8” & 9” rear end? Poor man posi w/washers? - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 5 (permalink) Old 03-18-2018, 05:36 AM Thread Starter
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8” & 9” rear end? Poor man posi w/washers?

Hey guys, looking for info about taking a differential and packing the washer, turning it into a limited slip posi kinda thing?

Anyone remember this trick? Experience?

Here’s the deal. I have a 64.5 that was mechanically completely gone through 20 years ago. I sold the car unfinished and the buyer never finished either. It’s now back with me to finally finish and I’ve forgotton what the deal was with the rear end.

A wizard with Ford set up my 8”. It was a differential 28 spline, 3:40 Richmond gears.

The guy who set it up really knew his stuff. He set my rear end up and told me it would be stiff and tight for about a thousand miles or so, but the “special washers” would wear and what I would be left with is a differential that is stronger than a locker or stock posi but would act as a differential “eventually” around corners and a posi under full throttle.

Anyone know about this trick? How reliable, how noisy?

When jacked up, I can turn one wheel and both turn together. It’s very hard to roll the car in and out of the garag. I mean very hard. It takes at least two grown men. It is the rear end, no drive shaft, front wheels spin free.
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post #2 of 5 (permalink) Old 03-18-2018, 06:04 AM
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Re: 8” & 9” rear end? Poor man posi w/washers?

I do a lot of 8 and 9 inch Ford rears. If you are only using it for the drag strip, maybe, but I wouldn't do it. In the end, as that 1000 miles wears those shims, the teeth on the gears, and works all the spider wear surfaces, the metal is going somewhere, and that is in your oil.

Additionally, it is generating heat, also not good. In the end, there is no free ride. It's basically a tight diff, how loose it gets, depends how it wears. If it were me, I'd pull the center section, buy a traction lock or some other posi, or set it to factory specs.

70 Sportsroof, 427 FE/489 cid, TKO-600, 31 spline 4.10, A/C. modified Mass-flo EFI/reprogrammed A9L/CnC ported Victor.
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post #3 of 5 (permalink) Old 03-18-2018, 10:19 AM
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Re: 8” & 9” rear end? Poor man posi w/washers?

I work as a trans re-builder and the shops I have worked in all do diffs etc as well.

I've been setting up diffs for years...

Young kids with Honda's want the latest trick stuff on their car..
One guy brought in some pieces while we had his FWD standard trans-axle apart.

One of the things was a "posi" kit that consisted of thicker washers to shim the side gears and make it a "posi".
Also some kind of pre-load thing that got jammed in between the side gears.
It certainly tightened up the diff... for awhile...

I had my doubts... but it's the customers money and parts so I put it in.

It lasted all of one road test and when we put it back up on the lift the diff acted as a normal open diff.

Right now we have a BMW diff on the bench.

From factory the spider/side gears are "tight" but no clutches in there.
The spider and side gears are all pitted on the tooth face and need replacing.
The rub is that you can't buy new replacement parts for this particular model.
All "good used" ones have the same problem.

The dealer can only supply some seals.
If you want internal parts they sell you a whole new diff for $1700.00.

Anyway... point being that My427stang is correct.

Shimming side gears is a losing game.
It puts excess stress on the spider and side gears
and fairly quickly "wears away" any posi action and puts metal particles in the oil.

Buy a "real" posi of some kind if you want posi action.

I would recommend the Detroit Tru-Track. Pricey but strong.
Works like a open diff under normal driving and locks solid when you hit the loud pedal.

I know some guys swear by shimming the side gears for a poor mans posi, but I wouldn't do it.

Sorry for the long post.
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post #4 of 5 (permalink) Old 03-19-2018, 04:24 AM
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Re: 8” & 9” rear end? Poor man posi w/washers?

I remember guys welding the spider gears back in the 50's and early 60's to make a locked rear for drag racing. Hope that's not what the prior owner did.

1966 Falcon, 347 stroker and C-4
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post #5 of 5 (permalink) Old 03-27-2018, 09:22 PM
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Re: 8” & 9” rear end? Poor man posi w/washers?

I did this to my 8 inch many years ago. It held up ok until I invested in a sticky set of street tires. The extra traction going around corners wore out the washers pretty quickly and turned it back into a regular open diff.

1968 mustang 408W Hybrid..Burns gas and rubber!
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