A Mamod Chassis Inspection

A very popular starting point for Live Steam. With their low cost comes a number of problems which can be discussed here
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dougrail
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A Mamod Chassis Inspection

Post by dougrail » Mon Mar 16, 2015 12:06 am

A Technical Update!

As related in my STWWW Shed, this loco has had two more trials. I have since disassembled the loco for a quick refit including the upgrade to an RWM Gas burner, a tank being repainted and relined and the lubricator being adjusted.

I took the time to assess how I build my locos and how I could improve the quality and lessen the time spent on the rebuilds. My experience shows that the chassis is often the fatal flaw with many Mamods - they either expand when heated and tighten on the wheels causing friction, or are not exactly square which again causes friction.

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With DRAGON I placed the wheels and cylinders on first. The wheels went on to provide a basic chassis alignment, plus went to show that when unhampered, they alone would roll freely and so they did. So, wheels to frames are fine.

You may note that I reattached the cylinders too - some would consider it early, I consider it time saving. The two nuts on each frame-to-cylinder portplate that are furthest away from the front are extremely awkward to roll onto the bolt once the loco is rebuilt - especially on the boiler-to-reverser steampipe's side inbetween the frames.

Usually it involves gravity, a steady hand, some oil and fairly pin-point needle tweezers to hold the nut in place and gently roll it onto the bolt thread between the frames and the steampipe!

By attaching the cylinders first this time, I halved my reassembly time. :)

Now we move onto a third fixing point for the chassis - a frame spacer or in this case, a gas burner's spacer.

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This can often unwittingly paralyse a loco. A spacer can distort a loco, especially if it is screwed in too tightly which can gently buckle/warp the frame inwards, creating friction on the bearings.

Furthermore, some spacers can be built shoddily or perhaps the loco in question was built with a little less care than standard [late 80's post-family decline Mamod, anyone?] so perhaps
the spacer needs amending. In this case, I have had to file down the spacer of the RWM gas burner on both sides equally in order that it does not force out or 'bulge' poor little DRAGON's frames - and again, restrict the wheels free motion. Once done and it slots in just about freely, it's time to secure it with a suitable bolt.

The trick...

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...is to gradually screw [bolt?] it into place as frmly as can go which doesn't restrict the freewheeling of the wheelsets. A quarter turn on both sides each step is the best way to measure it. This took me about 10 mins to achieve but I have a night, tight but not overtight bolt in which still allows the chassis to roll freely.

It is always important, at each and every stage to check the wheels free motion.

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Hold on side and run a hand quickly over the connected wheels, see how well they spin. You will note by this point I used the rear dragbeam to hold the chassis square and true to test the wheelspin.

I had also checked the conrods to ensure theor crank holes were the right size and that they nor the circlips were affecting the motion.

Now to secure that rear dragbeam in place with its 6BA bolts...
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And there we go. A chassis pretty much ready, square and true. The added bonus is having the cylinders [and gaskets] attached with the most difficult nuts and bolts of the entire assembly already fitted as well - which will save an immense amount of time in the overall rebuild of your Mamod / J-class loco.

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The 'top half' can then be built around the boiler and then the reverser block attached to the steampipe, held by said pipe and smokebox casting all ready to slot neatly into the chassis.

~

By ensuring the wheels are still free at each and every stage - and by installing them onto the frames first, you can eliminate a number of causes of why these little locos lock up - and increase your chances of building a good little runner.

Perhaps this is rudimentary basics, but I hope it may be of help, even if a reminder.

~

As for DRAGON...? Sandpots and conrods require repainting but I aim to steamtest this week with the super new gas burner. :)

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