Repairing a Kato SD40 - Step By Step How-To

For those of you who read my previous blog entry, the last few evenings of modeling time has been spent repairing locomotives.  While initially I just wanted to see my two "gymnast" Intermountains run again, I started with a pair of stubborn Athearn units before working my way up to the Intermountains.  After getting over the initial intimidation factor of tearing apart a small precision machine, I decided to take on all my old locomotives as well, documenting the process closely so hopefully anybody else with the same fear of gutting a locomotive as myself won't have to be quite so afraid of what's under that plastic body shell.

While an old Kato SD40 is the model that I documented, the mechanisms for all the locos I've taken apart in the last week are strikingly similar.  I've tried to include most of the differences I've noticed between the mechanisms, as well as some things that are different between a DCC (or "DCC Ready") frame vs. a traditional DC-powered frame.  So, without further delay, let's get into it!

The Locomotive

 The particular loco I chose was one that I've had for probably around 15 years, if not more.  It's an old Kato Burlington Northern SD40, which has a traditional DC frame.  It would come out to run on the under-the-Christmas-tree layout with it's identical twin (another BN SD40) every year.  After a few years, the mechanisms on both became very loud, with a squeaking, almost shreaking noise.  I was about to write both of them off after my first repair attempt failed and I found some of the newer Kato SD40-2s on sale, but ended up deciding not to spend the money.  I decided to give repairing the SD40s another crack, as I really like them and want to see them run with all my newer stuff.

Tools & Supplies

It doesn't take a lot of tools and supplies to take apart and fix a loco, but it's important to have the right things.  Here's what I used:

• Two small jewelers screwdrivers - One flathead and one philips head
• Rubbing alcohol - You can substitute any of your favorite cleaning liquids, as long as it is one that will evaporate without a residue!
• Some paper towels, or better yet, a lint-free cloth
• LaBelle's Oil - Available at any hobby shop, an absolute must.
• Optional - Compressed air can which you can get at most electronics shop.

Removing The Shell

Probably the single scariest thing in all these repairs is actually removing the shell.  To get the shell off, I gently inserted the flathead jeweler's screwdriver between the frame and the shell.  Pry out very gently while squeezing the top of the shell just below the dynamic brake housing.  Squeezing at the top forces the shell out just enough at the bottom that it will begin to slide off.  I was a bit of a nutcase doing this as I didn't want to force anything apart.  Eventually with some patience, it began to slide off easily.

• Athearn difference:  To get the shell off an Athearn, you won't squeeze at the top.  Instead of prying the shell away from the frame like the Kato, insert the screwdriver between the bottom of the shell and top of the fuel tank, and twist it a bit.  The shell will begin to lift off.  Start at the back of the fuel tank and work your way towards the front on each side.  Once the shell is off about 1/8" or so, you can begin to separate them with your hands.
• Atlas difference:  Not much.  I took the shells off the Intermountains much the same way as the Kato.  Just be careful.  If something feels like it's snagged, proceed gently.

Removing Control Boards - Lights, DCC Ready Board, or DCC Board

After the frame and shell have been separated, you need to remove the light boards on each end of the locomotive.  On the Kato, remove the front black light guard and then each light slides out easily towards the ends of the frames.

• Athearn Difference:  The Athearns have a full-length lightboard installed in them.  To get it out, push on the front of the light board forcing it towards the rear of the frame.  Be careful as you go, and eventually it will pop out of the frame molds that hold it in, and you can remove it with your hands.
• Atlas/Intermountain DCC/DCC Ready Difference:  The light board that comes installed is DCC ready, meaning the motor contacts are on the light board.  This is how drop-in DCC works...Simply replace it with a DCC chip and you're ready to go!  In the Intermountains (which are an atlas mechanism) the lightboard is held in by the sides of the frame, so it won't just slide out.  To remove it, loosen the screws located at the front and rear of the frame, gently pry the frame apart enough to remove the lightboard, and lift it out.

 Splitting the Frame

To split the frame into its respective halves, remove the screws on each end of the frame, and gently pry the frame apart.  You will need to remove the fuel tank cover first as it wraps around both frame halves.  At the top of the frame you will notice some small plastic clips that hold the motor and each frame half.  You may need to insert the flathead screwdriver and press down on them gently as you pull the frame halves apart.

• Athearn and Atlas mechanisms are pretty much exactly the same.  A single screw on each end of the frame to remove.  The Athearn doesn't have a cover as the fuel tank, so you won't need to remove it.  It's molded as part of the frame.

Trucks and Other Parts

After the frame is split, the trucks will practically fall out on their own.  I would recommend removing the brass contacts strips on each side of the frame as well as they can be a bit fragile.

