tach drive 61

My question today is I have a posi rear pumpkin loaded with 308 gears. i think it was out of a 63 impala . the case has a casting # of 3789812 and its a P case its date code reads J 1862 there is also a stamped # on rim where it would bolt on A X 10 . a flat section near the pinnion has 4 stamped there seems like a cast and no following #s , I did see a rear listed on ebay. and after the 4 was a stamp of 11 showing a 4 11 rear , OK Now the question can i have 411 or 456 ring and pinion installed in the 3789812 case Now my c 1 is running its open 370 unit and we have ran this case with the 308 set .

MikeM

I don't know about all those numbers you're tossing out there but you can use a 4.XX gear on a 3 series case if you use a readily available ring gear spacer.

vt65

the 4.11 that I just removed has 3733908 9-37 stamped on the crown gear, also GM 6-64-1

tach drive 61

what is the differance between the two cases ?

MikeM

The four series gears are thicker to space the teeth over closer to the smaller pinion gear on these ratios. That means the three series cases are offset a little further than the four series because of the fatter pinion gear. The spacer makes up the difference.

DZAUTO

If you're referring to the difference in the OUTER case between a 3-series and a 4-series, THERE IS NO DIFFERENCE!
The difference between a 3-series and a 4-series is the posi unit itself.
The flange where the ring gear bolts on, is further away from the pinion gear on a 3-series posi unit (as already pointed out). Also, as mentioned, to install a 4- or 5-series ring gear on a 3-series posi unit, a spacer plate can be used to move the ring gear closer to the pinion gear.
Both of these are non-posi units, but they show the comparrison between the flange position of a 3-series (left) and a 4-series, which will be the same for posi units.



Tom Parsons