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Post by terry1956 on Oct 18, 2015 11:57:50 GMT
hi, for sometime I have been playing around with my B1, trying to understand why it would not run on air. Today I found that the piston valves are allowing air to bypass the valves heads so allowing air into both sides of the piston cylinder at the same time. This of course was leading to the motion locking up. As the piston valves are a good fit at the start of both sides of the cylinder, it must be down to wear in the mid section. So what is the best way to repair, rebore the valve cylinder and make new valve pistons, or add piston rings to the valves. if piston rings what is the best material to use, and the best way to complete this job. thanks. michael
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Post by Roger on Oct 18, 2015 18:49:56 GMT
Hi Michael,
Are you certain this is the cause, because it's also possible that air is passing outside of the liners to reach the passages? This is the subject of another thread currently running.
If you have cutouts for the port opening, you can't use piston rings. You need to support piston rings in several places all the way round the bore where the port is, else the rings will open out into the available space and jam up.
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Oct 18, 2015 21:04:28 GMT
hi michael,
it is possible you can lap/hone the piston valve liners. first of all you need to turn up some thin plug gauges and check the bores. anything on a rod will do for the plug gauges.
how you then proceed will depend on whether cast iron or gunmetal cylinders.
new piston valves will be required.
all the above is quite a precision job and will require a major strip down.
i personally doubt if the existing liners leak between the liners and cylinders, but anything is possible and as Roger says the modelworks/winson liners prove that the impossible does sometimes happen!
new piston valve liners is quite a big awkward job and IMHO requiring absolute precision. but if new ones are required you can incorporate the type of port shape used by Roger with very good advantage.
i have never liked solid piston valve bobbins.
cheers,
julian
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Post by terry1956 on Oct 20, 2015 9:56:13 GMT
hi. thanks for the replies. the cylinders are cast iron. I have found that the wear is around 20 thousands of an inch, so the thickness of a sheet of printer paper all round the valve. There is know blow pass the liner.I think that following julians advice I will sort out some lap/hone tools, and make a new set of bobbins.thanks, michael
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Post by ejparrott on Oct 20, 2015 10:18:43 GMT
I'd take it to a professional honers and get them to do it. They can also tell you exactly what size they have made the bore to within tenths of a thou, something which a model engineering can't usually do. This will enable you to manufacturer far far superior fitting bobbins, and hopefully achieve perfect sealing.
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Post by terry1956 on Oct 20, 2015 14:14:24 GMT
not a bad idea, but I have know idea of the location of any professional honers, most of the engineering here in the S/W as long gone. what tolerances should I be looking for to allow for expansion between piston valve and cylinder. michael
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Post by ejparrott on Oct 21, 2015 8:54:24 GMT
Remind me what part of the world you're in?
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Post by terry1956 on Oct 25, 2015 15:24:47 GMT
somerset, but I travel around the S/w a lot. michael
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Post by bobpendleton on Oct 25, 2015 16:22:34 GMT
not a bad idea, but I have no idea of the location of any professional honers, most of the engineering here in the S/W as long gone You might take a look at hamlinsengineering.co.uk/Their web page suggests a very competent organisation but I have no first hand knowledge of them. They are in Bridgwater, near the junction of the Bath and Bristol main roads. Bob |
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Post by terry1956 on Oct 25, 2015 19:09:02 GMT
Thanks Bob, I will give them a ring Monday. michael
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