With the A units, there is a light board that can be used to connect the decoders red and black wires to the frame but what about the B units that don't come with a light board ? Here is the method that I use.
I use a small table top drill press that I purchased from Micro Mark. Because the fuel tank part of the frame would make it hard to hold the frame square, I use a scrap of plywood to support the rear part of the frame.
Then tap each hole with a 2-56 tap. Kadee part number 246 includes both the drill and the tap.
The motor brush to frame contacts that are in each Kato engine made great electrical mounting rings. I find them handy for a number of projects and save all of the I remove when doing installations.
This is the method I use to remove the motor brush to frame contacts on all Kato locomotives. Lay the motor on edge on a flat surface. Using a flat blade jewelers screw driver, gently pry the brush holder up using the edge of the motor housing as the fulcrum.Red / Blk / Yel / Blu / Wht = 1-9/16"
Gray = 1-5/16"
Orange = 1-7/8"
Solder the removed brush caps to the orange and gray wires, and the removed brush to frame contacts to the red and black wires.
Re-assemble the motor brushes with the decoder wires now attached to the brush caps.
Then place the motor, worm gears, and bushings into the right side of the frame. Lay the orange motor wire into the slot on the right side of the frame that has milled for this purpose. Take special care that the motor mounting rings are seated correctly into the frame. Last install the two black shoulder washers.
Connect the left side of the frame together with the right side again being careful to correctly seat the motor mounting rings. Then install the nuts and bolts that hold everything together. Lay the decoder in the pocket that's milled into the top of the frame, attach the red wire ring to the the hole on the right side frame and the black wire ring to the hole on the left side frame with 2-26 flat head screws. Lay the wires out flat in the milled groove down the middle of the frame and secure with scotch tape as needed.