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Author Topic: Graham Farish 3MT 372-327 valvegear has jammed!  (Read 208 times)

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Offline northwest99

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Graham Farish 3MT 372-327 valvegear has jammed!
« on: February 16, 2020, 04:36:10 PM »
My Graham Farish 3MT 372-327 valvegear has jammed! It doesn't appear to be bent but it is not seated in the plastic cylinders correctly as far as I can see.
This is the second 3MT with jammed valvegear. The last one I tried to repair I bought replacement valvegear but was unsuccessful as I knackered the replacement valvegear too! 
Has anybody else experienced this and successfully repaired it.

Online Dr Al

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Re: Graham Farish 3MT 372-327 valvegear has jammed!
« Reply #1 on: February 16, 2020, 07:07:12 PM »
I've rebuilt numerous new tool models with damaged valve gear - sometimes this is just the crosshead releasing from the slide bars (which need adjusted) or if the slide bars are bent inwards, clashing with front crank pins can occur.

In some cases the crosshead can loose the retaining washer, causing the connecting rod to be dropped. Often the washer is lost, and in these cases I usually completely replace the pin with a new brass one with rounded head to retain the crosshead.

Feel free to PM me if you are having trouble.

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces. Dr. Carl Sagan

Offline northwest99

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Re: Graham Farish 3MT 372-327 valvegear has jammed!
« Reply #2 on: February 17, 2020, 12:14:39 PM »
Thanks for the advice Alan.  :) At least I now know they are repairable. I'll give it a dose of looking at. I have nothing to lose!
Regards

Peter

Offline Fredastaire

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Re: Graham Farish 3MT 372-327 valvegear has jammed!
« Reply #3 on: February 18, 2020, 09:50:08 AM »
Ive had the same problem to sort. The front driver wheels have left / right side float plus poor riveting of the washer that Dr Al mentions. The factory dimensions are just too tight.
.
Ive added thin black plastic sheet spacers glued to the chassis block just behind the front wheels each side to prevent the excessive side play, then slightly tweaked the slide bars out just a smidgeon and filed the riveting pin head smooth.
.
This was to a colleagues Loco at the club, its now worked for many weeks with these simple mods.

Online Dr Al

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Re: Graham Farish 3MT 372-327 valvegear has jammed!
« Reply #4 on: February 18, 2020, 09:59:39 AM »
The factory dimensions are just too tight.

This fundamentally isn't true - the design is fine, and if correctly aligned there is adequate clearance.

The execution can be the issue, or handling bending everything - if the slide bars get bent then they can end up too close to the crank pins. The slide bars should latch into the plastic motion bracket, and often these have been compressed out of this position. I've only seen this on secondhand models, i.e. ones that folk have (incorrectly) handled - as new it doesn't seem an inherent problem.

The cast crosshead that's rounded over to create the rivet is a weak part of the design. Castings are weaker, and this is where handling can actually cause damage if the pin or washer break off. Again, rough handling will greatly increase the risk of this kind of failure occurring.

In short - handle models with ultra care - your fingers can exert a lot of force inadvertantly!

Cheers,
Alan

Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces. Dr. Carl Sagan

 

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