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Post by jeffhills on Jun 29, 2007 18:33:29 GMT
What would the more experienced members recommend for a piston clearance on a 1.3/4" bore gunmetal cylinder? I intend to use two PTFE rings in individual grooves, my thinking is a reasonably large clearance is needed as the piston rod and cross-head have small clearance, I fear the whole thing will just bind up if a third tight fit is included, no matter how much care is taken to get them in alignment.
Jeff
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simonwass
Part of the e-furniture
Cecil Pagets 2-6-2 of 1908. Engine number 2299. Would make a fascinating model....
Posts: 472
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Post by simonwass on Jun 29, 2007 22:30:18 GMT
When I use O rings in gunmetal bores, I machine the piston at least 30 thou undersize and super polish the bore. The O ring does the sealing and the piston is just something to guide the O ring. I'd do the same with the PTFE rings, you dont want any chance of the piston scraping the bore as a scratch will soon start to wear the ring away.
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Post by Boadicea on Jun 30, 2007 7:48:31 GMT
Jeff, I think Simon has it about right.
However, I hope you will not mind me saying I think you have the cart before the horse a little. It is the pistons and bores which must be right, and any compromises that have to be made should be done with the other components.
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Post by jeffhills on Jun 30, 2007 12:16:33 GMT
I think it all has to be right. its easy to get the back cover concentric and I have allowed extra material on the 'jaws' of the guide bar brackets, I will make plugs for the ends of the cylinder bores to mount a ground rod back through the brackets, then after measuring,machining of the 'jaws' can be carried out. I am a little puzzled on how to get it right laterally, I wish to harden and grind the guide bars so drilling in situ is not that practical.
Jeff
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