1st difference is (As River says..If you see Brass you have a W50).
W50 case has part number 33111-20091
P51 33111-20090.
There is a clear distinction between the W50 and P51.
To my untrained eye the P51 is a slightly stronger gearbox with similar bearing design.
The main shaft gears have a tad more thickness on the P51 than the W50 and might be stronger.
(Which makes sense seeing the 18RG of that era had 145 ps not 85 and twice the torque).
The main bearing goes in through the front in the P51 where it would be possibly to replace the centre main bearing by removing the rear housing in a W50.
The syncro methods and the anti-chatter scissor-gear are really the only differences.
The gearing on the P51 clearly shows a larger diameter in the intermediate gears reflecting the closer ratios.
The gearboxes start to make noises by 140-150.000 km but it is mainly the input bearings.
I think if we changed our oil more often one of these boxes would last 300,000 km or so easily.
Top W50 Lower P51 main shafts
(disassembled and W50 busted countershaft).
Viewed through the inlet hole you should see the brass rings and the different 1st selector on the W50
Whereas the P51 has a lighter fork and alloy brake bands.
Also looking back to the main plate if you see bolt heads (12mm) that's a P51.
There is also a 10mm gap between 1st gear and the main centre bearing plate in a P51
A W50 hass the gear flush agaainst the centre main plate and has no oiling nozzle.
the W50 main bearing is mounted from the rear.
So if you can't see thru the oil hole, drop the extension housing off
If you can see a main bearing plate with four Philips head screws
you have a W50.
There is nothing externally to tell the difference.
Second difference is the counter shaft input gear
which in the P51 has an anti-chatter scissor plate
and nothing on the W50 countershaft.
P51 countershaft u/s with Scissor Plate and how it works.
W50 countershaft (damaged)
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