• Athearn Difference:  The contact strips for an Athearn are completely different.  Instead of two long strips feeding each side, there are four small contacts held to the bottom of the frame by a small plastic piece.  Remove the plastic piece, and each contact strip.  The contacts on the Athearns are MUCH smaller, too, and when reassembled, should bend outward.
• Atlas Difference:  None really.

 Removing the Motor Mount

After the frames are in half, you need to remove the motor mount.  The Kato motor sits in a plastic mount that holds it to the frame.  You DO NOT want to remove the motor from the plastic mount, but want to remove the the entire assembly from the frame.  To do so, you need to press in the clips that hold each end in place.  You can do it with your fingers, or flip the frame over and gently force the clips out from the other side of the frame with the flathead screwdriver.  I found the screwdriver method much easier.

• Athearn Difference:  The Athearn doesn't have any clips to hold the motor in place.  The motor mount is sandwiched between the frame halves, and will lift right out when the frame is split.
• Atlas Difference:  The motor mount seems to be very similar...Not enough difference between the two to not anything.

Removing And Cleaning The Worm Gears

After the motor has been removed from the frame, the worm gears will pretty much fall right out of each end of the flywheels that are attached to the motor.  Each worm gear has two bearing blocks on each end, and the shaft that runs to the motor.  The bearing block on the far end of the worm gear shaft removes easily, and you should take it off.

• The worm gears on an Athearn, Atlas, and Kato are almost exactly the same.  Outside of the shaft to the flywheels, you'd be hard pressed to tell any of them apart.

With a cloth that has been dabbed in some alcohol (or whatever cleaner you feel appropriate) clean the end bearing block well, making sure there is no debris inside of it.  Next, clean the worm gear itself.  Make sure to remove any debris that is in the gear teeth, as well as anything around the worm gear shaft.  I found some carpet fibers that had wrapped themselves up near each bearing block, along with a fair share of dirt.  Make sure everything about the worm gear is as clean as you can make it.  As you can see in the picture below, a lot of junk can build up on these things.

Cleaning The Trucks

While the trucks were free from the frame, I threw each truck on the tracks and ran it back and forth with my hand to see how well it free-rolled.  Each truck seemed to roll fairly well, and a visual inspection seemed to uncover nothing obvious in the way of dirt.  I'm not sure if you can take the truck gear boxes apart, but I'm not going to be the one to find out.  If anybody else has, by all means, add a comment with a how-to.  Don't forget to inspect the bottom of each truck, as the gear mechanisms are clearly visible from below too.  You may also want to use this as an opportunity to clean the wheels of your loco as well.

• Athearn, Kato, and Atlas trucks all seem to have slightly different gearing mechanisms, but overall, the trucks are pretty much the same.  They are all in a single assembly, and the gear at the top will mesh with the worm gear, ultimately making things go.

Lubricating The Worm Gears

The one thing that is necessary with your loco that cleaning will remove is any lubrication.  You will want to make sure any area where two pieces of material are moving against each other have some lubrication between them.  Now, here's the caveat:  A little bit goes a long way!  Don't overlubricate your loco.  In fact, if you do, chances are you'll get the privilege of repeating this entire process over again!  For lube, I used LaBelle 108 oil which I picked up at my local hobby shop.  It was recommended in an earlier forum post I made, and after the results I got, I won't use anything else.

For the worm gears, make sure that BOTH bearing blocks are lubricated where they meet the shaft.  I drizzled a little bit along the shaft on each side (it only takes a drop) of the inner bearing block and worked it in a bit by twisting the worm gear in my hand.  For the outer bearing block, I put a drop of oil on the shaft and then slid the bearing block back on, twisting it around a little bit.

• Athearn and Atlas worm gears are almost identical to a Kato, so the above applies to all three.

Reassembling the Motor and Worm Gears

After lubricating the bearing blocks, you can begin reassembling the motor and the worm gears.  Before putting anything back in the frame, give each frame half a good cleaning with a cloth and some alcohol.  There were a few bits of dust and gunk on each frame half of my SD40 that were trivial to clean up.

Start with the motor when reassembling.  Line up the clips along the top half of the frame and snap ONLY the top half in.  If you snap in the entire motor, getting the worm gears back in is next to impossible.

• Athearn Difference:  Since there are no clips on the motor mount for an Athearn, just make sure the small tongue-and-groove matches up with the frame.
• Atlas Difference:  None, really.

