M47-->m46???

The M47 transmission in my new-to-me 89 245 yowls like a cat with its tail stepped on, especially in the lower gears. I have available 81 and 86 parts cars with M46s, but when I searchnoobed I found one post that indicated the input shafts were different diameters and different spline counts so I couldn't just substitute (incl drshft and crossmember). What say the collective TB experience? Am I looking for another M47?

1979-on transmissions are all the same, spline count. Diameter is the same. The different diameter is on the output flange for the 240 M46's (some 1976-82 boxes have the small flange, everything else has the larger flange). Internally, the M46 was revised in 1987 to use the P-type overdrive, which uses a larger output shaft from the M46 to drive it.

With respect to the two cars, use the transmission that has the lowest mileage (amount of km) on it. If it turns out to be the earlier box, and it's from a DL, then use the 1986 flange and shaft. You'll need to use a freeze plug to plug up the hole where the speedometer drive is on the '81 box and will also need to modify the bellhousing for the crank position sensor on either box.

If you can use the 86 M46 it already is a deleted speedo drive and the bell housing is already thin in the area where the sensor goes. Or you can swap over the bell housing from the M47. Also the 86 uses the newer throwout bearing so it is one less thing to change as well. Easy to change all that back and forth though if needed because you use the earlier trans.
Use the whole driveshaft from the M46. The rear section is dampened with a rubber insert on those driveshafts.

The disadvantages of the 1986 box:

As it's based on the D version of the M46, originally used in the Diesel and used in the Turbo cars as well, it frequently has worn synchros, especially on 3rd, and sometimes even on 4th. Seems the D version M46 is more fragile than the non-D 1979-82 non-turbo box. It also has the short 4.03:1 1st gear, so the canyon between 1st gear and 2nd gear still exists.

So, I still recommend going with the better condition box of the two. If the '86 box is lower mileage and shifts nicely, then go with that one. If the '81 box is better, then use that one.

Wow, thank you both for your detailed and comprehensive remarks!

Both transmissions seem to shift ok, a little stiff from sitting, but both cars rusted out before they wore out. The 86 has fewer kms on it, so I think I'll go with it, driveshaft and all.

My 89 is a NA DD around town; the M47 has the same short first gear which doesn't seem to be objectionable in use. I'll take the cover off the 86 and have a look at the synchro rings, though I remember it shifting fine when the car was still alive.

Again, thank you for this information, this has been very helpful.

Not a problem. You might want to fill it with some Redline MTL. Takes 2.4 quarts. You'll want a Volvo p/n 3502464 cover gasket for the overdrive. By doing that, you should notice an improvement in shift quality. Also, consider getting the STS Machining bronze/delrin bushing kit. Installed it on my 242, and the side-side movement of the shifter is much reduced.

-J