What do I Need to Have an Adjusts FPS?

[NathanW8]

Hi all,

I'm upgrading my Tokyo Marui Sig 552 and looking to fine tune the FPS. I’ve seen online that on some AEG's you can adjust the FPS with just a hex key. This allow for upto a 40fps adjustment. It looks like you can adjust the preload on the spring by moving the spring guide. But no matter where I look, I can’t find anyone who sells an adjustable spring guide for a V3 gearbox. The only place I’ve found is ROA, who use to make one for a V2 gearbox? Even the JG version of the Sig 552 has this feature. I’m half tempted to buy one just to pull it apart to steal the adjustable witchcraft inside. Can anyone point me in the right direction to what I'm looking for?

If there isn't an aftermarket part available, are there any recommendations on the best method to fine tuning FPS? I’m aiming for 340 fps but the two M100's I fitted measured 350 – 360 fps and the M90 I just fitted is measuring 310 fps. I’d like to get within 5-10 fps of 340 without going over. Is it better to preload a weaker spring or weaken a stronger spring with heat/cutting/compression? Or should I just keep buying springs until I find one that is right?

Thanks for any help!
 

View attachment 69915

 

[Adolf Hamster]

adjustable springs are something that pop up occasionally, but usually a factory proprietary system.

tbh they're not the greatest, it sounds like a great idea until you realise the reality that the adjustment has a nasty habit of backing itself off mid-game making the whole thing pointless.

in terms of tuning a standard box, you have a few options.

you can cut, but personally i'm not a fan of that option, but it's there nonetheless.

the other option is bearing spring guides, and bearings in the pistons. generally you want at least one (on the spring guide) but if it's not a fast enough build where piston weight is a concern then a second on the piston can bump the fps up a little.

you can also space the spring a little, although not by much, i've done it occasionally with some bushings (i wanna say hpa baja differential bushings was what i used), careful whatever your using doesn't interfere with the free motion of the piston because a lockup is gonna be a bad day for the gearbox.

however, with all that said you don't need to get too close, whilst yes energy is important, the effective range of an airsoft gat is a multi-factor problem and things like ammo weight/quality, barrel quality, hop setup, compression etc are just as important as energy for good performance.

 

[NathanW8]

@Adolf Hamster

Thanks for the advice.

I just thought it would be nice to have that adjustability without having to keep rebuilding my gearbox to chase a few fps, especially as the springs and seals degrade over time.

I'm already running bearings on both the spring guide and the piston. I'll try removing the one in the piston to see if that drops the fps enough on a weaker M100 spring.

If that doesn't work, would you recommend fitting a spacer on the spring guide or the pistol end of the spring. I've heard negatives for both, increased piston weight vs a potential piston catch point. I'm aiming for a rate of fire in the low 20's, no not sure how concern I should be about premature engagement on a M100 ish spring.

I'm know fps isn't everything, but I'd like to get somthing closer. 

 

[Adolf Hamster]

another option, depending on your barrel length, is you could consider short stroking by a tooth.

you should be fine at 20rps with a full stroke but dropping a tooth will lower the fps and make it very slightly snappier.

easiest way to check is if you have a ported cylinder- as long as there's enough stroke to clear the port then you won't have a voluming issue.