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Could someone post a picture of obviously worn pistons? I've got a few old tubas/sousaphones at the school where I just started teaching and was interested to compare them to some obviously worn valves to see how bad off they are. Thanks!

I bet if you also posted pictures of the valves the TNFJ would give their $.02.

An easy test would be to pull the appropiate slide and listen for a nice pop or gurgle no pop or gurgle may indicate a worn piston. You could also remove one slide at a time not the main slide and block the end of the slide recievers with a finger and blow if you feel lots of pressure the valve is not worn if the air leaks away quicky they are worn.
Another easy one is use some thick oil on the valves and if the tone and pitch improve your valves are worn.

Generally... you will see spots on the piston that have the nickel worn away to expose brass.

Try to move the pistons side-to-side in the casings. Take the top cap off and 'wiggle' the stem while watching for clearance around the piston. Any clearance you can see is usually too much for the horn to continue to play well.

Kings are especially prone to leakage because there is so little 'seal' area between the ports.

Sometimes pistons appear to have great plating but leak. New pistons might have been dropped into worn casings.

I use Ultrapure synthetic oil on the valves of my approx 20 year old Besson Sov EEb. Within 24 hours the valves are dry and need re-oiling. Is this a sign of worn valves, meaning the gap between valve & casing is too large to hold the oil up on the valve and, as a result the oil is draining off the valves? Should I use a heavier oil to fill the gap? I should explain that I use a Besson grime catcher clipped to the bottom valve caps, and it's hard to tell what proportion of the moisture that collects is oil.