Piston clearance?

[dave2288]

Whats the general rule of thumb when making a new piston for a chamber that originally was inconsistent(hence making a new one).  Trying to smooth up the action on a few of my spring guns, but im very much a noob with making new parts.  My machining/clearance knowledge is all automotive

[Motorhead]

Really depends on STYLE piston in question ... Some are indexed and don't rotate, where some do.
pending run out specs of tube, wall thickness & degree of shape change with heat / cold all influence how snug / loose a fit is best.  Wear pads, buttons or rings ... metal on metal again part of it.

Whole lotta nuttin specific ... sorry 

[dave2288]

Well, this piston doesnt rotate, but i have no good response for the rest of the things you brought up lol.  I can get measurements of the tube, but i have no idea on degree of change with temp.  I can freeze it and meaure id and then warm it to 100 degrees or so and remeasure i suppose.  Right?

[Motorhead]

Well, this piston doesnt rotate, but i have no good response for the rest of the things you brought up lol.  I can get measurements of the tube, but i have no idea on degree of change with temp.  I can freeze it and meaure id and then warm it to 100 degrees or so and remeasure i suppose.  Right?

Non rotators .... always contact the same place every cycle and can generally be set up with minimal clearances pending contact surfaces ARE NOT galling prone or have buttons or pads in use to mitigate the wear-n-tear.
As with general mechanics, tighter the fit ... lower the viscosity lubricant can be.



 

[Machinist]

I'd try .0015"-.0025". Shoot for .002"

Most tubes are tapered and or out of round.  Make a test slug and see how it fits in the tightest spot of the bore.  If the test slug doesn't move freely, take off .001 and try again. When you have a slug that is free, but has a slight drag, make the piston .002" smaller.

I've only made a few pistons for guns, but this clearance worked for me.  If you were going to use wear bands, I'd try .001" clearance.

Steve