How loose should the piston head be?

[digital_blue]

I just scratch built an MP5 from various parts and I slapped in the mag expecting at least 250-300 FPS from the stock well gearbox. What I got was a less than impressive 170 or so. I opened up the gearbox and it appears that the piston head was very loose in the cylinder and the O-ring wasn't sealing against the sides. Should the piston head be this loose?

[Darklen]

There should be no movement between the piston head and the piston. Any movement could lead to failure of the connection and you'd end up damaging or even destroying the piston, piston head, cylinder and/or cylinder head. Make sure the piston head is tight to the piston.

[Gigaknight]

Mechbox.com suggests using loctite to ensure that the piston head won't move unless you try removing it manually. What do you think of that?

[Syn]

To add- check how you screwed in the tiny bolt to the piston head, cross threading it, it wont screw all the way in to tighten the piston head.

For the o ring, I like to strech it out by hand and lube it making sure I get no lube in to the piston head airholes. Then put your thumb on cylinder head nozzle hole and push the piston head quickly forwards into the cylinder. If the piston+piston head immediately creates air pressure resistance, youv'e got a good seal. Then test the nozzle when it is at its farthest forward like it would be by the tappet plate and test for compression again.

[Darklen]

Locktite will cause polycarbonite plastic to crack and fail in a very short period. If the piston head has a metal female thread embedded in it or is made of metal itself, it should be ok so long as you put just a dab on the threads, soak up the excess with a tissue, then thread it in.

Avoid using too much and avoid allowing it to contact and plastic parts. If you wipe it off right away you'll be fine, but if you leave it on, you could have the plastic component fail on you in time. Also, I'd recommend using Locktite 222. It's low strength, milky purple in color. It will hold the part easily, but will still allow you to remove it whenyou want. If that's not strong enough, use Locktite 242, it's a milky blue color, is stronger than 222 but will still allow you to remove the part, though it will be hard to get out. If you never want the part to come out again, ever, use Locktite 271. It's clear red and permenent.

[Gigaknight]

Thanks Darklen, that could very well have saved me a lot of future difficulties, heh heh. Though aside from loctite, is there anything else that can be used on polycarb piston heads?

[Darklen]

I used to use silicone chaulking when installing scope mounts on high powered rifles (real steel). It seem to absorb the shock well and the screws didn't back out. Never tried it with a piston head (I use locktite all the time at work, so I have everything I need in my home shop), but I can't see it hurting. Just wait a full 24 hours for it to cure properly and make sure there's no oils, silicone or otherwise on the curing surfaces. Hell, you could even try a wrap of teflon tape (plumbers tape), it'd probably work fine.

[Gigaknight]

Originally Posted by Darklen

I used to use silicone chaulking when installing scope mounts on high powered rifles (real steel). It seem to absorb the shock well and the screws didn't back out. Never tried it with a piston head (I use locktite all the time at work, so I have everything I need in my home shop), but I can't see it hurting. Just wait a full 24 hours for it to cure properly and make sure there's no oils, silicone or otherwise on the curing surfaces. Hell, you could even try a wrap of teflon tape (plumbers tape), it'd probably work fine.

Well, if you say Loctite works, then Loctite it is! That should help my piston assembly out a bit.

[Crunchmeister]

Remove the o-ring from the head and stretch it. It should be loose around the piston head, not tight. That will give you amuch better air seal.

[Darklen]

Dummy me, now that I've reread the original post, does the piston head even have vent holes? I know TM does not and I always drill them in whenever I open a TM box. There is usually 4 knockout pin marks on the head face I use as a guide for the first four holes, then I measure and drill another four holes in between these for eight holes spaced equally running just inside the outer rim of the piston head.

SYN outlined the proper procedure for testing compression, though since this box is a Well, I'd probably just replace the oring with a quality part, slightly larger, that doesn't have a seam around it. If you don't have that option, try getting an oring lube that will expand the orings, like DOW 55. This can help the sealing factor quite a bit, but it could also destroy the oring as I have no idea what orings Well use.

Without doing the compression test, like SYN outlined, the piston head will appear quite loose in the cylinder. It had to be loose on the return or pulling stroke as the motor is already fighting the spring and you want as much air in the cylinder as possible before it hits the compression stage.

[CDN_Stalker]

Originally Posted by Gigaknight

Thanks Darklen, that could very well have saved me a lot of future difficulties, heh heh. Though aside from loctite, is there anything else that can be used on polycarb piston heads?

Nail polish. Seriously.

[digital_blue]

Originally Posted by Darklen

Dummy me, now that I've reread the original post, does the piston head even have vent holes? I know TM does not and I always drill them in whenever I open a TM box. There is usually 4 knockout pin marks on the head face I use as a guide for the first four holes, then I measure and drill another four holes in between these for eight holes spaced equally running just inside the outer rim of the piston head.

SYN outlined the proper procedure for testing compression, though since this box is a Well, I'd probably just replace the oring with a quality part, slightly larger, that doesn't have a seam around it. If you don't have that option, try getting an oring lube that will expand the orings, like DOW 55. This can help the sealing factor quite a bit, but it could also destroy the oring as I have no idea what orings Well use.

Without doing the compression test, like SYN outlined, the piston head will appear quite loose in the cylinder. It had to be loose on the return or pulling stroke as the motor is already fighting the spring and you want as much air in the cylinder as possible before it hits the compression stage.

The piston head is not vented. I actually took off the O-ring and put it down the cylinder and there seems to be a lot of play. Should there be this much spacing between the O-ring and the cylinder?

[grimreefer]

What is the idea behind vented piston heads? Or not the idea...the mechanism, rather? I don't mean to try and steal the thread, but feel it's relevant.

[Syn]

Originally Posted by grimreefer

What is the idea behind vented piston heads? Or not the idea...the mechanism, rather? I don't mean to try and steal the thread, but feel it's relevant.

Having the symetical holes eg 4 or 8 in you piston head causes air to rush into those holes (when the piston accelerates forward with high velocity by the spring). THe Air goes into the holes and this air creates pressure against the inside surface of the "loose" oring. As the air pressure builds up the orring is stretched out(pushed out from all sides) and this makes the oring outer surface contact the inner surface of the cylinder resulting in a seal and good compression.

digital_blue is it possible to drill symetrical holes in your piston head. I think this would help you greatly. Also how were your parts matched up? Did they come in a kit or are you mixing and matching?
As far as the loosness, My piston head oring has much play, but it is the oring streched out which causes the gool seal.

If you are not getting good compression:
1) get a vented piston head or drill your own holes (NOTE the holes do NOT go straight through the piston head to the back side of the piston head, the holes are drilled part way through to the Oring groove in the piston head. Drilled hole straight through to the back side caused very bad seal.)

2) Goto home depot with your old washer, piston head and lubes cylinder and go to plumbing dept and try on a bigger washer. Find a rubber oring that fits the piston oring groove and moves freely but has a greater outer diamter than your old oring. You can also use thicker 100% silicone paste from the plumbing dept to help fill the gap especially if the cylinder lube you are currently using is very thin.

But the Piston head holes help SOOOOOOOOOO much to get good compression.

[Skruface]

Originally Posted by digital_blue

I just scratch built an MP5 from various parts

Just to throw this out there....since you built a gun from a pile of parts (of dubious origin?) there's always the possibility you have a bore-up cylinder....in which case, no amount of fiddling with the piston head will ever make it fit properly.