Spring size - Sig 552

[etd]

With some help from some ASC members I've Identified a problem with my M120 in my Sig 552 mechbox.

The issue is that the spring doesn't have enough room to fully compress in the gearbox due to a sorbo pad, Bearing spring guide and bearing piston head being added.

Someone suggested I swap out my M120 for a PDI type spring as they are normally shorter. Which spring/type of spring should I use to achieve no more than 400 fps? Keep in mind it needs to be as short as possible (length wise).

[Schlyder]

You may have a little overkill going on with the bearing spring guide and the bearing piston head. All of that takes up space in the area the spring needs to compress in. If you go back to a regular piston head you may get the required space back. And you will be fine with just the bearing spring guide.

[Con Murder]

I use both bearing spring guide and bearing piston head in my 552 with a m110 so that is an option. I have not compared springs of the same strength for size so I couldn't help you with a brand. In your case I would test without the bearing piston head, see if that works for you. If you still have trouble your motor may not handle a m120 and both bearing sets?

[etd]

I already removed the bearing spring and there is still not enough room. I know as a FACT that my stock V3 JG mechbox wasn't built right as the cylinder space was also way to small but I'm working with what I have.


M120 spring still does not have neough room to compress with just a sorbo/bearing spring guide.

Motor is a systema Magnum and I'm running a 10.8 nihm so its not a power issue.

Just need someone to answer my question please. The problem has already been diagnosed so I would appreciate if I could get some answers as opposed to theories.

[Schlyder]

Looks like that spring isn't going to work. You will have to try another. Which spring? Let us know when you find out.

[CDN_Stalker]

Avoid PDI springs at all costs. They have been crap for a few years now, and you have no way of knowing if one spring is shorter than another. I compared three new PDI 150% springs together a few years back, they ranged from 6 3/8" long to 7" long. All rated "the same", but they weren't. Buy a Modify 100 from airsoftparts.com, it should sit you around 370fps.

[venture]

PDI springs traditionally have lots of coils. This can result in a large length before set. Be careful if you are going to change springs. Shorter looking springs can have a longer length before set due to having more coils. I would recommend that you cut away a little of the spring. Make sure you have both end ground smooth and flat so the ends won't get stuck in the bearings. Then you can use both of your bearing for free spring rotation.

[etd]

Okay I will try getting an M110 or something... I mean its not that expensive to buy a new spring so I won't mind taking the chance.

As for cutting down the M120 and making the end as flat as possible, that's going to be a last resort in my books. My other AEGs can use the M120 so we will see how things work out.

BTW thanks for the heads up on the PDI springs.

[CDN_Stalker]

Originally Posted by etd

Okay I will try getting an M110 or something... I mean its not that expensive to buy a new spring so I won't mind taking the chance.

As for cutting down the M120 and making the end as flat as possible, that's going to be a last resort in my books. My other AEGs can use the M120 so we will see how things work out.

BTW thanks for the heads up on the PDI springs.

No problem. Also, different brands of springs output different fps regardless fo the rating. I bought a few MadBull M110 springs thinking they'd output about the same as the Prometheus 110 springs I was used to (hey, all are rated for the same meters per second, right?) only to find out in a few guns I installed them in, they'd sit around 425fps!?!?! Is why I suggested the Modify 100, it's cheap yet good quality, and is very predictable for it's velocity output. Most other springs rated the same, you're looking at anywhere between 360fps and 430fps. At least what I suggested will give you some headroom to deal with depending on the other parts working together.

BTW, I upgraded a TM SIG 552 a couple years ago, a simple install of a tightbore and Modify 100 spring, shot around the aforementioned 370fps, but the thing was a freaking tack driver at 30ft, easy 1.5" groups! Best I'd seen before was 2" with my MP5s, and every Armalite was well over 3" easy, even on aimed semi auto.