Upgrading the N scale Intermountain SD45-2 with a Kato chassis

[kiwi_bnsf]

I'm modeling the East slope of Tehachapi circa 1999, and so I have need of a couple of BNSF (ex-ATSF) SD45-2s.

I also model the Lone Pine branch from Mojave as far as the interchange with the Trona Railway at Searles, so I also need five Trona ex-ATSF SD45-2s (rebuilt by Montreal Locomotive Works as SD40-2Rs).

I've acquired a bunch of Intermountain SD45-2s over the years, but I've never been a fan of their chassis, drive trains, nor the OEM Atlas trucks. Even with a lot of tuning, they don't run as well as Kato, and their appearance when viewed from low angles or side on leaves a lot to be desired — with large air gaps around the trucks and fuel tank.

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With the SD45-2s sharing the same 43' 6" truck centres as the SD40-2, I've always wondered if I could "lazily" upgrade these to a Kato chassis — and I'd once read somewhere that you could do a straight swap.

Unfortunately it turns out the Kato chassis is 1mm wider than the internal width of the Intermountain long hood at its widest point (at the area surrounding the motor cradle). Both the Kato Early SD40-2 (DCC ready with frame halves screwed together) and the Kato Mid Production (screwless frame halves) suffer the same width issue. The Kato frame also projects upwards too far either side of the decoder slot. Finally, there are some shell supports that interfere with the Intermountain shell.

Of the Kato Early and Kato Mid chassis, the Early variant features metal supports for the shell and pickup strips that interfere with the Intermountain SD45-2 shell sills. Even milling recesses for these, or removing them altogether just leads to a shell that rides directly on the pickups of the Kato trucks — this didn't work for me.

The Kato Mid chassis has a design that is more adaptable to the SD45-2 shell with some careful milling of recesses in the shell sills. It also has the big advantage of including a sub-frame with an integrated fuel tank, air driers, and traction motor cabling.

Kato SD40-2 Mid Chassis modifications

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I would not call this an easy swap — I would recommend using a milling machine for the required precision (I use a Proxxon MF70). Once modified, the frame halves become significantly weaker under vertical load (e.g. in a vise), so I have found a specific milling order that works for me…


1. Rear Shelf

I find it fastest to get the majority of the heavy milling out of the way first, using a 3mm bit at 12k rpm, before the sides of the frame are weakened.

I clamp the complete chassis in modified Kato Mid fuel tank holder (sides milled flat) and with 0.5mm styrene spacer to preserve the frame geometry under heavy pressure.

I mill away section behind rear decoder clips first until this is level with the top of the rear shelf, and then I then mill away the entire rear shelf down to the level of the first parting line on the rear "nose". This leaves a sizeable area for a keep alive tucked under the rear of the decoder, and a 3D printed speaker enclosure…

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2. Top

I then mill away top section down to the level of the top of the decoder clips…

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3. Left and Right Sides

I swap to a smaller 2mm bit @ 12k rpm, and gently clamp the sides, and lightly screw down retaining arms to stop vertical play, and then mill away a 1mm notch from the sill at the front of the fuel tank on both sides. I then mill down side panel to the thickness of adjacent primary chassis. Take special care on the thin vertical motor cradle members.

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4. Shell underside

Even after all of this milling, the underside of the Intermountain shell has several points that foul the Kato Mid chassis.

It is necessary to mill away some shallow recesses for the chassis to snug into (marked in orange). You have to be super-careful doing this, as one false move and you will mill straight through the sill (don't ask me how I know this). I find a faster 20k rpm mill speed helps minimise the vibrations, and I keep one hand on the shell to stop any vertical play.

It is also necessary to mill down the six nubs that clip the body to the underframe (marked in red).

The notches in the fuel tank milled in step 3 (marked in blue) allow the fuel tank section to clear the large structural clips that hold the shell to the underframe.

