trans is an m21! came like this from the factory.

and I LOVE it.

keep the 3.08 the big block atleast mine has plenty of power with the 3.08 if it were a sb I might want more gear bu the 3.08 works great with a bb.

you can just keepi that 3rd gear pulling and pulling on the hwy if someone new mustang wants to play or something. It's a hand full at hwy speed for even newer muscle cars.

[Duke W.]

Re: Axle ratio clues

Pat


With respect to Duke I would not use a new Eaton as the best choice for a replacement,nor would I use an Auburn cone type. If that posi is good I would rebuild it using solid steel clutches. Pollish & tune it. The 71-79's had the "snowflake" clutches which are not the best, also I don't buy into the fiber coated clutches.



\

It's my understanding that Auburn is the spun off old Borg Warner division that manufactured cone type LSDs. If fact, the Positraction in my Cosworth Vega's 7.5" axle is a Borg Warner cone type, and the same axle was shared with X-bodies, F-bodies beginning in '82, and S-10 trucks of the era.

It's also my understanding the the cone type is not as durable as the clutch type, but they are less expensive. The CV's Positraction has held up well at 80K miles (including about 5K miles of race track hot lapping), but it has a torque shy high revving 2L engine, so it doesn't place too much demand on the LSD.

In the past I've checked prices and the Auburn is cheaper than the Eaton, notwithstanding the fact that they want a ridiculous premim for what amounts to nothing more than stiffer springs for more preload.

I don't have any recent back to back experience with both in Corvette, but assuming you do, could you go into a little more detail including pricing differences.

I believe in the distant past I have recommended looking at the Auburn in lieu of the Eaton for Corvette axle overhauls. Given the fact that most vintage Corvettes see light duty service the Auburn may be a better choice, especially if it is still less expensive, but I can't quantify it.

One of the problems with the Eaton is case cracking at the window. Magnaflux inspecton of mine showed a crack there, so I replaced it with a new Eaton from GMPD, but that was back in the seventies when you could buy either an empty or loaded Positraction case.

As far as checking the axle is concerned, side yoke end play is a key measurment. If it's more than about .030" either the clutch pack or yoke ends are worn. A check of ring and pinion backlash should also be done. If the backlash is within spec and yoke end play within the above limit, it's probably okay to skip the axle overhaul other than cleaning any sludge out of the bottom of the carrier and installing new oil and Positraction additive.

Duke

[Gary R.]

Re: Axle ratio clues

It's my understanding that Auburn is the spun off old Borg Warner division that manufactured cone type LSDs. If fact, the Positraction in my Cosworth Vega's 7.5" axle is a Borg Warner cone type, and the same axle was shared with X-bodies, F-bodies beginning in '82, and S-10 trucks of the era.

It's also my understanding the the cone type is not as durable as the clutch type, but they are less expensive. The CV's Positraction has held up well at 80K miles (including about 5K miles of race track hot lapping), but it has a torque shy high revving 2L engine, so it doesn't place too much demand on the LSD.

In the past I've checked prices and the Auburn is cheaper than the Eaton, notwithstanding the fact that they want a ridiculous premim for what amounts to nothing more than stiffer springs for more preload.

I don't have any recent back to back experience with both in Corvette, but assuming you do, could you go into a little more detail including pricing differences.

I believe in the distant past I have recommended looking at the Auburn in lieu of the Eaton for Corvette axle overhauls. Given the fact that most vintage Corvettes see light duty service the Auburn may be a better choice, especially if it is still less expensive, but I can't quantify it.

One of the problems with the Eaton is case cracking at the window. Magnaflux inspecton of mine showed a crack there, so I replaced it with a new Eaton from GMPD, but that was back in the seventies when you could buy either an empty or loaded Positraction case.

As far as checking the axle is concerned, side yoke end play is a key measurment. If it's more than about .030" either the clutch pack or yoke ends are worn. A check of ring and pinion backlash should also be done. If the backlash is within spec and yoke end play within the above limit, it's probably okay to skip the axle overhaul other than cleaning any sludge out of the bottom of the carrier and installing new oil and Positraction additive.

Duke

Hi Duke,
Well the last Vega I was around was 30 years ago, a 72 with 70 LT1 in it. I don't recall what was used in the diff but the original one was long gone. I kind of like those old 72 vega GT's with a small block in them.

I agree the Auburn cone posi is ok for a mild application, most of the guys I work with run 400-1000hp in their vettes so those aren't going to last very long. They are less expensive then a new loaded Eaton. I'm guessing around $400 but I would never use one. The added cost of a custom built Eaton is worth it to me over a Auburn. Auburns are not rebuildable and the endplay varies with them.

The new Eaton posi's are available from any part supplier,ebay,vendors,etc. They make the 3 and 4 series. They run about $500. They are used by a lot of rebuilder because they are available, save time and they can make more off them- not because they're better then building one. They use one less clutch/plate per side, use the fiber clutches(which have failed in track use and smudged in my own hand), the strength of the spiders is also questionable in my opinion. The case is pretty good and once polished is a very nice base to build from. Again this is my opinion, gather several rebuilders and you're going to get different opinions. What I've found is few if any are polishing and tuning them, instead following standard rebuild procedures. This is ok but really doesn't address the crack concern you're aware of.

The early design eaton used in the corvette started back in '65 and ran through '68. It is the square window type and yes they were/are very prone to cracking. The 2nd design starting in '69 - '79 was much better. I have found them to crack but no nearly as much as the '65-'68's. The clutches in the '65 -'70 are the solid steels- or were until possibly rebuilt. The clutches in the '71-'79 are what I coined the term as "snowflakes" as they were cut to allow more additive in between the them to eliminate clutch hammer/chatter. It didn't work and many of these in cars that were beatup broke. The tolerance on the spider lash is 001-008 and the springs and plates,in my opinion, were nothing but a band -aid for loose tolerance on the line. Get a 007-008 lash and the springs tightened them up, get one in the 002-003 range and a lot hammered. They both provided "posi" just the combination of the tigher lash and constant spring pressure on the clutches caused the very common hammering. Using the GM additive may hel a lot of them but posi set is usually the best answer if the additive doesn't work.
As you can see in my link I don't use the springs or plates in any of my builds, this is something that has been around 40 years that I'm aware of dating back to Pepe Estrada and Tom Watt, now me. It works and is track proven- when setup correctly. If not then you'll have an open diff or a broken one.

With all that said, I prefer to build a posi from the best parts available, either in 10 or 12 bolt configuration. This is what I recommend.

Last note on the 63-64's they used either an open or Dana posi. The open I can get spiders for but if someone still has an original Dana posi they are not rebuildable. Part kits are lone gone and even if they were they're really not worth rebuilding as many failed under load,hence the Eaton use in '65.

Endplay is caused by:
1- worn axles (side yokes)
2-posi clutch setup or wear ( big area where some rebuilders are sloppy)
3- worn cross shaft case hole. I machine the case to install a 12 bolt cross shaft and usually can save the case and make it stronger.
4- Combination of all/any of the above.

Many times once there is 050" + endplay installing new or rebuilt axles will not get the play under 020" which is too much for a rebuilt diff. I find the tolerance on rebuild axles varies as much as 010" I usually have to machine grind them to fit a tuned posi and set the endplay to 005". With the 30 spline axles I build they require some more machine work and again set them to 005. These use the standard 12 bolt C clip not the thin snap rings used on the 17 spline axles.

I guess that's about all I can say, now go out and light up those Vega tires!