Westlotorn

I pulled the numbers off my differential today, from what I can see, which was hard to get a clean read, it looks like my differential is marked " A O 10565 The last 5 is very difficult tor read but I think this is what is stamped on the rear Differential.

From what I have read I think this is a 3.70 Posi, 4 speed 327.
I will look again to make sure it is not an A Q 4.56 posi.
It is a L79 4 speed original car but I had never checked the differential.
I have not yet driven this car. It is a long project waiting for time and money. I have been aquiring parts and planning ( dreaming ).

Some of the rear diff ID numbers indicate Wide or close ratio on the 4 speed, any idea which this car should have, I suspect wide ratio M20would be the standard.

Thanks

postalpete

I think Your "AO" is probably right. The "AQ" would I think be a rare find and sure not something I would want to drive on the street for very long. My 66 L-79 has a "AN" (3.55) and a M-20. I think that is pretty common among L-79s and a good combination on the street.

Vet65te

My 66 L79 has the same AO (3.70) designation on the diff. I think I have a pic of it somewhere. The trans in my Mosport Coupe must have gotten destroyed long before we got the car in '89 so it now has a wide ratio Muncie in it but from what I've seen, most L79's with 3.70's came with close ratio boxes but didn't think the diff code would shed light on whether the car came with a close or wide ratio box.
Mike T.

Westlotorn

The cheat sheet I have shows differential codes for small block and big block, some of the codes indicate which 4 speed and some do not. Some show the car could have had a powerglide. The AO code says Positraction 4spd 327. Thanks for your imput.

Plasticman

My 66 L79 Muncie 4 speed was AO. I thought the AO (3.70 gears) were the "standard" with that combo. Other gears could be ordered, but......

Plasticman

jdk971

remember how old these vehicles are. some one could of changed the gear set over
the years and left the tag. chances are they would remove the tag. but ya never know.
jim

Westlotorn

Mine was only 12 years old when it got parked in 1978, I bought it in 2007. Still parked in a garage lying in wait. Full restore needed.

KS69Coupe

Same here, 1966 Coupe with AN 3.55 differential with 327 engine and M-21 close ratio. Pretty nice combination for driving. AO is probably correct. AQ is unlikely.

65tripleblack

Don't trust the code numbers. The gears could have been changed. The only way to verify what you have is to rotate the wheels and mark/count the shaft revs.

Westlotorn

Thanks guys, I will go through the differential once the frame is done and for sure I will know the ratio when I pull the cover but I was curious today and pulled the numbers just to check. I appreciate the help.

JohnZ

The '66 L-79 could be ordered with either the M20 wide-ratio or M21 close-ratio box (first year that choice was available); 3.36 axle was standard with the M20 with the 3.55 optional, and with the M21, a 3.70 was standard, with a 4.11 optional. No other ratios were available with the L-79.

tuxnharley

I guess that was changed for '67, then? My '67 L-79 was built (I have the window sticker and original bill of sale) with a 3:36 axle and a M-21 transmision (2:20 1st gear ratio). Not a good combination for making a clean smooth start! I've got a Tremec 5spd with a 3:27 !st gear in it now - a much better combination for all around driving, especially with the .67 OD 5th gear!

JohnZ

'67 was the same as '66. The 3.36 wasn't available with the M21 per the restrictions on the '66 order form, but some otherwise "non-available" ratios could be requested as an over-ride if the Zone Office approved them; that's what was apparently done on yours.