As I put my ES back together, I have the option of installing either an M 41 or an M 410, as I have both, including bellhousings. As I understand it, the 410 is a stronger unit, but i have no information on the gear ratios. Anyone have the ratios for both? Any other comments on this swap?

M410: (according to 1972 164 green manual)
1: 3.54
2: 2.12
3: 1.34
4: 1:1
R: 3.54
OD: 0.797

M41: (according to 1972 140 green manual)
1: 3.13
2: 1.99
3: 1.36
4: 1:1
R: 3.25
OD: 0.797

While the M410 is "stronger" it has several flaws. For one thing the
transmission cannot be removed without the bellhousing, as the bellhousing
is also the tranny front cover and can only be removed from the inside of the
bellhousing. Also there is a lube problem on the bearings and the tranny
becomes noisy after about 80,000 miles. Additionally it is bigger and
heavier. Add to this that the M41 is a slightly more "close-ratio" box,
given the choice I would choose the M41 every time for a 4 cylinder engine.
I've been running one for about 18 years with absolutely no problem of any
kind on a fairly hot B20 engine. I am on my second noisy box in my 164
(although it works OK).

I beleive I'll be happier with teh M41 ratios as well. The engine has a big bore kit, F head, and modified D Jet. Not sure of the cam yet. I bought the car dis-assembled with LOTS of spares, so have no idea what kind of power I'll have. The PO had run this engine with the M41, said he had no problems, but he wanted the 410 for strength. If it is really not necessary, I'll stick with the 41, as I like the ratios better.
Thanks for the info!

What car is the M410 coming from? Early 164 & 1800 M410's have the same ratios as the early M41's.

I don't know...it is with all the parts that came with this car. How can I identify it?

Put it in to gear & see what the ratios are. The early M400 is 3.14, 1.97 & 1.34. I reckon that the wide ratio box does not come in the 4 cylinder box as they didn't make them in 164's until '72. Also, you should measure the diameter of the bellhousing because the 6 cylinder housing is slightly bigger in diameter & it's not obviously bigger until you get 4 & 6 cylinder versions side by side.