Correct rear differential for 1966

[Vincent A.]

All,

I'm looking for advice on the correct rear differential for my 1966 427, powerglide coupe. Options for the car are L36 (427/390), C60 (Factory AC) M35 Powerglide, Side Exhaust Corvette coupe. Other options are Pwr windows, Pwr steering and brakes. Build date of the car is May 9, 66. Very few where built this way, approx. 12 coupes.

My question is in regard to the rear differential. The differential is dated AL 6 14 65 (June 65 327 3.08:1) and the folks who owned the car before me believe this was the original rear differential as there was no specific code for the BB/PG combination in 66 and this RD may have been used from the prior year.

Does anyone have insight as to what the correct RD codes would be for this car, or have a 66 BB/PG that they can share the rear differential cast and assembly codes/build dates?

Thanks very much!

Vince

[Jack M.]

Re: Correct rear differential for 1966

Vince- First, CONGRATS on a fantastically optioned '66. I can't be certain of the validity of the information I'm presenting... you may have seen it before, so take it for what it's worth. It seems the majority of early '66 power team options restrict the powerglide transmission to the base engine. I did find one document (dated March 16, 1966) which confirms the possibility of a powerglide with the L36 engine. As for rear axle ratios, it lists them as similar to the 427/390 with a 4-Speed Close Ratio. For details, take a look at page #18 of this PDF document (compiled by GM Archives): CLICK HERE

[Vincent A.]

Re: Correct rear differential for 1966

Thanks very much Jack for the info and nice words. Yes, its a cool car and I'd like to get it back to as ordered (tough without the original paperwork). However this spec sheet is very helpful, based on this 3.08:1 "Economy Cruise" was an option for the L36, so the question is the dating back to June 65 date and AL (327) code.

Thanks

Vince

[Gary R.]

Re: Correct rear differential for 1966

Just about all the diff's I built/build used a 308 gear ratio with automatics. Some used 336's and 355's.

[Vincent A.]

Re: Correct rear differential for 1966

Thanks Gary. Any thoughts on the dating? Have you seen other instances where the diff build date was quite a bit in front of the car build date?

[Gary R.]

Re: Correct rear differential for 1966

Hi Vincent,
To be honest I generally don't pay much attention to numbers unless there is a problem or question. Typically the assembly stamping is 2-3 months after the casting date of the carrier (housing). If you know the history of the car I would guess you have a 308 however, the 66 diff's had some weak areas, notably the posi case and spiders so it could be changed around if the history is unknown.

Case in point a local guy brought his "original" 79 diff in to me to custom build. When I opened it up I found an open case in it, then I checked the housing casting date and numbers and found it was a '67 open diff. When I cleaned off the outside I saw "67 open for sale" painted on it. The guy was told the car was 100% original and untouched. Another example, I have in a 67 BB box that was rebuilt before but supposed to be correct. It was correct but the worm nut was flipped to try and save worn out parts. It didn't work. So I would recommend you check it close and see, you can always mark the pinion yoke and count the number of turns at a wheel to see if it is close to ratios of the day, 308, 336,355, 370.

[Vincent A.]

Re: Correct rear differential for 1966

Thanks Gary. Unfortunately I don't have a complete history of the car. Suspect it's not posi. Will definitely take it apart at some point and then try the ratio analysis you suggest.

Vince

[Gary R.]

Re: Correct rear differential for 1966

Vince you don't have to take anything apart to check it, just get the rear wheels off the ground. To prevent u-joint binding you can put a deep well socket under the TA pivot point to raise them up to ride height. With the trans in neutral you can mark and count it. You can also check for stock posi by holding one wheel to see if the other will spin free or not. This holds true for stock posi builds as mentioned in another current thread but if someone was to check a tuned posi built by myself or Tom's differentials it would appear as an open diff.

[Jack M.]

Re: Correct rear differential for 1966

However this spec sheet is very helpful, based on this 3.08:1 "Economy Cruise" was an option for the L36.

Keep in mind, your build date is close to the start of the 1967 model run... and in '67, the ONLY rear axle ratio available with a 427 + Powerglide + A/C = 3.08 (no other axle options were listed for that combination). Unfortunately, it also seems that in '67, those 3.08's were required to be positraction. During your '66 mid-year change, a number of possibilities exist... without original documentation, one may never know your specifics.

[Vincent A.]

Re: Correct rear differential for 1966

Thanks Gary

[Vincent A.]

Re: Correct rear differential for 1966

Thanks Jack, I'll keep at it

[Bill L.]

Re: Correct rear differential for 1966

All,

I'm looking for advice on the correct rear differential for my 1966 427, powerglide coupe. Options for the car are L36 (427/390), C60 (Factory AC) M35 Powerglide, Side Exhaust Corvette coupe. Other options are Pwr windows, Pwr steering and brakes. Build date of the car is May 9, 66. Very few where built this way, approx. 12 coupes.

My question is in regard to the rear differential. The differential is dated AL 6 14 65 (June 65 327 3.08:1) and the folks who owned the car before me believe this was the original rear differential as there was no specific code for the BB/PG combination in 66 and this RD may have been used from the prior year.

Does anyone have insight as to what the correct RD codes would be for this car, or have a 66 BB/PG that they can share the rear differential cast and assembly codes/build dates?

Thanks very much!

Vince

I would offer that there were probably "FEWER" than 12 1966 coupes set up this way --- it just all comes down to how you "run the numbers" ---

I have personally seen three coupes and one roadster with these options A/C, Auto PS, PB etc --- the last one I inspected was a coupe and it had a 3.36:1 rear end...with an automatic.

Regards
Bill

[Bill L.]

Re: Correct rear differential for 1966

Duplicate post deleted ---

[Vincent A.]

Re: Correct rear differential for 1966

Thanks Bill, yes the 12 count is just for the L36/M35 (BB/PG) format. On top of the options it's trophy blue, 1966 only. The rarity is one of the reasons I really like the car.
I've noted in some rd tables that 3.36:1 was the default or "all other" configuration. Based on the spec sheet Jack sent above the 427's should of had a posi rd, but hard to know. I'll stay at it.
I should look to see if there is a 1966 registry out there...
Vince

[Joe R.]

Re: Correct rear differential for 1966

Vince,

try the C2 registry.