MTH Smoke Motor Screws Too Long

I'm in the process of replacing a dead smoke unit motor from a Premier H-3, 2-8-0, #20-3142-1, which has a different style smoke unit then normal. I'm using a new motor that I purchased from the MTH parts stand at York. The problem I'm having, is the two screws that hold the impeller housing to the motor are too long and hitting the armature inside the new motor, not allowing it to turn. The new motor is the same size but looks to be from a different manufacturer and must have a slightly larger armature inside. Does anyone know the thread size of these screws so that I can search the web for shorter ones? I believe trying to shorten these will be a failure and be too difficult to dress the threads after their damaged from the cut and loosing them while trying to shorten them is extremely possible. My only other option I could think of, would be to cut off the top of the old motor, drill the threaded holes out, then use it as a spacer between the new motor and plastic housing. I thought of trying to find tiny washers for spacers but the plastic of the housing is not ridged enough to keep everything straight so that the impeller won't rub inside. I really hope I can find shorter screws because I don't think the impeller will have enough holding power on less shaft. This impeller is not as robust as the common impeller used in the standard smoke unit.

Also, would anyone know which wire, green or gray, is the positive connected to the motor? Thank you all.

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Dave Z


Photos (5)
Original Post

If that's the Mabuchi FF-N20 style DC motor used in many MTH smoke fans, that's a metric M1.4 screw - maybe compare other dimensions to confirm same motor. 


My LHS carries the NWSL (NorthWest Short Line) line of hardware who have bags of M1.4 in various lengths for less than a buck.

metric screws

I figure MTH probably sells the screws (if you can find the right part number!) but when you add shipping, etc..



Photos (2)

I believe Stan has it, the Lionel motors use the same screw as the MTH, I've interchanged motors from MTH into Lionel stuff without any issues.

FWIW, I get my stash of tiny metric screws from taking apart old disk drives. they used a lot of neat little screws in those.

Measuring the diameter of the threads with digital calipers, they measure 1.3 mm.  So I'm thinking they must size the screw by the size of the shaft of the screw before it is threaded. The length measures 3 mm.  I'm going to open up a Lionel tonight, after I put a new nozzle in my oil furnace, and borrow its screws if their a bit shorter.  Good advice John.  Thanks for posting that info Stan. I'm glad to see that 2 mm are made and must be what Soo Line has found that Lionel uses. I will do a search of NWSL and see if someone locally carries them.

Dave Z

There was a run of motors that had issues with long screws.  First, gray is 5V, Green is DC ground control.  Second invest in a small pair of clipper pliers.  I just cut the excess length off an reinstall.  You do not need screws that long.  G

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Thanks GGG, I was afraid I would mess up the threads cutting or grinding the screws and then mess the threads up in the motor holes trying to insert them. Normally I would put a nut on the screw then cut it and remove the nut to fix the threads, but I don't have nuts that small. (Almost choked on my egg nog laughing at that one). I was going to borrow some from a Lionel because Soo Line said they were a bit shorter, but decided to order some shorter ones and wait. In the mean time I'll get the wires soldered to the motor. Knowing the green goes to the positive of the motor will save some time guessing. Thanks again.

Dave Z

UPDATE:  I ordered some 1.4 dia. x 0.3 pitch x 2mm long Wafer Head screws which solved the problem. The MTH screws I removed, that were too long, were also wafer head. They have a much lower profile then a pan head which helps to prevent the fan from rubbing them in the base of the fan housing. I found the screws at, part #20581, .65 each.

Dave Z

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