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differintial question



I was told I had a 4.11 posi in a 61 driver I recently purchased, but would like to confirm. Cant make out the stamp on passenger side of axle but have this on the drivers side
GM 1/2 P
Anybody got any clues?

Also can't get to the engine codes on front of block but have 3970010 6 GM on drivers rear and f220 on other side. Supposed to be 1970 LT1 any other way to confirm?
Mike, An easy way to see what you rear gears are is to mark the yoke with chalk, or somthing, and then spin the wheel around one turn and count howmany times the mark goes around. If it goes around just past 4 times, it's a 4:11. If it goes around just past 3 1/2 turns then they are 3:55's etc. As far as the "P" that means that it is a Posi housing. I'm not sure what the other numbers are (no book with me) But I bet Tom or one of the other guys can tell you. As far as the motor goes, that is a pretty comman casting number for Corvettes and other Chevys I would guess that the f220 is the build date which could be June 22 1970 if I was guessing. The numbers you really need to proove anything are the number on the front pad......I hope this helps.........Steve

With the rear axle off of the ground when you turn one wheel, if both wheels turn the same direction you have a posi rear. If the other wheel turns in the opposite direction you have an open rear. The spamped code for a 1961 with 4.11 gears is AP for manual trans and positraction, AT for man trans, posi and HD brakes and suspension, and FL for man trans and metallic brakes. The other numbers are casting numbers and dates, ect. Of course some previous owner could have changed the ring and pinion at some time to a more streetable gear or even removed the posi unit.

The 3970010 350 block is a common block used in cars and trucks. It was available in 2 and 4 bolt configurations. Steve has the casting date right. You need the numbers off of the rh front pad to determine what it is. If it is a LT1 the number will start with V for Corvette engines built at Flint in '67 and later. Next will be the build date. 4 didgets with the first 2 being the month and the last 2 being the day. The stamping will end with a 3 letter suffix code that tells the usage of the engine. A '70 LT1 will be CTU or CTK (new code interduced during the production year.

For other specifications on the C1 Corvette check the specification pages on the front home page.


[Edited by 59Tom on 02-04-2001 at 12:48 PM]
The K663 is the casting date for the case itself.
This means it came out of a 1964 passenger car that was probably built in Nov, maybe Dec, 1963.
The 55-64 passenger car and 56-62 Corvette rear center sections (pumpkins, carriers, whatever you choose to label it) are totally interchangeable.
If the P that you posted is a great big P, that designates Positrack. If so, this is what people commonly refer to as a "P" case.

[Edited by DZAUTO on 02-28-2001 at 01:28 PM]
Hey just head out on the highway, if your pushing about 3 grand on the tach and only doing about 55 mph. and everyone is passing you giving you a weird look because you have this sports car that can't do over 60 miles per hour then you have the 4.11. But it sure is a lot of fun on the side streets. If you want to change the gear then most likley the pig will have to be machined to fit the new gear. A good ring and pinion shop should be able to set you up.

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