I pulled my engine and trany today. I found the trans tag still on the top side cover bolt bent to the top of the trans. It reads 3890534. I did some searching and it comes up L78 M20. I know my car is a original L34?

Luke,
Check the rest of the casting numbers and stamped numbers on the transmission to see if it is original to your car. Most important would be a partial VIN stamped on the transmission to match your car.

Does the vin #' on the maincase match the vin on the the pillar? If vin is not there does the assembly date predate the build date of the car? Maybe the side cover or entire transmission has been replaced?

The numbers stamped on top of trany match the vin# and the numbers stamped on the side are also dated correct for the built date of the car.

I'm surprised the tag was on an upper bolt rather than a lower bolt, but at least it is there. What is the rear end ratio? I'm thinking that the transmission numbers may have to do more with what speedometer gear is installed, rather than what engine was installed, although the engine also helped determine what rear ratio was standard. Whatever, I guess an M20 for an L78 should be compatible with an L34 if there was a shortage of the 3870357 M20 at the time your car was built. We can't question the originality of the transmission, possibly only the tag, if the tag could have been switched at some point in time. That thought is why I made mention of the location of the tag.

Code on the rearend is 3:73 posi. The car is a 64K mile original built in Fremont Cal. The drivetrain has never been out of the car until Today. I'm the 3rd owner.

Code on the rearend is 3:73 posi.

I would speculate that the 3.73 could be the reason. I wonder if the difference between the two part numbers could be attributed to whether a 7 or 8 tooth worm gear is installed? M20s were normally used with 3.55 or lower numeric gears. The 3.73 ratio was the standard ratio with the L34, but that usually was teamed with an M21. Interesting! Have you verified that you have an M20?

Yes, 2 grooves on the input shaft.

Was the L78 even available from Fremont? Trans and tag would have been installed at the Towanda plant before shippment to Fremont. Tag would have been installed on transmission to identify it; but when they put the trans and engine together they probabley just used the input shaft to identify wether it was a M-20 or M-21. So the wrong tag was put on at the muncie plant or someone had the cover off and put the wrong tag, or wrong cover and tag back on. So do you have the pop or original paper work to identify 1st owner and sale to 2nd owner.?

Luke, being the third owner it is very hard to know what is original and what is not. I am the original owner and my trans does not even have a tag. I have forgot all the work that I had done on the car over the years. I had the radiator recored at some time and neither I nor my brother can remember ever having it done.

The trans part no. had nothing to do with rear end gear ratio. The Assembly instr. manual (Sec. M-20 page A-8) refers to a chart which specified the speedo gear to be inserted into the trans by the assembly line worker. The speedo gear usage depended on the combination of the rear end ratio and the tire size, so there would have been dozens of possibilities. But there are only two 66 big block M-20 transmission part nos. So the appropriate speedo gear got inserted at the final assembly plant, not at the Muncie plant.

If the engine and trans are both verified beyond a shadow of a doubt to be original to the car, then I think there are three possibilities on the trans in question:

(1) Somebody took a tag off another trans because the original was damaged or missing, or

(2) The Fremont assembly line worker grabbed the wrong trans by mistake, or

(3) As someone else suggested above, there was not a correct 3870357 trans available at that particular moment. They were supposed to "trade up" to the closest "better" part in that situation. I don't know what the exact difference was between the L34/L35 Muncie (# 3870357) and the L78 Muncie (# 3890534), but presumably the L78 version was "better" in some way than the L35 version, and that is therefore the one that would have been substituted.

The other possibility is that the engine in the car has been changed and restamped at some time in the past and the car was really an L78. Or the trans has been changed and restamped. Sounds from Luke's posts like he's confident that's not the case though. Do you have the POP for the car? What is the trans assm. date and the cowl tag date?

Was the L78 even available from Fremont?

Yes, there are numerous documented examples.

Trans and tag would have been installed at the Towanda plant before shippment to Fremont.

Did you mean at the "Muncie" plant? Tonawanda didn't build the Muncie transmissions.

All I can say is this car came from the factory with that tag on the trans, The original owner had the car until 1986 and I bought the car from the second owner in 2003 and he had no clue what numbers even were. Hey maybe the car was a L78, not a bad problem to have.lol

If engine is original you should be able to determine if it is a L78 or L34.

My goof, original tag (wrong or correct) would have been installed at muncie plant. Then trans installed on engine before shippment to Fremont. Thus the engine pad would have the T for towanda, the date, then the suffix code like EJ for a L34 or EG for a L78.

Guess my information on a L78 being from Fremont was incorrect. I was told they were not available due to the smog regulations ; but was told there were L78 cars sold in CA that were built at different plants like Kansas City, Atlanta, etc.

Engine pad stamped T1208EJ 6Z1153XX Trans stamped [side] P1115 [top] 6Z1153XX

Where did the info come from that tag 3890534 is L78 M20.

czeto- What were you and your brother smoking back in the 60's. LOL

Looks like as suggested, someone at the towanda plant grabbed the wrong M-20, if there was even a difference between the L35/L34 M-20 and the L78 M-20. Or as also mentioned maybe the cover was off and wrong tag replaced because of location of tag. A couple of my books had the number for the M20 as different for the L78 as opposed to the L34/35. As always information could be wrong. Since you have a Fremont built car, maybe you have or can find the build sheet.

I did a little research on speedometer gears. It looks like an 8 tooth drive gear could cover the possible variation of rear end ratios from 3.07 through 4.11, and my parts book confirms that a black 23 tooth driven gear would be correct with 3.73 ratio and 7.75 tires. So it is possible that both transmission part numbers had the 8 tooth drive gear. So I don't have an explanation of the difference between the two transmission part numbers. The two part numbers are listed in the Assembly Instruction Manual, Luke.

Jim, just a correction; the transmission would be mated to the engine at the vehicle assembly plant (Fremont in this case), not the engine assembly plant (Tonowanda).

Thanks! for all feedback.

Luke, How did you determine that # goes with L-78? Just curious because my Fremont L-34 has the same number and I don't remember finding that # associated with L-78 when I researched it 5 years ago.

http://www.chevelles.com/forums/showthread.php?p=1783494

http://www.chevellecd.com/66cd/etr/transmission/manual.htm

Check these links out.

Mark, what was your cars built date?

Mark, what was your cars built date?

Interesting. My car is a late Dec 65 build. I also have the 4 bolt main 962 block.