MTL GP Chassis shortened length of MTL F unit.

Some months ago I completed the FP7A shell and mated it to a GP chassis. I wanted to add a F7B unit to the A unit so sourced a GP chassis and some parts I needed and finally over this Thanksgiving Weekend here in Canada I completed the modification to the GP chassis to shorten it to the length of an F unit chassis.

I used the same method as David K Smith did on his MP15 project, but I shortened only one end of the GP chassis to get an F unit chassis length I needed with GP power. I removed the small fly wheel and then drilled out the long end of the worm gear axle shaft with a #69 drill and then used a reamer to get a tapered fit to the ceramic shaft. And then press fit the worm to the shaft. This is delicate work as the ceramic shafts are very fragile. (I understand from Loren that current motors in used in production now have metal shafts) For removing the small fly wheel I used the NWSL box section wheel and gear puller but substituted a piece of brass "U" channel with a slot for the ceramic shaft for the plate to push against and used a custom pin made from a dentist dental reamer fitted into the end of the screw in the puller.

In the pictures you can see the section I removed and the reconnection using a 00-90 screw down from the top of the chassis under the PCB with the lights. I made my cuts in such a manner that I left the retension post for the PCB in place. In the alignment process I used a shim of .040 inch styrene custom cut to fit the complete chassis when fully assembled. I screwed the two main chassis pieces in place, then screwed the two nose pieces to-gether and lid them onto the shim and aligned them up properly. I used a small piece of the .040 shim stock forward of the nose spacer to provide leverage to close the nose piece on the styrene shim. Otherwise it won't tighten up and stay in place for the next step. Once I had the nose piece aligned using steel straight edges I then put a drop of ACC in each cut line to hold the pieces in place and fill the gaps. Even cutting with the finest saw blades there is some loss of material in the cut. After a few minutes I repeated the process of filling the gaps with ACC. After letting it sit for half an hour I started the drilling process to provide the electrical connection between the two halves of the chassis. Without the screws you would only have power pickup from one truck not both as the ACC glue provides an insulator in the gap between the nose and the main chassis pieces.

Once I had completed installing the screws I then used my excel scroll saw to cut the clearance from the fuel tank area to clear the front truck and used a file to clean up the cuts. A word of caution when drilling and tapping the 00-90 screw. If you go down exactly centered in the square lug on the main chassis you will come out of the side wall of the chassis in the area where the rear worm gear shaft bearing was located. This in itself is not a problem as you won't be using that bearing, but requires caution when drilling and tapping as this sort of location for te end of the hole can lead to breakage of the drill bit or later the tap. So you could really bugger the project if it happens. I used safe tap fluid for both drilling the holes and running the tap into the hole. I also back and clear the drill bit and the tap several times in the process. The screw I used was the one from MTL that we use for body mount couplers as they are the right length.

The FP7A has a full pilot and body mount for 905 and I have filled in the lower head light as this soon to be CP engine only had the upper light. In the picture of the two units coupled to-gether you can see some of the pieces removed in the process of shortening the B unit chassis

So the project continues.

will try this again and see if we can get the complete picture loaded

Garth, This is a great modeling job. I should teach you how to make trees for me now that you will have some spare time. Cheers, Jim CCRR:woohoo:

Good job, Garth,

yes, it's a bit nerve breaking pulling the fly wheel off knowing it's the ceramic shaft. I broke one too....

Nice work, Garth. As I read your description, it all seemed sooooo familiar...!

Glad to see I am not the only one Gerd. Pissed me off I had just pulled the flywheel coupling off the other end and I dripped the motor and the floor did not give. Tried to sleeve the shaft but that did not work too well. So I will used this motor on a repower of a Marklin Steamer. I have been playing around with Beyer Garrett design for awhile now and hope to get at it this winter.

Where you lead David others will follow. I enjoyed the challenge and the result. Got to do something about the painting and detailing though. I have had the pair out running with a string of passenger cars already and what a quiet smooth pair they make.

Jim I would rather build 100 of these chassis than make one tree. Though I suppose I would get as good as you and Loren have if I made as many as you guys have. Practice makes perfect they say. I guess I have done more engines than trees lately.

