fitting piston valves to liners with molybdenum disulfide.

Hi all

I am just reading through some build instructions from Don Young and it involves fitting some cast iron piston valves to cast iron valve liners. Don suggests turning the piston about half a thou bigger than the liner and then coating it in molybdenum disulfide powder or grease and then hammering the piston through the bore of the liner. He repeats this a few times and reckons that both components will have a mirror finish. Has anyone ever tried this or had any success with sort of thing? I am a bit sceptical about this and really don't want to trash my cylinder liners.

Thanks

First you need the calibrated measuring instruments to do the inside /outside measuring to HALF a. THOU!

I’m intending to try a very light drive fit on a friend’s Springbok. The ports are large slots which precludes rings on the piston valves, which are vastly superior. Plain bobbins wear and start to blow eventually. I’ve made a series of plugs for him to try in the bores. Measuring is not accurate enough as Bernard says. Half a thou is far too much interference I would think.

Thanks that all makes a lot of sense. Yes someone else said that is far too much and best to go for a tight push fit. My liners also have a large internal slot in the middle so I am thinking of scrapping them and putting it on the outside so I can get iron rings on the valve.

Don’t want to speak ill of the dead but Don done very little and wrote a lot. An industrial method for finishing a bore is to grease it well and press a hardened ball through it, not sure of how much underside the bore should be but no doubt someone will tell us in a while.

I have heard of the ball method but only in plain bores as when there are ports the bore size must change as the pressure is on less material!!

I remember the late Geoff Wigzell of the Gravesend club trying this method on his Don Young designed class 2, on the advice of Don himself. Geoffs comments on the method would get me banned from here for life, suffice it to say I believe he made new liners, which were ruined by the process, and made fluorosint valves. I know Don used them in the K1/1 he built but I was told by someone who knew him well did very little running so the method was really unproven. Geoff was a good engineer and if he couldn't make the method work it must be difficult to achieve success.

Ball-sizing works very well in industry - including engine manufacturing - but they have the equipment including very sophisticated metrology departments and dimensional controls!

Piston-valves in full-size do have piston-rings but the ports are not simple radial slots. They are commonly triangular or parallelogram holes so the rings are always fully-supported on the diagonal bars, a bit like the wheels over point-blades.

Not something I've ever tried but I know this has been used successfully on miniature locomotives. I think they can be plain round holes, too, with a slight loss in port area perhaps.