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5. Fuel Tank

It is necessary to remove approximately 1mm from the highest section of the Kato Fuel Tank so that this can snug up against the shell. I do this using the milling machine to ensure it's straight.

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This is all a bit fiddly, but with some time and care, you can get an Intermountain shell that will snug down nicely on the Kato chassis…

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(apologies for the silver trucks — this was a test unit).

[kiwi_bnsf]

Here's a Clinchfield unit I upgraded for @Doug W

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I also took a gas soldering torch to my OMI Brass SD45-2B that has up until now been a shelf queen (Brass collectors please avert your eyes)…

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This is now upgraded with the same Kato chassis (wrapped in Kapton tape to isolate the frame) and is Code 40 compatible 

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[ednadolski]

Beautiful work!   <inserts sound of jaw hitting keyboard>

I had one of these IMs that is now sitting, disassembled, stripped, and abandoned in a drawer, since the factory drive & frame ran so poorly and I could not fit it onto a Kato frame.  Maybe I will have to see if next year's budget can allow for a proxxon (with the needed tooling)....

Ed

[tehachapifan]

That is some high-level machining!

[kiwi_al]

Oh Mate!! That is some awesome work!
The sd45-2 B unit, is that a Baldylox shell? I had been thinking along these lines (using a Kato SD40-2 mech) but never got around to it
Very well done!

[kiwi_bnsf]

Oh Mate!! That is some awesome work!
The sd45-2 B unit, is that a Baldylox shell? I had been thinking along these lines (using a Kato SD40-2 mech) but never got around to it
Very well done!

Thanks for the kind words.

The SD45-2B shell is an Overland Models Brass model that I've removed the mechanism from (because it is a very poor runner and the definition of non-DCC friendly), and then de-soldered components to allow the Kato fuel tanks to fit.

They occasionally come up on eBay for reasonable prices. They have grab irons cast onto the shell, but otherwise they are quite a nice model, and a quick way to get a decent SD45-2B. I've always been fond of the "Hammerhead" ATSF B-unit conversions, and this is a good solution for me.

I'm looking forward to seeing how big a speaker enclosure I can add to one of these, as there is a lot of room in the font dynamic section, and the shell has open grills that should help.

Cheers

[kiwi_bnsf]

Thanks @ednadolski  I'm a big fan of your work on your super-detailed locos and track, so your compliment means a lot (I'm not sure why it justified a down vote, but I've cancelled that out with an up vote!).

Thank too @tehachapifan.

Cheers

[CRR Chase]

This is a cool approach
I was looking at updating my SD45-2's using styrene on the underframe. however the rideheight  looks much better on the kato frames. nice job

[oakcreekco]

Great machining, nice job.

[peteski]

Clever and well documented conversion!

[lock4244]

This is excellent, Tim!

I've a rake of SD40-2W's that require a similar transplant, and have actually shoehorned the IM shell over the Kato SD40-2 mech without modification... it was bowed out so it fits at the ends without milling, but will need trimming along the sides. I cannot recall if I used an early or mid-production chassis as I have both sitting around, acquired along ago for various reasons, this being one of them.

[Ed Kapuscinski]

Are you using these to fill up the air gaps?

https://www.keystonedetails.com/keystone-customs/2020/1/27/n-scale-intermountain-emd-sd4045-2-dress-up-kit-zjcdg-abh5m

[kiwi_al]

Noooooooooo I have 6 SD45-2's so I need 6 of these 

[kiwi_bnsf]

Are you using these to fill up the air gaps?

https://www.keystonedetails.com/keystone-customs/2020/1/27/n-scale-intermountain-emd-sd4045-2-dress-up-kit-zjcdg-abh5m

I'm using the standard Kato Mid Production fuel tank — as this includes an integral subframe that hides the gaps (see the photo in Step 5. Fuel Tank in my original post).

Cheers!

[CRR Chase]

i am curious as to what would need to be milled on the Inter mountain frame to achieve similar results?? couid it be done?