BackInTheGame

A couple questions for the general audience:

The 1968 CSM states on page 7-619 that you can differentiate between the M20 and M21 by locating "HP" (M20-wide ratio) or "HF" (M21-close ratio) on the right/passenger side of the case. The letters are supposedly 1" high.

While still mounted to the engine in the car, I've not found these markings on my trans. Has anyone else found them? If so, did the designation hold true?

In another document (I don't recall where at this moment) I came across some info that the factory would make a decision which trans you received (M20/M21) dependent upon which differential gear ratio was ordered. My '68 4-4-2 came with 3.23 plain-Jane non-limited slip. Any ideas from the crowd as to which trans the factory would have selected for mine, if they did indeed made the selection?

allyolds68

A 3.23 would only be an M20. The 3.42 was the only rear ratio that you could get an M20 or M21 in 68 (domestic cars)

Octania

what?
No 3.73, 3.90 or 4.33 option, of those were Auto Trans only?

I think painted ID might have been handy when they were manufactured, but hard to spot these days... though I DID just see the ghost of "OH" in huge letters on a '68 OH trans I sold...

If you want to know for sure, count turns of input/ engine vs. turns of output, in first gear... or remove side cover and count teeth.

oldsmobiledave

Since the M-20 & the M-21 use a different 1st gear ratio you can use a GPS speedo app on a smart phone along with your known rear axle ratio, your tire height and a tach to discern which trans is in the car. Take the car to a quite location & observe your mph & rpm while driving in 1st gear. From there it is grade 9 math to figure out which 1st gear ratio you have....2.20 or 2.52. If the former you have an M-21 & if the latter you have an M-20.


PS...you might have to find a grade 9 student to help with the math.

allyolds68

I thought the OP asked about MT. M20 is 3.42 and numerically lower & M21 is 3.42 and numerically higher per the assembly manual.

BackInTheGame

LOL! Thanks OldsmobileDave! BTW I actually excelled at math in 9th grade. Now that 9th is about 35 years behind me, I find tracking the variables to be problematic, LOL! About all I'm good at nowadays is the ratio of two-stroke oil to gas for my dirtbike. This one I can figure off the top of my head, HAHA.

I actually already have a GPS/Speedo app on my smart phone. I had to figure out why cars were passing me on my sport bike when the OEM speedo was showing me to be speeding. It turns out that the tire/drive-/driven-gear combo is not calibrated to the speedo. By a factor of about 8%.

Allyolds68, would you please share the section/page where you found your info in the Assembly Manual? I must have overlooked it in mine.

Thanks for the input, guys! Much appreciated.

allyolds68

Section 4 page 104