Correct fit for PML piston

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ArthurSull

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First time using a PML piston ejection system tomorrow. It's not too warm over here in the UK so I know the QT body of the rocket could shrink a little outside, but how loose/tight should the piston be.

At the moment, it doesn't fall out, it takes a gentle pull of the shock cord to drag the piston out. Should it be sanded down to be a looser fit? Should there in fact be a small gap around the diameter to let air past?
 
mine are 'gentle pull' but you're likely safe enough if you have up to 1/32" clearance, as that is still tight enough that flaming bits will not get past the piston until the chute is out.
Rex
 
I sanded mine so that it slides in and out with gravity. There is no visible clearance, but if you tip it over without the NC it will slowly slide out. Only flown three times, but all successful with ZERO damage to the unprotected chute above. Just make sure to have a way to clean out the inside of the airframe between flights, as there will likely be significant BP residue. I use a microfiber cloth taped to a 1/2"x36"wooden dowel, also works well for cleaning tough residue on motor cases.
 
+1 on cleaning. With the water-resistant phenolic motor mounts and plastic tubing, you can basically pour water in and scrub it.

Test before each flight, and as long as it slides freely you are golden.
 
The sticking point on QT is around 40 degree F; below that mark you might have problems.
There is a chart somewhere that was generated after testing it.


JD
 
Because of the temperature fluctuations I've found the best fit for mine is in my parts box on the basement floor. I all most always have problems eventually with the rocket if using pistons.
 
I have good success with pistons on all my models from 3" diameter to 6" diameter. I usually sand the piston sides with an electric sander (often using a mouse sander) so that when I pull on the shock cord it comes out easily. It also helps to rub the body tube inside to remove soot. I have never had trouble with pistons, but have had failures when not using a piston.
 
I ditched mine due to the temps we got here in the UK, every launch I've seen then having to be sanded. Cleaning not a problem but the sanding yeah I want to fly.
 
I never used the pistons on PML kits- always built them without. Your mileage may vary.


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
I tend to build in the winter. When I set up a piston I lay the finished rocket in the stairwell to my basement on a stupid cold day. After a few hours, I go out and sand to fit so if you turn the tube over the piston slides slowly out. After that it will work in most any wx I am likely to want to go outside and fly.

One note. I do not assemble as PML instructs

1) I build the piston as a discrete assembly with a very short strap and use a quick link to connect it to the main motor strap. That way repairs/cleaning are easier.

2) I install the piston upside down. That leaves more room for the recovery gear.​
 
I watched one of my cert people sand for quite a while before he took my suggestion to ditch it. Adding a small piece of nomex made for a satisfactory certification.
Pistons are best left in your car engine where they fit all the time and have oil lubrication.
 
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