Author Topic: Piston heat expansion?  (Read 434 times)

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Offline Bigdude

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Piston heat expansion?
« on: June 11, 2019, 03:26:42 AM »
Does anyone know if the piston expands (diameter enlarges) after it gets hot? I was just looking at my custom red dot mount and there is only approximately .001" or .002" gap between the piston and bottom of mount. Basically my mount straddles the piston and I am concerned that after it heats up the mount and piston will be touching. I am afraid to shot it actually.




Offline mattra1

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Re: Piston heat expansion?
« Reply #1 on: June 11, 2019, 10:50:36 AM »
Normally, when something like a rod heats up it expands lengthwise and because objects maintain their same volume, it would shrink its diameter. However, it could bend slightly or flex during operation and make contact where it is close. You could put some ink on the area and check if any is removed after firing. That could give you an idea if any contact is made, but it won't be due to expansion.

Offline mattra1

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Re: Piston heat expansion?
« Reply #2 on: June 11, 2019, 10:52:53 AM »
You could also relieve an area under your mount in the center providing slightly  more clearance if you are concerned.

Offline Bigdude

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Re: Piston heat expansion?
« Reply #3 on: June 11, 2019, 10:54:05 AM »
You could also relieve an area under your mount in the center providing slightly  more clearance if you are concerned.

I did using my dremel but I need a mill to properly relieve it.

Offline Bigdude

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Re: Piston heat expansion?
« Reply #4 on: June 11, 2019, 10:55:04 AM »
Normally, when something like a rod heats up it expands lengthwise and because objects maintain their same volume, it would shrink its diameter. However, it could bend slightly or flex during operation and make contact where it is close. You could put some ink on the area and check if any is removed after firing. That could give you an idea if any contact is made, but it won't be due to expansion.

When barrels heat up they expand, right? But when the piston heats up it doesn't? Slightly confused.

Offline RSR

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Re: Piston heat expansion?
« Reply #5 on: June 11, 2019, 02:52:24 PM »
Does anyone know if the piston expands (diameter enlarges) after it gets hot? I was just looking at my custom red dot mount and there is only approximately .001" or .002" gap between the piston and bottom of mount. Basically my mount straddles the piston and I am concerned that after it heats up the mount and piston will be touching. I am afraid to shot it actually.




Towards the rear (photo above) it looks like the piston has greater clearance under the rail than under your mount.  I'd try to ensure those are roughly the same -- double check the piston w/ calipers but piston shaft should be pretty much the same diameter throughout the middle shaft section in question.

Also check and see how much movement there is in your piston with the upper rail removed, at a minimum account for that.  But yeah, looking like you might want to mill the bottom of your mount a bit.

And definitely shoot the gun prior to installing an optic to ensure no interference/hits that could kill the optic.

Offline Bigdude

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Re: Piston heat expansion?
« Reply #6 on: June 18, 2019, 02:40:04 PM »
I purchased a new piston from Bonesteel Arms and it has a slightly smaller diameter so I installed it. Now there is actually visible clearance between the bottom of the mount and piston. It's not much but it's something.

Here is new piston installed:



Here is old piston installed: