The German firm Minitrix, now owned by Markin, used to produce a range of US and UK steamers as well as their native German locomotives. Most of these early models date back to the 1970s and are still running strong. However there are a few parts that are prone to breaking and in this post I will share with you how to repair the crank pin on a Britannia class locomotive.
Minitrix used the same chassis or a variation of the chassis on a variety of locomotives. In the US it was the 4-6-2 K4 and 2-10-0 Decopod. In the UK it was the 4-6-2 7P Britannia, 4-6-2 A3, 4-6-2 A4 & 2-10-0 9F class engines. These all had metal side rods, drive rods and eccentric cranks but the eccentric pin that holds it all together is plastic. And it’s this part that has been known to break. If you look at the model in the picture below I have circled the crank pin in red. The side rod connects all three driving wheels together and is the nearest to the wheel. The connecting rod or main rod is fixed to the center wheel and runs into the cylinder. The crank pin holds the connecting road and side rod to the center wheel. The eccentric rod connects to the cranked end of the pin and drives the valve gear in the cylinder.
The crank pin is designed so that the side rod and connecting rod can easily rotate without binding on the pin but are also held in place. So you can see how this works I have modeled the crank pins below. The square shaft on the end of the pin fits into the wheel. Because it’s square this will ensure it will rotate with the wheel. The side rod and connecting rod fit over the round section and are held in place by the wheel and cranked top. The holes in the side and connecting rods are larger than the pin to allow free moment.
What normally happens is the metal rod assembly becomes jammed and the weakest point is the plastic crank pin. The pin breaks where the round section meets the square section and falls out. The connecting rod then flails about and can become bent. With this 4-6-2 that is exactly what has happened, although luckily the connecting rod wasn’t bent.
So to fix this engine the first thing needed is a new crank pin. Spare parts for these engines ran out years ago so my next option is to 3D print one. I printed some using the 3D model above in Shapeways Frosted Extreme Detail material. It will also print in their Frosted Ultra Detail material.
Next the old square section on the crank pin needs to be removed. For this I used a small drill in a pin vice as you can see below. I selected a drill that was a bit smaller than the pin. Once I had drilled through the pin the remaining materiel was so thin and weak it pulled out with a pair or tweezers. You can see the remains of the pin under the new one.
In the image below you can see the old and new parts next to each other. The old crank pin is connected to the eccentric rod by a metal pin which has a flared end and cannot easily be removed. One option is to cut off the old plastic crank and un-flare the end of pin hoping to fit it into the new one. However that is very hard to do.
To make this job easier I did cut off the old plastic crank but I did not un-flare the pin. Instead, using a very sharp craft knife, I cut through the loop on the end of the new crank pin. This then forms a C shape although the two ends still touch. Carefully spreading the new C shape I was able to push it over the metal pin as you can see below.
The connecting road and eccentric rod were then refitted to the loco.
The new crank pin simply push-fits into the driving wheel, ensuring it passes through the hole in the connection and side rods.
It is important to make sure the crank pin is pointing in the right direction, which is towards the center of the wheel.
The next part is to test the the new assembly and what could be better than running it on a layout. Below is short video of the engine running demonstrating that the new crank pin works perfectly.
The rocking motion is caused by the traction tires as the loco is supposed to be moving.
The last thing to do is to add some silver paint to the new crank pin to help blend it in. It’s best to put on as little as possible as you don’t want to paint the parts together or cause a bind.
A pack of four replacement crank pins is available from Shapeways here.
Alternatively if you would like this repair made to your locomotive for you, you can contact me though the contact page or directly at firstname.lastname@example.org and we can arrange to get it done.