Hard to Diagnose Compression Issue

[cpldave]

Hi All,

I recently bought a replacement cylinder, cylinder-head, tappet-plate, piston + piston-head and air-nozzle and fitted them to my gearbox, along with an M120 spring. The idea was to get the rifle shooting about 400fps and lock it to semi so I could use it as a single-shot only DMR at the site I play on.

Unfortunately, after fitting the parts, my rifle is only shooting 318-330fps, which was what it was shooting with the stock parts. I've taken the compression system out and tried putting my finger over the cylinder-head nozzle and pushing and it doesn't compress, so I don't think it's the cylinder or piston heads causing any issues. It's an ICS rifle and all the parts are Ultimate, so I would assume they're all fairly well mated. I read somewhere that the air-nozzle on ICS guns is different, but I've tried the old ICS nozzle and the FPS doesn't change.

Could it be the fit between the cylinder-head and the air-nozzle itself? Or the air-nozzle -> hop unit?

A couple of notes:
- Using a 460mm barrel + full cylinder
- Standard ICS rotary hop unit and bucking
- All upper-gearbox parts are now Ultimate

Does anyone have a good way of testing whether air is leaking between the nozzle and cylinder-head? Same goes for the join between nozzle and hop-unit? (I can't think of a good way of testing it :P)

Cheers,
Cpl Dave

[a_flea]

Re: Hard to Diagnose Compression Issue

-Try the old tappet plate, not sure on the compatibility of ics gbox and aftermarket tappets, it might not allow the nozzle to fully return to the back of hop...?

-What's the compression like with the nozzle halfway along the cylinder head shaft?

[Genesius]

Re: Hard to Diagnose Compression Issue

I have an ICS m16 and I'm doing exactly the same thing.
I'm shooting 375fps wiv the standard string.

I've got a double o ring guarder cylinder head, guarder bearing piston head and stock piston.

Try getting those bits and using silicone gease on the cylinder head and piston head, not the spray stuff.

Try coving the holes in the ported m4 cylinder with a thin layer of ptfe tape.
Renew the air nozzle with an ICS one. Try a new ICS tapper plate.

Check that the hop up is free to move on the spring. So take out the upper gearbox and see if it springs in and out freely, if not then that can cause sealing issues.

If your still getting nowhere try gutting 3mm off of the hop up bucking and wrapping up to where the C clip goes with ptfe tape to make sure air isn't escaping around the bucking and down the outside of the barrel.

I'd say try all those free options first using ptfe tape because it costs pence then c if that improves it.

Post back and I will see if I can help you from there.

It could always be that your motor isn't up to the job of compressing that spring. What motor do you have? I run a turbo 3000 as its high torque.

To be honest though even without the new cylinder head and piston head (so just standard parts) I was getting 365fps and it jumped up only when I added a TB barrel. Less space for air around the BB means the same amount of air pushes the BB faster in a tighter space.

[Skolob]

Re: Hard to Diagnose Compression Issue

If you have good cylinder compression the the only other places are the nozzle interface to the cylinder head and hop and thr hop rubber to the hop unit.
Ptfe tapr on the hop rubber to get a good hob rubber to unit seal will resolve thismin most cases, then as said above you last place is some function of the nozzle/tappet plate.
I wouldn't recomend a thin line of tape on the cylinder vents as any compression would blow that straight out and could end up being drawn into the cylinder. The vents are there to get the piston up to speed before compression starts.

[richardy]

Re: Hard to Diagnose Compression Issue

yeah i vote tappet plate not being super compatiable, alot of the time with the variety of gearbox shells theres often mods needed to brand new parts, try removing all gb bits apart from tappet plate and nozzle and screw together the gb shell, try sliding the nozzle back and forth with your hand feeling for resistance or difficult to move. Compare the new air nozzle with the ics one for length to make sure there no big incompatibility there. And exchange the two while the tappet plates in the gearbox you looking for a difference in final resting postion between the ultimate nozzle and plate and the ics versions maybe indicating a small gap as the pistons released allowing air too escape, sometimes in these cases a filing of the end of the tappet plate can help, or weak tappet plate return spring. The test results should help narrow things down.

