Im not sure a double lead thread will mesh in place with a single lead thread. so when you machine a double lead thread the pitch moves twice that of a single lead thread. exp. a 32 pitch thread moves at .03125 per revolution where a double lead 32 pitch will move a .0625 per revolution.

John,    I cannot specifically speak for Lionel's twin helical worm / worm wheel vs single helical however the Samhongsa twin and single helical worms will interchange with their respective worm wheels and seemingly work fine even though they don't mesh exactly the way they do when the correct worm and worm wheel are used.  I am sure a mismatch causes accelerated wear but they do seem to work without much fuss.  I have some photos in my other computer and will post the photos showing the skew angle on the two different worm wheels when I get home.           j

Well, I am going to pull the sucker apart AGAIN and check, the worm is the only thing that makes sense as the issue.  It's running with a perfect 2 chuffs instead of 4-chuffs, and since the tach is working and the RCMC is the original, I can't imagine anything but the gear ratio being the issue.  I'm just amazed that an older Atlantic would have a different worm, but I'm running out of other ideas.

I'm wearing the screws out on this thing with all the issues.

Mystery solved!

It is indeed a dual-lead worm that was in the older Atlantic, and the Legacy Atlantic has a single-lead worm.  This explains the jumpier speed steps and the two-chuffs, the gear ratio is half what it should be!

This piece is STILL not finished, I can't believe it!

@Trains and Car Guy posted:

Im not sure a double lead thread will mesh in place with a single lead thread. so when you machine a double lead thread the pitch moves twice that of a single lead thread. exp. a 32 pitch thread moves at .03125 per revolution where a double lead 32 pitch will move a .0625 per revolution.

Well, in this "sample of one", it worked perfectly, just didn't have the right gear ratio.  There is no binding of any kind with the wrong worm installed.  The only apparent effect is a 2:1 reduction in the gear ratio resulting in bigger speed steps and half the chuffs/rev than I should have.