Volvo M4 M40 M41 M400 M401 transmission
Volvo M4 M40 M41 M400 M401 transmissions
The 4 speed M4 came out about 1958 or '59 for the 544. All of the 122 cars with manual transmissions I've worked on had the M40 as did the 140 series and the 1975 240. The M4 and M40 4spd transmissions shared some common parts but the cases weren't interchangeable because of different bolt patterns and the M40 had a longer clutch shaft.
Most of the M40 units had a shift lever coming up directly out of the top cover with the shift lever held in place with a screw down cap. 1972-3 had an extension going back from the top cover with a shorter lever held in place with a screw down cap. 1974 models had a similar extension housing with the shift lever held in place with a circlip and were unique to that year. 1975 models were of similar design but were unique to that year having a rubber cushion about 3/4” / 19mm thick between the cover and extension housing. There were two sizes of rear flanges for the M40.
The biggest weakness on the early M40 was that the needle bearings on the countershaft would wear out and the surface of the cluster gear they rode would deteriorate leaving you with a useless cluster gear because of the defective bearing surface. I experimented with grinding out this bearing surface and using other needle bearings. With some ingenuity this could be solved by grinding out a place for a bearing.
The M41 was the same M40 transmission except it was made to take an overdrive.
You can't just bolt up an overdrive unit because the rear shaft needs to go into the overdrive and the case is also a little different so that the oil can flow between the transmission and overdrive.
M400 transmissions were the 4spd that went in the 164S/E models. The M401 was the same as the M400 except made to take an overdrive. I thought these transmissions were really sturdy but when they needed parts there were basically none to be had because most 164 models had the BW35 automatic making it a short production run.
Here are some M40 notes I found about gear compatibility:
Cluster gear 380621 / small shaft / 15/20/24 tooth. Compatible gears: 1st 380150; 2nd 380174; 3rd 380619.
Cluster gear 380172 / small shaft / 15/20/23 tooth. Compatible gears: 1st 380150; 2nd 380174; 3rd 380140.
Cluster gear 656568 used in M4 / 14/20/25 tooth. Compatible gears: 1st 656572; 2nd 380174; 3rd 380140.
Cluster gear 380894 large shaft / 15 (wide)/20/24 tooth. Compatible gears: 1st 380895; 2nd 380174; 3rd 380619.
Cluster gear 380807 large shaft / 15 (wide)/20/24 tooth. Compatible gears: 1st 380150, (the rest no notes).
This information was in my collection of notes from working on these cars. Additional articles to follow.
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