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Valve surface finish

Postby WillDellinger » Sat Jul 20, 2019 8:17 pm

Ever since I saw that a typical Hetman lapping compound is just 1200 grit, I've wondered how smooth a piston is supposed to be.

I got MAW valves for my horn and after being lapped in by a local shop, I can see that patches of the valve are very shiny (fingernail moves very smoothly over it) and other patches are grey/dull/slightly pocked when you look really close.

But I know oil - bearing surfaces shouldn't always be very smooth. Like hand - scraped mill/lathe ways that need to slide vs super smooth jo blocks.

I will say...the valves work okay but not great.
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Re: Valve surface finish

Postby WillDellinger » Sun Jul 21, 2019 2:37 pm

I looked at everything I could figure on the horn before taking it to the shop. I did measure the guides to be a bit "taller?" (Further from the center of the piston) than the OEM valves. But filing them down to match the others did not make the difference. And their other dimensions matched. When first purchased, the valves would go in the cylinders, work alright for about 15-30 strokes, and then get quite stiff/stuck. Both the cylinders and the pistons were clean

In as many places as I could get the micrometer, the old and new valves were the same size.

I'll end up putting in a purchase request to have the valves fitted better by a different shop since it's a gov horn, but I was curious to get some input on what the optimal piston surface is.
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Re: Valve surface finish

Postby WillDellinger » Sun Jul 21, 2019 7:03 pm

I considered the lime. But I cleaned well, let it all dry, and saw no white.

15-30 may have been generous. I think it was something else. Especially considering that all 5 sets of old valves (3 OEM, 1 Maw) from among the three horns that use that valve set worked interchangeably, but the new maw set wouldn't work in any of the three horns.

...actually the horn they are currently in was the horn in which they were closest to working in the first place.


I suppose this has turned into two separate questions...
But does anybody know about the surface finish thing?
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Re: Valve surface finish

Postby hup_d_dup » Sun Aug 04, 2019 6:58 am

WillDellinger wrote:But does anybody know about the surface finish thing?


It looks like you will have to ask the question in a different form.

Or maybe start all over in a different thread.

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Re: Valve surface finish

Postby roweenie » Sun Aug 04, 2019 5:01 pm

I haven't lapped any valves (not only have I been "lucky" to not need it in any of my projects, but if they did, I would only trust that to a seasoned pro), but for lapping valve slides, I've used the Dupont #7 that bloke recommended, with good results. Used with a light oil, it works well.

There is another lapping compound I've used on slide tubes called "Timesaver", that I've also used for lapping the babbitt in crankshaft and connecting rod bearings, where tolerances need to be just as precise as piston valves. I usually use their number 100 yellow, made for soft metals:

https://www.newmantools.com/lapping/time.htm

The thing I like about it is that it is so fine that it is designed to degrade quickly and won't embed into the work.

FWIW, Meinlschmidt is the "Cadillac" (or should I say "Mercedes-Benz") of valve manufacturing, and I've personally never had an issue with anything they've made.
"Even a broken clock is right twice a day".
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Re: Valve surface finish

Postby iiipopes » Sun Aug 04, 2019 6:02 pm

Pre-cyborg King would leave the valves "rough" and marketed that as micro-grooves that would better hold onto valve oil, or something like that. Now, whether that was actually what they did on purpose, or just gave a marketing spin job (pun intended) for lack of valve polish to get more instruments done and out the door quicker, is probably not discoverable. The valves on my early '70's Super 20 trumpet are smooth, and look to have been replated and lapped in properly. I have no issue with oiling.
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