Oversized coupling rod crank pin holes

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jon price
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Oversized coupling rod crank pin holes

Postby jon price » Mon Sep 11, 2017 4:04 pm

Can anyone tell me what the implications are of having oversized crank pin holes in coupling rods on an 0-6-0. The holes should be 1mm, but are 1.5mm. Is this disastrous, or liveable?
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Philip Hall
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Joined: Mon Aug 10, 2009 7:49 pm

Re: Oversized coupling rod crank pin holes

Postby Philip Hall » Mon Sep 11, 2017 6:07 pm

Jon,

It might run OK, but then again might be prone to uneven running. Many RTR engines have this kind of slop and are OK, although I usually take out a bit when converting them.

I am, though, a bit mystified with your dimensions. The standard (at least on Ultrascale and Alan Gibson crankpins) is 1.5mm over the crankpin bush, and I open the coupling rod hole out to 1.6mm once all is aligned. A 1mm hole sounds to me like the diameter of a Romford crankpin, designed for their small diameter turned retainer. Are you using them in your wheels?

Philip

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Paul Willis
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Re: Oversized coupling rod crank pin holes

Postby Paul Willis » Mon Sep 11, 2017 6:21 pm

Philip Hall wrote:I am, though, a bit mystified with your dimensions. The standard (at least on Ultrascale and Alan Gibson crankpins) is 1.5mm over the crankpin bush, and I open the coupling rod hole out to 1.6mm once all is aligned. A 1mm hole sounds to me like the diameter of a Romford crankpin, designed for their small diameter turned retainer. Are you using them in your wheels?


Philip beat me to it...

I'm sitting here with a box of wheels and crankpin bushes, and the micrometer. It's showing 1.5mm as the outside diameter of the (Gibson) crank-pin bush. Well, 1.48mm, but that's my rotten measuring skills.

It sounds to me that you don't have a problem, you have the basis of an accurate and smooth running chassis :-)

Cheers
Flymo
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www.5522models.co.uk

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jon price
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Re: Oversized coupling rod crank pin holes

Postby jon price » Mon Sep 11, 2017 9:52 pm

thanks for this reassurance. I think I was misled by the holes in the etched crankpins being 1mm, obviously because you can open them out to 1.5 for the AG crankpin bushes, but you couldn't close them to 1mm for the Romfords. I never thought to check the actual crankpin bushes though. Good, I can now get on with the comedy task of attempting to produce a well running chassis. So far only a (well running) rigid chassis under an RTR body has emerged from the works, but if this chassis comes out OK the dam will burst and finished sprung locos will start pouring off the workbench. Or something.
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