No Compression?

[desperado]

I have an M4 parts gun that i'll be putting back together. The seller claims that theres no compression in the cylinder. could this be the Cylinder Head thats leaking? or the O-ring around the Head?

please, no flaming me

:flame:

[Syn]

Well you won't know for sure unless you inspect it physically for wear, damage, or even mismatched parts like a small diameter oring. It could be caused by something as simple as the piston head holes being clogged up with lube and the oring wont expand out to maintain compression.

So inspect cylinder head, nozzle, oring would be a good start. Check compression by: http://forums.airsoftmechanics.com/i...hp?topic=908.0

"
Part 1: compression inside the mechbox

First of all, excessive grease is going to do negative results. Be sure to clean your cylinder, cylinder head of grease. Only the piston heads' o-ring should have less than a half fingertip of preferable thick silicone grease on it.

First, we test piston assembly + cylinder assembly without nozzle
If you put in the piston into the cylinder unit and block the cylinder head pipe with your finger, it should block instantly after the hole in your cylinder (if any). If not, you may try a different o-ring or a more thick grease (For example, those thin cylinder greases from G+P don't work well).

Next, try with nozzle on. Note that most nozzles will seal perfectly if you push them towards cylinder head. Use the tappet plate to push the nozzle to its most forward position and try again.

If you have any compression issues at this point (cylinder head vs. nozzle) your options are:
- try to apply a bit thick lube between nozzle and cylinder head (normally works on minor air leaks), but dont't overgrease
- try a different cylinder head / nozzle combination
- ignore this issue (it may cause some consitancy issues)

Some people actually use teflon tape for sealing the nozzle. I won't recommend it, because the tape can block the nozzle and cause misfeeds or damage to nozzle and/or tappet plate.

Part 2: detecting air leaks between mechbox and barrel

Note: this is an advanced troubleshooting option, if you get lower results than expected. If you want to get any results, you must be sure that your cylinder unit compression is perfect.

In some cases, you may want to detect where air leaks outside the mechbox.
For the next tests you need something to block your barrels' end. You can use your finger, but for myself I made a barrel plug out of hot-glue using an old stock marui barrel. Be creative to make your own, but be sure that it doesn't leak

Apply your barrel with hopup unit to your opened mechbox and the barrel plug installed.
First, check if your nozzle gets far enough inside the hopup unit. It should not leak air towards the BB feeding tube.
Next, test the systems compression by pushing your piston in. Normally, you should have no air leaks. If you have any leak on non-moving parts, you can try to seal them for example using teflon tape."

[desperado]

will do. will keep this thread updated..

[Skruface]

Check to make sure the piston head is in good shape and perfectly round, esp if it is plastic one. I just found that the p.head on a AK mechbox I was working on was "flattened" a little - lots of lost compression. Replaced with an Area1000 one and compression returned to perfect. I didn't really notice it until I took a real close look....

[desperado]

the mechbox is apart. putting it together soon. it's got a metal head.