Repowered Hallmark PA's with GP chassis. Did 6 in total.
built a Japanese Doodle articulated doodle bug using a GP chassis
a 2-8-8-2 North American Mallet
a Japanese 0-6-0
a Japanese C57 Pacific
A No Am 0-8-0 switcher
an Indian YG Mikado
An Nn3 Cape Gauge 4-8-2
a passel of Lumber camp cars
a CP caboose
a 3 bay covered hopper
an FP7A shell
modified a GP power unit to go under an F7B shell
and finally modified a bunch of MTL switches for power routing on my layout and used Hoffman switch machines to power them.

Guess this is why my trees suffer a bit.

Garth, I saw your tool list on TrainBoard, and I can recommend some substitutions for anyone else who is game to try the chassis modification and doesn't have all of the tools you do.

Instead of a gear puller and special holder, you can use a plain bench vice, a broken drill bit, and a small hammer to safely remove the flywheels. I've removed quite a few so far with no mishaps.

I don't have a dentist's reamer to enlarge the hole in the worm. To drill the hole in the first place, I mount the worm in the chuck of a 1/4-inch drill, hold the bit in a pin vice, and run the drill at the lowest speed. This causes the drill to center better. Then I leave the bit in the worm while it continues to turn for a moment or two; this enlarges the hole just enough to press-fit on the motor shaft.

For cutting the frame, I use a jeweler's saw. I also do not cut right on the mark; I leave just a slight bit of the frame behind, then file down to the mark with a plain flat file. It's slow going, but the fit is good.

David K. Smith wrote:

Garth,

I don't have a dentist's reamer to enlarge the hole in the worm. To drill the hole in the first place, I mount the worm in the chuck of a 1/4-inch drill, hold the bit in a pin vice, and run the drill at the lowest speed. This causes the drill to center better. Then I leave the bit in the worm while it continues to turn for a moment or two; this enlarges the hole just enough to press-fit on the motor shaft.

For cutting the frame, I use a jeweler's saw. I also do not cut right on the mark; I leave just a slight bit of the frame behind, then file down to the mark with a plain flat file. It's slow going, but the fit is good.

Hi David;

The dental reamer is for making the tool to go in the push screw of NWSL's "The Puller" The reamers I use are in the yellow plastic case in the picture. I found the shaft on my motor had a bit of a taper to it. That is why I used the drill I choose and one of my long reamers

As for using your method of removing the flywheel/coupling I used the first time and did not catch the motor and broke the newly exposed shaft. I found this method was much easier and less problematic.

I will have to try your drill method for boring the axle on the worm gear. I had to clean off the end a couple of times before I got a good center start for boring down the axle. Anyway end result is wobble free and runs fine in the bearing at the outer end of the worm.

Like you I tend to cut short and hand dress the mating surfaces with a file to fit but I found I needed a bit of distance between the pads and the top of the nose to get the nose bottom edge level with the rest of the chassis. Since I would be putting a screw through this gap for electrical and mechanical connection I did not use any brass shim stock to fill the gap as the ACC is more than adequate for the 2-3 thou gap.

I made three cuts to remove the nose and shorten it. First cut was level with the top of the nose and went back under the pad almost to the motor spring slot. The second cut was down the face of the pad starting with the PCB retaining post at the bottom. When I reached the PCB retention pin I just rotated the saw blade through the bottom and back up under the pin. The third cut was then just forward of the motor spring slot. The last four cuts were to shorten the fuel tank and create the clearance required for the truck under the new shorter nose. Just followed the side profile of the original forward edge of the bottom of the chassis ahead of the fuel tank. The bottom cuts were done after I had aligned and fixed the new shorter nose to the rest of the chassis.

garthah wrote:

As for using your method of removing the flywheel/coupling I used the first time and did not catch the motor and broke the newly exposed shaft.

I almost did that the very first time, too--mostly because I hadn't expected it to work so easily; thankfully I had quick reflexes. Since then, a towel under the vice solved the problem.