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Post by Malcolm on Feb 13, 2006 17:41:47 GMT
An interesting difference of opinion happened the other day. The subject was clearance of cast iron pistons in cast iron bores with cast iron rings. I have always used one thou per inch, same as bearings. Another suggested as little as possible. yet another, a respected constructor, suggested about three thou per inch, presumably on the basis that the rings do the sealing and you don't want the piston to seize if it expands quicker than the bore on start up. What views do the group have on this?
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Post by the_viffer on Feb 13, 2006 19:05:43 GMT
Traditionally so that you can't get a worn silver sixpence down between the bore and the piston. It is the kind of accuracy I get.
I've not done CI pistons with CI rings in a CI bore since I made a Stuart 7A in '75. I've not seen it for 20 years so I can't measure it for you
I think there are two issues. One is about ring fit and side wall pressure and the other about piston fit.
In a ringed piston I'd say it is the ring which is making the fit and piston could be a bit away from the walls. I'd buy 3 thou per inch.
In an unringed piston a thou per inch sounds good.
As little as possible is clearly not good because you could make it a dead fit which would be way too stiff and worse when warming up.
For ring fit I think Tubal Cain's ME Handbook has some sensible advice too long to repeat here unless requested.
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Post by chris vine on Feb 14, 2006 1:12:22 GMT
I think it depends on whether the piston is in a steam cylinder or in a steam valve chest.
In the power cylinder the piston will be somewhere between the temperature of the incoming steam (which is already cooling having come through ports) and the exhaust. However in the valve chest I think the piston valve will be pretty much at the temperature of the incoming steam and so will require more clearance.
I have noticed this in practice, has anyone else noticed it?
Chris.
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waggy
Statesman
Posts: 747
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Post by waggy on Feb 14, 2006 14:19:31 GMT
Never used cast iron for pistons, always aluminium. This only because my local supplier sells cobs of the stuff at very reasonable prices. When I made my first pistons I thought I'd look in one of the Haynes manuals for some idea on clearance. Small, its hardly worth bothering with! I leave two thou' 'twixt cylinder wall and piston skirt, never had any problems.
Waggy.
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