Royal Scot Chassis

[ColinB]

I recently acquired a Royal Scot on EBay, R2633 "Planet". It supposedly just had a con rod screw missing. I replaced the con rod screw but to my surprise when I ran it the motor didn't mesh with the gear. After a lot of changing parts (motor gears, wheels), I came to the conclusion that there was something very wrong with the motor holder or the chassis. Anyway the chassis was a bit like metallised chewing gum, so I suspected it had succumbed to the rot, this was further endorsed when the motor holder disintegrated when I tried to file it. Fast forward and there is a guy on EBay selling chassis fairly cheaply (I don't think he knew that they were like hens teeth), so I bought 3, in the hope one was ok. To my surprise the one that looked the best and had a motor holder, fitted perfectly, except the height of the motor mounting was reduced by about 1 mm. In my case this was perfect as the motor ( a new one) now meshes properly with the gears. The loco now runs properly.

Firstly, did Hornby change the design to make the motor sit lower or is it that when the Mazan rots it expands enough to cause a 1mm difference. The new part had a machined flat where the motor holder sat, the other one was just a raw painted casting as are the other two I bought. I also changed the design of the motor retainer so it sits better and puts less stress on the post.

[The son of Triangman]

Mazak (Zamak) expands due to the impurities in it (Mazak Rot), this has caused the height difference most likely. I recently replaced a modern 9F chassis that was turning to dust and was like metallic chewing gum in the middle, Mazak rot to be sure.

[The son of Triangman]

Those familiar with the old Lima UK Steam outline range will know all about Mazak rot, the weights in the Lima UK steam outline locos were prone to it. It usually distorted the plastic chassis the mazak weights were in, causing running issues with wheels up in the air on the diagonal in worst cases.

[ColinB]

Thank you that explains a lot, I just couldn't understand how the motor would be so far out. Now I know, I will go and measure the other two chassis I bought.

[Going Spare]

And in more recent times, in China-made Hornby class 31 locos and I believe others, expanding chassis have caused bodies to crack in the corners from the base up to the cab windows.

[atom3624]

There have been quite a lot of different locomotives which have succumbed to Mazak rot.

I purchased a 'spare' Royal Scot chassis off Peter's Spares.

Purchased for my beloved 46100 in case it too did fall foul, but it hasn't, and I've since used it in making good 2 hopeless 6100 LMS Crimson Lake Royal Scots into one good one - and it's great.

I did notice the screws were all very tight, so perhaps it was 'home grown', but all is well.

Dimensionally its spot on.

Al.