Transmission pops out of second

[Tim Ayers]

Originally Posted by Bored&Stroked

You'll not be able to diagnose the issue by looking at it - I only wish it were that easy. You'll have to take it all apart and start the process of really checking clearances, wear, etc..

Outside of incorrect synchros, the most important are:

1) The stack-up of second gear (end-play)
2) The condition of the front of the main-shaft and the bearing it goes in
3) The tightness of the 2nd gear bushing.

It is kind of a "compounding" situation - the more slop in all three of these areas, the more chances to pop out of second.

As noted earlier, I have Charlie NY re-bush second to exactly fit the specific main shaft I'm using. Also, I frequently test fit a couple different main shafts to front input shafts - to see where I have the least potential deflection (I do this on a lathe).

I'm not sure if NOS main shafts are available for your trans, but mic the front caged rolled bearing surface. It needs to have no wear! If it is possible to find NOS main shafts - without rust all over them, I'd pick one up for sure.

I think Dale is spot on. A little out here, a little out there, adds up to a lot of out everywhere.

I was surprised on how tight some of the tolerance are and thank goodness Mac VP sells thrust washers of varying thicknesses to help get things on the low side of spec.

I'm not familiar with this trans, but I'm thinking you'll need to go through it again unfortunately.

[Automotive Stud]

Just an update, trans is back out and the syncro assembly seems a little more beat than I thought upon initial inspection. The one spring keeps popping out of it, and the three blocker bars look fairly worn. Also it doesn't slide on the hub smoothly. I sourced a NOS synchronizer assembly. While I wait for that I'll double check my clearances on everything else...

[Automotive Stud]

Follow up repair, I wound up replacing the second gear with another and replaced the synchronizer assembly with a NOS replacement. Reassembled and double checked tolerances and it's good. I'm not really sure where the fault was since I replaced everything in question. And yes we now know Zephyr gears will work in the '49-50 Merc overdrive case!

[Bored&Stroked]

Please let us know if after a bit of break in, does it stay in second gear upon deceleration from higher RPMs? We're all trying to do our best with these old cog-boxes and parts are getting harder to find!

[Tim Ayers]

Originally Posted by Bored&Stroked

Please let us know if after a bit of break in, does it stay in second gear upon deceleration from higher RPMs? We're all trying to do our best with these old cog-boxes and parts are getting harder to find!

Not to answer for Automotive Stud, but I'll add my two cents to my recent rebuild.

Everything I used was NOS. Everything: case, clusters, bearings, top, shafts, rails, forks.

I have approx. 500 miles on it and it still shifts firmly and crisply. I've been purposely decelerating hard in 2nd to see if it will pop out and the stick stays firmly in place. Doesn't shake around like I've seen some boxes that are a tad worn.

Fingers crossed I got it right, but I'm happy with the rebuild. I am going to be doing a 26 tooth LZ box build up with winter and may swap it in for the 28 tooth box that's in there now. I'd like to get more legs out of 1st, but I'm also going to be installing a quick change so that will help with gearing as well.