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I have the choice of putting a 3 speed or a four speed (both are 46 units) in my truck. You people with experiance, which one would you use? Also are these syncro transmisions? Anyone care to vote? Thanks for you input!

On the 4 speed, I assume you have the one with the reverse lever on the stick. That's not the tranny I would choose. Both trannies are synchro except for first gear, and since second gear in the 4 speed is about the same as first in the 3 speed, that is an advantage. Reverse in the 4 speed is considerably lower than the 3 speed, which I like, but that's a personal preference. Granny gear is rarely used, but nice to have. If it were me, I would get the SM420, 4 speed from the 48 to 66 model, which doesn't have the lever, and is a good tranny.

Thanks for the reply. I know wgere there is a 57 5 window. Would this truck have this trnas? Also what is the advantage to convert using this one. One more thing, what engine would the 57 likely have? It is 1 3/4 ton.

The '57 should be an SM420 if its a 4-speed. That transmission was used until sometime in the '60s.

Be aware that the '57 had an open driveline, and your '46 would not.

If it''s a six, the 57 should have the 235 with the SM420. Open driveline, too. Assuming your truck is a 1/2 ton, I would also find a later model differential and convert. You will have to do some driveshaft work, too. Iirc, the 4 speed had a bigger rear shaft than the3 speed.

red rock:

If it were me, between the two I would choose the 3 spd. I think you'll find yourself diong a lot of double clutching with that 4 spd. It's not syncro in any gear. I've got the 3 spd in my 46 1/2 ton and it works just fine. Still no syncro in 1st, but thats not a big deal.

Go with what makes you happy, if you do decide to go with a newer trans and you are going to dump that 3 spd, let me know. I can always use a spare.

George

I've never had to double clutch the SM420 in my 61, or any other 4 speed I've driven for that matter. SM420 4 speed is definately the transmission of choice. There's one going on ebay right now.

http://cgi.ebay.com/ebaymotors/ws/e...1&category=33733&item=2443546206

The stock 41-46 4 speed trans does NOT have synchromesh
gears and does require double clutching or skillfull manipulation of engine RPM to shift. The 41-46 3 speed is synchro in second and third. The 3 speed is much easier and faster to shift and weighs less than half the 4 speed.
If you elect to use the later model trans, you will need to change the rear end, at least the pumkin, the driveshaft, and your brake and clutch pedals and master cylinder will have to be custom mounted or changed as they normally mount to the 41-46 transmission.

You can avoid changing the rearend by using a pre-'55 SM420 out of a 3100.