Thankyou.  That is the problem. 

I have a second K4 where the running gear broke and can NOT be repaired, so I was going to swap the drivers of the broke K4 to the K4 with bad drivers.   Sounds like trimming the tires is the easiest. but I would like to disassemble the bottom frame to remove drivers.  Never seen a model so complicated to access and lube.

-dmiller

I have a K4 Spectrum series that "wags" from side to side . I can not figure out how to remove the shell so I can lubricate and check for bent axles.  Also would the valve gear binding be more likely to cause this . ( It looks good and I can not see any issues) I can attach pictures to another email   (the pictures are to large to attach )
Thank you
dmiller

Sorry I had a typo   I meant   difference between  81903  and 81907  ( no 9)   Am I correct it is just the name on the Loco , but the same physical model locomotive?  Thank you .

sorry typo  I meant  81903 (mine)  and 81907. (NWS part ID)  Thanks you answered my question  -the difference is in the name on the Locomotive  and not and part difference ..
Thank you !

Does anyone know the difference between an Item# 81903 HO Scale 3 truck 80 ton Shay (Greenbriar and Elk River) and Item# 91907.   I am looking at a NWS replacement 3 truck gear set for: Item# 91907 Which is also  a HO scale 3 truck 80 ton Shay  . What is the difference between the two Item # s?

I am Needing to replace the gear to the front truck  front tire  and NWS lists it as: Bevel Gear, HO Bachman 3-truck Shay (81907) 80-ton, 3 Truck upgrade bevel gear (6)  Part Number:  2800-6   ( No picture available )

https://nwsl.com/products/bevel-gear-ho-bachman-2-truck-shay-81907-80-ton-3-truck-upgrade-bevel-gear-6


Thank you all!!