[essexmerc]

Re: Hard to Diagnose Compression Issue

Also check the air nozzle, as you will find that the is a exact match for the hop up as i had the same problem when i upgraded my upper gearboxes. The tappet plate is not causing this due as i am running the asg ultimate tappet plate with the standard ICS nozzle and was getting a constant 360 fps on a M100 spring.

[dadio]

Re: Hard to Diagnose Compression Issue

nozzle issue,even 1/2mm difference in length will cause a loss of seal.

[cpldave]

Re: Hard to Diagnose Compression Issue

Right so, got home last night and stripped it all down.

- Tested the cylinder compression again and it was fine.
- Switched to Ultimate cylinder-head and ICS nozzle - lost 100fps over the 328 original.
- Switched to ICS cylinder-head and ICS nozzle and gained 60fps over the original.

I'm still not convinced it's shooting 100% though. With an M120 spring in it, it's chrono'ing at 348-350 fps with .25's. The spring is from my friends JG M4 and his was shooting 450-470 with the same spring. I wouldn't expect .25's to knock 100fps off though. I'll try looking at the hop-unit next and making sure it's mating with the nozzle correctly. I'll also try Skolob's PTFE hop-rubber mod to ensure no air-loss there.

Thanks for the help everyone! It's most appreciated.

Cpl Dave.

[Figjam]

Re: Hard to Diagnose Compression Issue

Just to say guys, this thread has really helped me, I've had a similar compression issue that has been driving me mad, I was down from 340 to 220. Sorted today thanks to you guys who said tappet plate. Swapped it out with a fresh one, back to full power. On inspection the old one had a tiny nick (maybe .5mm deep) on the surface that engages with the sector gear about 2/3rds the way up.

So cheers guys!

[richardy]

Re: Hard to Diagnose Compression Issue

Right so, got home last night and stripped it all down.

- Tested the cylinder compression again and it was fine.
- Switched to Ultimate cylinder-head and ICS nozzle - lost 100fps over the 328 original.
- Switched to ICS cylinder-head and ICS nozzle and gained 60fps over the original.

I'm still not convinced it's shooting 100% though. With an M120 spring in it, it's chrono'ing at 348-350 fps with .25's. The spring is from my friends JG M4 and his was shooting 450-470 with the same spring. I wouldn't expect .25's to knock 100fps off though. I'll try looking at the hop-unit next and making sure it's mating with the nozzle correctly. I'll also try Skolob's PTFE hop-rubber mod to ensure no air-loss there.

Thanks for the help everyone! It's most appreciated.

Cpl Dave.

Personally id get a guarder m120 or such rather than relying on a spring which worked in another gun esp a jg jobby. In fact better would be one your aiming at ie an sp100 if your going for 350.

Defo try the change in tappet plate like i say worth a try, and you can always mod it for more travel, just sand the flat end furthest from the gears. They are cheap enoough nowadays even aftermarket bits are considered consumable.

removing the hop unit and pressing it agasint the gearbox when the tappet plate and nozzle and spring installed will show you its postition in the hop at final travel, the concept is that the Lip on the end of the hop rubber forms round the end of the air nozzle. so as it fires the nozzles in the rubber and air should go down the barrel. Its gotta be a pretty bad fit of the hop rubber, or more irregular cut barrels like maybe vsr's for ptfe to really see a big improvment, the hop unit should hold it in place unless the barrels like a wand up gandolfs sleve in your hop unit.

If the rubbers baddly installed ie sometimes when you press it in the barrel can sit in the lip to far in, sometimes when lube or somethings used to fit the rubber. ie tight pushing into the hop unit the barrel will then force it open.. ohhh errr..

So a reseat of the hop rubber, maybe a replacing of it if its still standard ics, maybe the lips not compatible with your new nozzle.

Thats the concept, its an area though it doesnt need to be 100% spot on you can have plus or minus as long as its somewhere in there when the piston releases. If its too long it will soon tell you by not allowing bbs to feed.

id experiment, if your 100% compression is spot on becuase theres very little else it can be..

super dirty barrel massively stepping out of the box here, that can knock 100 fps off, ive seen some nasty barrels... real neglected jobs with half bbs jamed in side, some you can put a rod through it and young airsofters with two tones fall out of it... wtf