66+ Differential Unit - Posi vs. 4-Spider Posi

[Rotary FP46]

Found this differential and transmission on a recently-uncovered shelving unit.

Running into a question about determining whether it is marked accurately.

I've read and been told that the quick way to ID a posi differential is to spin a drive tire / axle. Idea being that if both axles spin in the same direction, you're working with a posi.

Does this hold true for 4-spider posi units, or do those behave differently?

Separately, if this is tagged accurately, what would have been the most likely range of applications for that setup?

Based on the shared mess pattern, the transmission parked right next to it was its mate. Will need to learn how to check the gearing on those.

[RexJohnson]

A working posi will spin both yokes the same direction no matter how many spider gears it has. Taking the cover off and looking inside can tell you more than a tag can.

[Rotary FP46]

I ended up pulling off its top cover. This is what I found.

The ring gear has 32 teeth. The pinion gear has 9 teeth.

When I spin just one axle, the other turns in the opposite direction.

While spinning just one axle, the carrier does not move.

While spinning both axles in the same direction at the same time, the carrier and pinion shaft spin.

[texas yenko dude 199]

That is a Crown 4 spider carrier and it does have a POSI cover so it is tagged correctly

[RexJohnson]

I used the term working posi for this reason. Several years ago a club member got a new project with the transaxle out so I was taking a look at it. He asked if it was a posi? I spun one yoke and just like yours the other side went the opposite direction. So I said not a posi. When I took it apart it had a posi that someone put the plates in upside down so that there was no posi action. It is always good to just pull the cover and count the teeth. I have changed to many gear ratios to trust the codes after all of these years.

[martyscarr]

Rotary FP46 wrote:

Will need to learn how to check the gearing on those.

If you remove the side cover of the trans, you should be able to count the teeth on the cluster gear. Front to back they should be 29-23-19-15-15

On the main shaft the tooth count should be 18-21-26-29
Modified transmissions will have a different tooth count.

what would have been the most likely range of applications for that setup?

A high torque/HP application, good for a V8 powered car.

HTH
Marty Scarr

[Rotary FP46]

Rotary FP46 wrote:

Will need to learn how to check the gearing on those.

If you remove the side cover of the trans, you should be able to count the teeth on the cluster gear. Front to back they should be 29-23-19-15-15 stock saginaw cluster.jpg On the main shaft the tooth count should be 18-21-26-29
Modified transmissions will have a different tooth count.

what would have been the most likely range of applications for that setup?

A high torque/HP application, good for a V8 powered car.

HTH
Marty Scarr

Removed the side cover. For the most part, gear tooth counts on both shafts match what you have listed. Couldn't find a reference point on the last gear of the cluster. It likely has 15 teeth, but I cannot honestly say I took the extra steps needed to get an accurate count on that one.

Just a normal 66+ 4-speed, it seems.

[jimbrandberg]

It's a little tricky getting that side cover with the forks back on.

[Rotary FP46]

It's a little tricky getting that side cover with the forks back on.

Not so bad, as it turned out.