The Stovebolt Site Forums The Parking Lot Big Bolts SM420 with no synchro?

I have a 47 1.5 ton. The original gearbox was filled with water running down the dash and dripping down the gearlever. I got another one just the same. Both are definitely SM420, 4 speed:
box #1 590474, date code j62
box #2 5834141, date code 11 67
Both boxes are full 4-speed crash. There is no synchro cones at all. It looks to me like they never had any. They both work fine but the teeth do the meshing.

Has anyone ever heard of SM420s with no synchro? Maybe we never got synchro boxes here in NZ.

In '47 the SM420 would not have been stock, the first year for the SM420 was '48. Do these transmissions have a release lever to allow getting into reverse?

Do you have the transmissions apart? The synchros in SM420s are not near as visible as they are in a Borg Warner transmission. If they are indeed 420, there has to be something that would take up the space of the synchros.

Good point. I was expecting to see the usual large synchros I'm used to in modern boxes. I will download a manual and have a look.
I have had the top off and there is nothing to stop the gears sliding together. They slide effortlessly up and down the shaft until the teeth engage.

Oldsub- neither box is original, according to what I think are the date codes. What came before the SM420? Something with identical dimensions that would bolt in where an SM420 appears to fit now? both my boxes have the pull up lever for reverse.

Thanks for your responses.

I downloaded a manual. I must have some old pre SM420 box? They look identical externally but there is definitely no synchro, also all my gears are straight cut, the SM420 manual shows synchro cones and helical cut gears.
Too bad, it will get it moving and stop my wife wanting to drive it.

I've never heard it called anything but a crash box. I'm sure the early transmission has a name or designation but I don't know what it is.

The release lever confirms you don't have an SM420.

That answers the question, having the little thumb lever for reverse indicates you don't have an SM420, but rather you do have the non syncro predecessor.

Grigg

Back in the 50's we referred to the old one as the "GM sliding gear 4 speed" and the newer one as the "GM synchro or SM420". They do not bolt to the same clutch housing.

Is that the reason why you have to double clutch?

Double clutching is to synchronize sliding gear transmissions.

I'll try to remember that. Back in the fifties I didn't know what a transmission was...

I cant explain the 60's date codes on the castings. Too bad.

This may bring on a repower and new trans a bit faster.