View Full Version : O-ring replacement and Spring Guide Question

02-03-2008, 11:54 PM
Ok, so last time I took my SCAR-L out for a skirmish, it was chronoing at an impressive 380 FPS, something I felt pretty proud of. That day, the tappet plate snapped, so I took her home and put in a Guarder tappet as a replacement. I noticed the piston head didn't look too great, and it showed Saturday, as the same gun chrono'd at 240 FPS, likely due to CA's bad compression (as usual). The Guarder hopup I had put in it made up for the velocity loss with hop-up assisted range, but I REALLY want to get back to 380 FPS next time I field my gun.

So, I've figured out that the issue is probably a bad O-ring, like CA guns seem to have a lot. What kind of piston head would you guys recommend I put in the gearbox as a replacement should I need it? Or should I just buy a stock TM piston and piston head and put the TM piston head on the CA reinforced piston, like Marimba suggested? I've also heard that #14 O-rings from hardware stores work very nicely as replacements; is there any truth to this?

And finally, I've heard some people say that O-rings need to be lubricated and stretched a bit prior to installation. Should I do this, and if so, what kind of lube should I be using (I'm assuming silicone oil, or white lithium, which I'd get from Home Depot as well)?

Oh, and does anyone know if the CA SCAR has a plastic or metal spring guide? I couldn't tell last time I was working on it.

02-04-2008, 06:44 AM
Try taking your stock CA o-ring and *gently* stretching it a bit. CA is notorious for using slightly undersized o-rings in all their AEG's resulting in compression loss. Many times, giving it a slight stretch is enough to seal the chamber and give you the compression you want.

As far as lubricant goes, don't over-lube the o-ring. Also, whatever you use MUST be petroleum free. 100% silicone is the recommended lubricant in either oil or gel-type form.

I've not used o-rings bought from hardware stores, so I can't speak to that. I believe there's a thread about that already here on LSA, so try searching for it.

For the spring guide, I'm not sure there either. CA has been using metal spring guides with a thrust bearing in most of their recent releases. I suspect the SCAR is one of those, but I don't know for sure.

02-04-2008, 07:03 AM
CA states that the SCAR-L comes with Spring guide w/bearing. I would think it is metal as I have never seen a plastic one with bearings. I could be wrong.

02-04-2008, 08:18 AM
O-rings that are properly-sized for AEG non-boreup cylinders, easily found at mcmaster.com:

Metric: 19mmID x 24mmOD x 2.5MM thick

English: AS568A Dash Number 116
Width 3/32"
Actual Width.103"
Inside Diameter 3/4"
Actual Inside Diameter .737"
Outside Diameter 15/16"
Actual Outside Diameter .943"
Material Viton
Durometer Hard
Durometer Shore Shore A: 75

Either one of those will work.

If you cannot find those or don't want to order them, stretch the o-ring over the outside of the cylinder. Then heat it up with a heat gun or hairdryer for a brief bit, and then allow it to cool. Then install to the pistonhead.

The cylinder/pistonhead o-ring need just a light coat of lube. Virtually every mechbox I open always has too much lube in there, and it just gobs up the o-ring which doesn't allow it to seal well.

02-04-2008, 09:50 PM
Wolv, those specs describe a #14 O-ring from the faucet/bath section of Home Depot to the letter. I got a pack of them for $2.18 today, and I'll install one tomorrow after lubing and stretching it with AI silicone oil.

I also found some white lithium grease at Home Depot, but it had petroleum in it, so I left it there.

Thanks a lot - I'll report on results when I get the chance.

02-28-2008, 08:34 PM
Well, I finally got around to opening the box today, and did the heated O-ring around the cylinder, cleaned out the cylinder (was this a bad move?), greased with liquid silicone oil, reinstalled everything...

...and still can't penetrate the side of a Diet Dr. Pepper can at point blank, while my TM Hi-Capa running on duster did that and left a nice dent on the other side.

For what it's worth, I did lube the barrel and swab it using silicone oil before testing, but still, I would think that I'd get a better response out of this gun than a flat rebound with no noticeable denting.

Any other ideas?

02-28-2008, 09:53 PM
Personally, I've never had any luck using an oil of any kind inside the cylinder. I get the best performance using a silicone gel lubricant. I find it gives a much better seal. However, you have to be careful to not use too much otherwise, the excess will end up inside your barrel and that's not a good thing.

Obviously, you're still not getting compression and/or you've got an air leak somewhere in the system. Unfortunately, this kind of problem is next to impossible to diagnose in this forum setting. If you're going to be at the Harwood game on Saturday, stop by and see me.

02-28-2008, 09:57 PM
Any other ideas?Confirmed yet again over the years......After working on several CA guns over the past couple weeks, the air nozzle and cylinderhead are crap and are not compatible with a few different aftermarket and stock parts I tried to swap in. Nice, leaks are fun. They shot below expected muzzle energy as a result.

cleaned out the cylinder (was this a bad move?),No. That is a good thing.

How much and what weight of silicone oil did you use? 80W is the absolute minimum to use in a cylinder. 90W is better. Personally, as shak covered, I use silicone gel.

Did you do the piston/cylinderhead compression test first? Then add the nozzle, tappet plate, tappet plate spring, and do it again?

Do those first to verify minimal air loss. After that, if you are having extensive low power, it is time to look at the nozzle and hop rubber interaction....

02-28-2008, 10:15 PM
I used 10W - yikes. I'll put in for some cylinder grease on an order later this week.

Wolv, do you think I should pick up a new cylinder head and air nozzle as well? I did notice some wear and literal tear on the cylinder head when I had it out. I'm also thinking about getting a type 0 cylinder while I'm at it.

02-28-2008, 10:18 PM
Do the compression checks covered earlier. If they don't seal, then yes.

10W is way too thin.

02-28-2008, 10:29 PM
Wonderful - apparently, the SCAR nozzle is proprietary. Should I get that new cylinder head anyway?

02-28-2008, 10:30 PM
In my recent purchase of M15A4 SPR (CA) It to came with a crappy o-ring that had a improper seal, and a air nozzle that seemed not sized correctly and seems to have some light "binding" on the cylinder head nozzle. (Just confirming more of what Wolv was experiencing)

02-28-2008, 11:02 PM
Raptor, no. Try to solve with what you have for now.

I say that because of the incompatibility I experienced with CA nozzle and other brands of cylinderheads that I referenced earlier.

02-28-2008, 11:25 PM
Ok then - I ordered a grease set earlier today; I'll try again when it gets here.