Once the motor is partially in, insert the worm gear shaft into the end of the flywheel.  Line up each bearing block to the grooves in the frame, and slide them down until they are snug.  Repeat the process for the other worm gear assembly.  After each worm gear has been inserted in the flywheel and the bearing block are lined up, snap the bottom of the motor mount into place.  You should now have one frame half, the motor, and each worm gear assembled like the picture below.

• Athearn Difference:  Since there are not clips to hold the motor mount (and as a by-product, the worm gear assemblies) securely in place, you'll find they don't always want to stay put.  Be careful as you move the frame around, but watch to make sure everything is staying together as you continue assembling things.

Lubricating The Trucks

Again, a little goes a long way, but it's important that the lubricant is spread through the gearbox.  I put a drop of lube in a few spots on several gears, being very careful not to over-lubricate.  As I did so, I ran each track along the table with my finger to help spread the lubricant.  I visually inspected things to make sure the lube was spreading across the gearbox, and noticed a few dry spots on the underside of the truck.  I placed a small amount of lube where it was dry, until I was satisfied that everything had a thin coat of lube.

• Athearn and Atlas trucks will be lubricated the same way.

Putting Things Back Together

Putting things back together is really just a few steps.  Assemble the other half of the frame, the trucks, and the light board(s).  I started by aligning the other frame half with the motor mount and clipping the frame half together.  After both frame halves were loosely attached (the motor mount clips will hold it in place) I lubricated the actual worm gear, working a very small amount of oil in each groove.  I did this on both worm gears.

• Athearn Difference:  You will want to lubricate the worm gear on an Athearn prior to placing the other frame half back on.  As there are no motor mount clips, the frame half won't hold themselves together, and the end result of trying to do anything without putting the frame screws back in will be that the bearing blocks and worm gear will move.  It's easy to re-align them, but it's annoying as all get out.  Also, make sure that as you assemble the frame halves that the brass contacts have all been re-inserted and that the plastic block that holds them in place are lined up.  My biggest complaint with the Athearn frames are those pesky contact strips and the plastic clip that holds them in.
• Atlas Difference:  None.

Next, begin screwing the frame halfs loosely together.  Make sure that the insulator between the frame halves on the holes for the screws are still there, and that the plastic nuts the screws go into on the other frame half is as well.  Before tightening everything up, place the trucks back into the frame.  On the Kato trucks, there is an arrow at the bottom that should point AWAY from the fuel tank.  After the trucks are in and lined up correctly, tighten up the screws on each end of the frames.

Replace the brass contact strips on each side of the frame making sure they go under the small metal piece of the frame that sticks out above the trucks.  That's what forces the contact strips down onto the truck pickups.  Make sure the contact strips are actually touching the top of the pickups as well.  They have a nasty habit of falling inside or outside of the pickups.  They should sit directly above them.  You can see how the strips should sit in the picture below.

• Athearn Difference:  See my earlier note about the Athearn contact strips.  They are a pain.
• Atlas Difference:  None that come to mind, except the contact strips tend to find their way off the pickups from the trucks a bit easier than on the Katos.

Lastly, place the lightboards back in and replace the light guard around the front lamp.

• Athearn Difference:  You will slide the light board back in from the rear towards the front.  Be careful pushing it back in, but make sure it goes all the way in as far as it can go.  Again, be careful.  It takes a bit of force, and you don't want to snap it apart.
• Atlas DCC/DCC Ready Difference:  You will need to replace the lightboard or DCC decoder as you assemble the frame halves together.  You can do this at this point in the process as well, but it's easier just to do it during re-assembly.  To get it back in, loosen the screws on each frame end and gently pry the frame apart until you can fit the light board in on the top.  Make sure you are putting it in the correct way and that the light board feeds are touching the motor contacts.  After, tighten everything back up again.  The trucks may fall out when you loosen the frame, and if they do, simply place them back in as you tighten things back up.

 After the lightboard is back in, you're ready for a test run!  Throw it on the tracks and see how she does!  If it's anything like mine, you should notice a MUCH quieter and smoother running engine.  Especially at low speeds.  Another test is to make sure you put the motor back in the right way.  Throw another "known" loco on the tracks and make sure they are running in the same direction.  If they are running opposite, you put the motor in backwards.  Take the whole thing apart, flip the motor around, and try again.

After a successful test run, put the shell back on carefully.

I hope this how-to of mine helps somebody with a pesky loco or improves the performance of one.  I'm flat-out shocked at the difference it made in both my Katos as well as the ressurection of my Intermountains.  The Athearns have been much more difficult with me, but we'll see how things go in the future.  My old Katos are now running just like new.  In fact, they are running together with some new Kato hardware.  My parting shot to this entire thing is both of my cascade green SD40s running around with a Heritage SD70MAC, MU'ed up together perfectly.