Two of the few US narrow gauge Mallets were Unitah 50 and 51.
This is a side tank saddle tanker that served a mining road and later a couple of timber roads. With the exception of the fact the Roundhouse 2-8-0 and 3 in 1 kits are outside frame locomotives they match up fairly well. I printed and did takeoffs from a Jon Davis drawing.
My ruling dimension for takeoffs is driver diameter which was known to be 42″ on the prototype. What follows is a takeoff table comparing HO dimensions converted from the drawing to actual dimensions from the model.
Note that overall length is almost identical as is the wheelbase between drivers in driver blocks. Driver diameter is a bit smaller on the model which will give it more of a hunkered down look. The wheelbase on the model is quite a bit longer but is adjustable due to bashing. The model makes up for the longer block to block wheelbase with a shorter front deck. As the wheelbase in the model is adjusted, room will be gained for a longer deck. Heights are close. The cab is longer on the model but needs to be bashed to add a door and bunker.
Distance from the rearmost driver to the rear truck and frontmost driver to the front truck are adjustable. The side tanks need to be scratch built so can be adjusted to fit the space. Overall, the model is close enough to the prototype for my purposes.
Here are some more prototype photos and in one case a LGB Unitah photo for details.
I will keep a builders log as I proceed with this project.
In order to create a 2-6-6-2T, two Roundhouse 2-8-0 frames are required. The first is used to create the rear 6-2T portion of the frame.
A number of cuts are required, along the red lines in this image. The front cutoffs are discarded. Click the image for a larger view.
The second frame is used to create the 2-6 portion of the frame. A single cut is required along the red line. The rear cutoff is discarded.
In the following photo the modified frames are placed in the approximate locations they will assume once the frame modifications are complete. The approximate location of the cylinders are also shown on this image. The exact placement of the rear cylinders is not absolutely crucial as the cylinders connect to the piston rods and the yokes. The driver wheelbase is crucial as it affects the side rods. But those distances are cast into place on the model. Note that the rear motor mount is intact. It is designed to power the second set of drivers on a 2-8-0. This drivers are the front set of drivers on the rear portion of the 2-6-6-2T frame. There is not sufficient room to place a motor to the rear of the gear mount in the front portion of the frame. I’ll deal with the motor mount issues in a separate post.
This view of the rear (top) and front (bottom) portion of the 2-6-6-2T frame shows the resting places for the cylinders. Note that this area is notched in the front portion of the frame. A similar notch will need to be filed into the top of the rear portion. Doing so will weaken the frame in the notched area as a slot for the fourth driver on the 2-8-0 runs through that area.
This shot shows the underframes that hold the drivers in place. The right underframe is in the correct orientation. They are screwed to the underside of the frame. In this shot the frames have been inverted. You can see one of the threaded holes that allow the under frame to be attached to the frame on the right frame. Because filing the recess for the drivers weakens the frame in the area of the front driver notch, the underframes will need to be modified to tie the front portion of the rear frame together to overcome this weakness. To accomplish this strengthening, a second hole will need to be drilled into the front portion of the underframe just behind the notch. A matching hole will need to be drilled and tapped into the frame.
The frame will need to pivot (articulate) just in front of the rear set of cylinders. So a suitable connector will need to be fashioned that ties the two halves of the 2-6-6-2T frame together. I’ll fashion that connector out of a piece of brass strip. Lets get to work on these modifications.