Lionel Modern Era Plastic Idler Gears

Even brass, bronze, or other alloy metal gears can crack. It's all mostly for the same reason- manufacturing tolerances or "stack up" of errors. When you machine a shaft and then often knurl it that changes the dimensions. Then you have a gear manufactured with a bore diameter and then press that onto the shaft- and that's where the tolerances and dimensions "stack up" to determine the pressure of how tight that gear is trying to be expanded by the shaft.

If too much, any gear will crack, even a metal gear.

I'm just trying to point out your concern is slightly misled. Sure we like metal gears, but they are just as prone to cracking in some models because there was a mismatch in the press fitment of parts and over time they still crack. You can say Nylon is weaker, but again, this critical press fit, the shape of the knurling, the bulk of the gear and number of teeth compared to shaft diameter all come into play.

One example of just such a failure in a metal gear is my 6-14532 Pennsylvania Sharknose Diesel AA Set with TMCC Century Club, I bought it second hand but basically new in the box. Right out of the gate, had a cracked metal worm wheel and would bind slightly every revolution due to the crack spacing the teeth further. I had to replace the entire truck.

@Vernon Barry posted:

One example of just such a failure in a metal gear is my 6-14532 Pennsylvania Sharknose Diesel AA Set with TMCC Century Club, I bought it second hand but basically new in the box. Right out of the gate, had a cracked metal worm wheel and would bind slightly every revolution due to the crack spacing the teeth further. I had to replace the entire truck.

I had the same problem.  Turns out, there is a gear that fits there, but I found out after I replaced the entire truck.  Actually two trucks, I had two split gears.  Now I have two spare trucks.

@Mikado 4501 posted:

Awhile back I had a Lionel SD60M from the mid 90's that I sold to another person recently.

When I bought it from the previous owner, he said it ran fine. When I ran it, the front motor would spin the armature, but it wouldn't turn the wheels.

I found upon opening the motor that the idler gear the armature was supposed to turn the axle had cracked and split. I did indicate this to the person who bought it from me, but I wasn't aware just how common the plastic gear usage was. I knew it was a widespread thing in the MPC era, but had no idea it trickled down well into the LLC era post-1996.

Don't get me wrong, I love the Richard Kughn era of Lionel, and this isn't an intense bash at the Big Orange L, but this kind of makes me wary of buying second hand models from this period. I've yet to find any steam locomotives with plastic gears, but I need to ask, how many Lionel diesels in the first 30 years of the modern era had this design? I just bought a set of F-3's from the early 90's, and I think can see one of these in them as well.

While the plastic gears do split and the brass does too, the nylon gears MPC used quiet  the drive train. It’s all in the slack…..

I guess the bigger take away is most definitely, there are some designs and series that run and work well, and then there are some others and at this point in the game- some of these well into 20-30 Years old, the problem ones have been talked about likely in a forum.

Again, there are designs like the Lionmaster diesel truck with the longer motor mount and D shaped bushings on the worm wheel shaft that are a royal pain to work on. There are lots of examples of both plastic gear and metal gear versions of locos and various truck designs that have a failure rate higher than we probably would like. There are ones we cannot get the parts for and can be an even bigger headache to try to find a way to repair. There are some that are easy to work on an run forever. It's just a mixed bag.