[GEmrsn]

Anyone know which color gear is correct for a 67' BOP (Pontiac) M20 3:90? Also, what is the difference between the BOP and the Chevy gears? I have the red with 21 teeth in there now but it's from a 3:36 car. Also have the natural one with 19 teeth but was told that is for a 3:55? Does the driven gear from the 3-spd Dearborn work also?

[jeff chmura]

Try this link....
https://images.app.goo.gl/Ry3yDGt8Fuep7h7a9

['ol Pinion head]

GEmrsn, the issue will end up being.....

99 & 44/100ths of all factory assembled M20's did not have 6 tooth drive gear on their mainshaft. Every factory assembled Muncie 4spd that I've ever looked, at or reassembled, that was originally mated with 3.90-4.56 gear rearend HAD to have a 6 tooth drive gear. The chart that Jeff posted, takes that's a given. Would bet your M20 has a 7 or 8 tooth drive gear.

[turbo69bird]

I’ve been told that the chart is wrong by guys who do muncies trying to find this info myself except w an M21

[unruhjonny]

Originally Posted by GEmrsn

Anyone know which color gear is correct for a 67' BOP (Pontiac) M20 3:90? Also, what is the difference between the BOP and the Chevy gears? I have the red with 21 teeth in there now but it's from a 3:36 car. Also have the natural one with 19 teeth but was told that is for a 3:55? Does the driven gear from the 3-spd Dearborn work also?

I will assume you know some or all of this, but will elaborate just incase anyone can use the following information;

This is a slightly flawed query (this is what OPH pointed this out);
To suggest that a given gear is for X.XX ratio differentials is actually wrong;
Its a combination of gears that nets the result you want.

There are two (and a half) variables when ever looking for the correct speedometer cable gear;
1) gear installed on main shaft (I believe this is called the drive gear)
2) speedo cable gear (I believe this is the driven gear)
2.5) presence or absence of an adapter.

The other variable is the wheel/tire combination, and the OD (outside diameter) therein;
I believe that different models had different wheel tire combinations, and as such, there were variations in the combination of gears used for a given differential ratio depending on the model.
I do not believe there were Chevrolet or a Pontiac drive or driven gears;
The gears are going to be specific the the transmission/drive train.

Since a given model only has (essentially) one OD for the wheel tire combination, then if you have an OE wheel/tire combination and a mainshaft/drive gear that aligns with a master parts catalogue application chart, you should be able to select a driven gear from that chart.

If you want to know what driven gear (or combination of driven gear and adapter to use) you would need to start by sharing with us which drive gear is on your M20's main shaft.
If your wheel/tire combination is not equivalent to a factory height setup, I would suggest that you should share that too.

[turbo69bird]

Here’s the chart I was referring too

[turbo69bird]

Must have been the green gear they said was incorrect on that chart.