1968 4-speed ID

[Michael F.]

I've looked through a lot of 4-speed ID posts in this forum, so I'm confident I have confirmed an original transmission in my car. But I'd have a question on number I haven't seen discussed and also need help identifying a character after the date code.

On the passenger side, the case casting number is 3925660.
On the driver side, the casting number on the cover is 3884685.

On the passenger side is my VIN together with the date code P8E13 with a little c or g next to it. I know I have a Muncie 4-speed, original to my car, with a date of May 13, 1968. But what, if anything, does that little c next to the date mean?

Also, does anyone know what the metal tag attached to the cover bolt signifies? It has the number 915090 (or 91S090) stamped on it.

Pics attached. (Sorry for the neck strain. I tried rotating pics and uploading but it didn't work.)

Thanks,
--Mike

[Joe L.]

Re: 1968 4-speed ID

Michael------

I don't know what the character at the end of the manufacturing code is. 1968 Muncies did not use a suffix code.

The tag on the side cover usually is embossed with the transmission assembly part number. In this case, the first digit which should be a "3" is either missing or got obliterated by the bolt hole.

[Michael F.]

Re: 1968 4-speed ID

I can't thank you enough Joe! That missing "3" and subsequent googling helped to complete the puzzle. I found out that 3915090 is the RPO for an M21. Several search results mentioned finding this info in the AIM, so I checked it out. Now I finally understand how this book is arranged! The AIM, in section M20, page A1, lists the 3915090 as an M21 with L79. I already knew I had an L79 so now I have a more complete picture. Thanks for the help!!

I just bought the car last month, so I'm obviously doing a lot of research and fact checking on it so that I can figure out exactly what I have here. It's been a lot of fun so far. I think this weekend, after I install my new fuel pump, I'll crawl underneath again and look for rear axle codes.

Thanks again for your help!

Regards,

--Mike

[Douglas L.]

Re: 1968 4-speed ID

Just to add, and for informational use only, the L79 designation associated with the 3915090 part# in the AIM is kind of confusing. That part # is for ALL M21s regardless of what engine it came behind. I don't know exactly why it says L79 in the AIM but I suspect that its because you couldnt get a M21 behind a 300hp engine, only available with L79,L36,L68 and L71. As an example, my original engine/trans 68 L36/M21 car also has the 3915090 trans tab.


Also, your rear gears should be 3:70 or 4:11 as those were the only 2 ratios avaialbe with a L79 and a M21.


Doug

[Joe L.]

Re: 1968 4-speed ID

Just to add, and for informational use only, the L79 designation associated with the 3915090 part# in the AIM is kind of confusing. That part # is for ALL M21s regardless of what engine it came behind. I don't know exactly why it says L79 in the AIM but I suspect that its because you couldnt get a M21 behind a 300hp engine, only available with L79,L36,L68 and L71. As an example, my original engine/trans 68 L36/M21 car also has the 3915090 trans tab.


Also, your rear gears should be 3:70 or 4:11 as those were the only 2 ratios avaialbe with a L79 and a M21.


Doug

Doug------

Not exactly. The GM #3915090 was used for all 1968 applications using the M-21 transmission and having a rear gear ratio of 3.55:1, 3.70:1, and 4.11:1. Applications using the M-21 and having a rear gear ratio of 3.08:1 or 3.36:1 used transmission GM #3915091. The reason was that the speedometer DRIVE gear (installed on the transmission mainshaft) was different for 3.55/3.70/4.11 versus 3.08/3.36.

By the way, applications using the 4.56:1 rear gear ratio (rare; some L-88 only) used a different speedometer drive gear.

[Michael F.]

Re: 1968 4-speed ID

Thanks for this info. I found a similar discussion regarding the AIM information being confusing here:
https://www.forums.ncrs.org/showthre...ette-4-Speed-s

Joe, it sounds like the 3 different DRIVEN gears (for 3.55/3.70/4.11) are all compatible with the DRIVE gear installed in the 3915090 while the DRIVEN gears for 3.08/3.36 are compatible with the DRIVE gear installed in the 3915091. In other words the number and angle of the speedo drive gear teeth is different from the 090 vs 091, an only compatible with the corresponding driven gear. Can't use the speedo gears for 3.55/3.70/4.11 applications in a 091 drive gear because the teeth wont mesh. Does that sound right?

Question for both of you: where did you find the info on what rear gears were available with L79/M21 combinations? Is there a chart somewhere?

[Douglas L.]

Re: 1968 4-speed ID

Joe,
Thanks for the clarification. The L79 designation in the AIM makes alot of sense now as in 68 the L79 was the ONLY M21 application where you were guaranteed to get the 3915090 as the only ratios available with the M21/L79 combo were 3.70 and 4.11 while 3.08 and 3.36 were optional with M21 equipped L36/L68 cars and 3.36 with the L71/M21 combo.
Thanks again, Doug

[Joe L.]

Re: 1968 4-speed ID

Thanks for this info. I found a similar discussion regarding the AIM information being confusing here:
https://www.forums.ncrs.org/showthre...ette-4-Speed-s

Joe, it sounds like the 3 different DRIVEN gears (for 3.55/3.70/4.11) are all compatible with the DRIVE gear installed in the 3915091 while the DRIVEN gears for 3.08/3.36 are compatible with the DRIVE gear installed in the 3915091. In other words the number and angle of the speedo drive gear teeth is different from the 090 vs 091, an only compatible with the corresponding driven gear. Can't use the speedo gears for 3.55/3.70/4.11 applications in a 091 drive gear because the teeth wont mesh. Does that sound right?

Question for both of you: where did you find the info on what rear gears were available with L79/M21 combinations? Is there a chart somewhere?

Michael------


Yes, that's right. However, you erred in your first use of the part number as I've shown in red. It should be 3915090.

Available engine/trans/rear ratio combinations can be found in the NCRS Specifications Guidebook as well as other references.

[Michael F.]

Re: 1968 4-speed ID

Michael------


Yes, that's right. However, you erred in your first use of the part number as I've shown in red. It should be 3915090.

Available engine/trans/rear ratio combinations can be found in the NCRS Specifications Guidebook as well as other references.

Oops, yes. Thanks for pointing that out. I'll edit to correct it.

[Douglas L.]

Re: 1968 4-speed ID

Question for both of you: where did you find the info on what rear gears were available with L79/M21 combinations? Is there a chart somewhere?

I use the sales brochure. Here's a link to both early(with mistakes) and late versions.
http://paintref.com/cgi-bin/brochure...&page=1&scan=1

Doug

[Joe L.]

Re: 1968 4-speed ID

Question for both of you: where did you find the info on what rear gears were available with L79/M21 combinations? Is there a chart somewhere?

I use the sales brochure. Here's a link to both early(with mistakes) and late versions.
http://paintref.com/cgi-bin/brochure...&page=1&scan=1

Doug

Doug------


That works unless one is interested in an L-88.

[Michael F.]

Re: 1968 4-speed ID

Slick. Thanks!