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Old 03-25-2020, 08:06 PM   #1 (permalink)
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Default Piston ring conversion from graphited yarn in old Aster

I hope all this makes sense and please refer to the images as they really tell the story at a glance and this may inspire some to replace the old yarn with something more lasting.
I acquired an unbuilt kit of the early Aster PLM Pacific that has graphited yarn for piston rings, as many older Aster do, and I wanted to replace this with piston rings. As the original piston has really big and deep grooves to accommodate the graphited yarn, I wasn't sure if just putting in a huge rubber o'ring or thick and deep piston ring on the same piston would be suitable. I thought a thick o'ring or piston ring would not be flexible enough as I have only seen thinner ones on other loco's so I asked Gordon Watson (of Argyle loco fame and a Jedi Knight of small steam) and he suggested making a teflon piston ring supported on by rubber o'rings underneath to provide some tension to the ring. I had heard of this idea before and as I didn't want to make new pistons I thought I'd give it a go.
So armed with a small piece of graphite impregnated teflon from Gordon to make piston rings I started first by measuring the piston that is 13mm overall diameter with a 2.95mm wide by 3mm deep groove giving a 7mm diameter ring groove.
As the o'rings are used as "springy tension" support under the teflon ring I found three o'rings of 1 mm thick would fit snugly across the existing ring groove leaving me room for a better teflon piston ring.
Loose o'rings are hard to measure and ideally they should be a slight stretch over the ring groove ID or a good friction fit. By making a dummy rod the 7mm diameter of the piston groove I found I had a good friction fit and the 'new' ID of the groove (with o'rings fitted) is now 9mm.
So now I had to make the teflon rings with an ID of 9mm, width of 2.7mm to allow some side clearance and OD of 13.1 that allowed some tension on the scarf cut Piston ring.
Once scarf cut these rings will squeeze down with a little tension on the o'rings underneath and on the internal wall of the cylinder.
For those who are wondering a scarf cut is, it's the diagonal cut from the top down to the ring groove ( see image) . This diagonal cut enables the ring to be opened for fitting but more importantly allows the ring to contract and expand to follow the cylinder bore without losing it's steam seal.
The engine is now assembled and is very smooth and works very well on even the smallest throttle opening and the self lubricating properties of the graphited teflon piston rings and the o'ring support should easily outlast the life of the engine and no replacing worn out yarn like an original one after many miles.
Russell
Attached Images
File Type: jpg Cylinder and pistons.jpg (168.9 KB, 57 views)
File Type: jpg Piston.jpg (815.5 KB, 53 views)
File Type: jpg Aster PLM.jpg (298.9 KB, 48 views)
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Old 03-25-2020, 10:36 PM   #2 (permalink)
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Nice job Russell.
Well thought out.
Regards,
David Leech, Delta, Canada
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Old 03-27-2020, 06:20 PM   #3 (permalink)
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Thanks David, I could have gone the easy way and just put in one big fat o'ring and see what happened as it may have worked.
If anyone out there has put in an o'ring to replace the graphited yarn on an early Aster standard piston or any other variation please let me know as I have a couple more to do and no more teflon left.
Russell
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Old 03-27-2020, 06:36 PM   #4 (permalink)
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I have just stuck with graphited yarn.
In fact those in my King Arthur have been in there from day one in 1979 when I built it.
Still seems to be fine.
Cheers,
David
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Old 03-28-2020, 10:53 AM   #5 (permalink)
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To use just an Oring means you need to turn a new piston as the groove depth is to deep. I replaced my Climax yarn with twisted up teflon tape, It took a number of tries with taking a layer off at a time to get the proper tension but once done it was a very smooth running engine.



If you need material for rings, there are a number of suppliers. You might want to try Garolite as Gordon/Argyle Locos uses it and is obtainable in Australia.
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Old Yesterday, 06:17 PM   #6 (permalink)
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Thanks Jason, you confirmed my thought of just using an o'ring to fit the large groove would be too much.
I will look at your idea of teflon tape to do my early Aster shay as the pistons are really small and the graphited yarn doesn't last that long as I have done it before.
Is the teflon tape the same as Plumbers use and do you twist it to make a string and wind into the groove or try to use it flat to build up in the groove? Any tips on installation would be handy.
Russell
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Old Yesterday, 07:28 PM   #7 (permalink)
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Russell.
On my Accucraft, I replaced the o ring with twisted plumbers tape.
I just kept filling up the grove until it was a nice tight fit in the cylinder.
If I remember it took me a couple of times to get it just right.
Not too much, and not too little, just right!!!
Maybe Jason has a better way.
Cheers,
David
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Old Today, 12:57 AM   #8 (permalink)
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Thanks David I'll give it a go soon on the three tiny shay cylinders.
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Old Today, 02:58 PM   #9 (permalink)
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Nope no better way, I just started with it packed in well and a small flatter wrap proud of the piston. Thats what I trimmed back until I was happy with the tension. Basically you want to be able to cover the ports and have a vacuum pressure on the piston when pulled. Also it needs to slide with little pressure. Too much will make it run poor and too loose and it will blowby. The suction test is the best way.
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