Replacing King arms piston

[Adam3088]

After many months of my M4 being useless (been shooting at ~200fps) I've opened up the gearbox to try and work out what's up.

Everything looks to be fine except the compression is total crap, putting my finger over the nozzle there's practically no resistance at all.

Stock piston is 15 tooth (1 steel) with a single o-ring ported plastic head, can I simply swap these out with similar aftermarket ones?

Also the spring guide is a simple plastic one with a washer, is it worth swapping this out for a metal bearing guide while I'm at it?

 

[Lozart]

Yes. Bear in mind that the spring guide will compress the spring a little more which will contribute to fps.

You can easily counteract this by fitting a sorbo pad and correcting the AoE.

 

[Adam3088]

Great, I'll have a look around on ebay.

Also how is the piston head attached to the piston, is it just glued? As my piston is in pretty much perfect condition is there any way to re-use by removing the head it or am I going to have to just get a new one?

 

[RR01]

Most piston heads are screwed on, so removal should be fairly straight forward.

 

[Adam3088]

Doesnt appear to be screwed on, there's no screw head on top or inside from what I can see.

Scrap that, had to use a torch to see it clearly but there's a hex screw hidden inside, I just wasn't looking closely enough!

 

[Sitting Duck]

uhmmm - you might just need a new o-ring, some ptfe tape on cylinder head and maybe an o-ring nozzle

bit of grease etc....

new piston will mean f*ck all - as long as teeth ok and is moving smoothly it is fine for job

get a few o-rings and ensure you can't compress the piston & piston head

if anything a new cylinder head but not always an absolute must - it is how parts:

top dollar top of the range or some old spare bits knocking about - if they work in good condition and fit well together.....

then it is all down to how them parts are fitted that counts

 

[Adam3088]

Did have a look around to see if I had a replacement o-ring, nothing of quite the right size though. Have ordered a zci piston head anyway, at £7 they're pretty damn cheap.

I'll ptfe the cylinder head once I've got the spring guide + piston sorted out, have removed the old head from the piston now so that's going to be re-used.

Will stick with the original nozzle for the time being, the lack of compression was definitely due to the piston head so I should be ok leaving the rest as stock for now.

 

[Sitting Duck]

2 main areas in box is piston o-ring and cylinder head seals

piston o-ring is crap - you can stretch it but it often shrinks back a bit so good compression will only last a short while

(some heat the o-ring to retain the stretch but ffs just get a new one or anything that isn't stretched)

you can buy a pack real cheap and try to stretch one of them with heat if still not brilliant seal/compression

as you will have some spares - always good to have, then if you snap or screw up the o-ring then you ain't boned

the new piston head should be better but consider a few spare bits n bobs at some point

(heck even buy a mixed box of o-rings on fleabay for a few quid, some may work and any smaller ones may come in handy

for other bits n bobs - eg tiny o-ring on barrel to reduce inner barrel wobble etc........