Jump to content
Sign in to follow this  
GSO-Geko

Marui M870 Fix and Tuning Thread

Recommended Posts

Thing is the stock one had it's own revision. First gen was all plastic and the post eventually sagged which leads to it misfeeding. Then they started churning them with steel posts imbedded in an otherwise plastic arm and solved the issue. If you have the old one it would be advisable to get an after market replacement.

  • Like 1

Share this post


Link to post
Share on other sites

Somebody know the o-ring sizes which are used in the gas cylinder?

 

There is two o-rings which seal in the two end of cylinder.

Front is around ID 9mm with 2mm thickness, and the back o-ring which is on the valve is around ID 10 or ID11?

 

And other o-ring which are seal on the valve at front-middle position.

Share this post


Link to post
Share on other sites

We made a new valve-nest to keep the sealing issues at the expansion chamber by the valve.

 

This new valve-nest:

- made from copper.

- we used much bigger o-ring to keep the sealing better.

- we designed it with spring guide for the valve-spring.

- drop-in fit to the original parts as you can see on the picture.

 

Photos

How looks like the valve-nest: 1, 2, 3,4

Fully assembled: 1, 2, 3, 4, 5

 

We tested it with HPA on 120psi pressue. It keep the sealing without any problem - now you can solve the black-hose sealing and the gastank-stock connection issues.

  • Like 1

Share this post


Link to post
Share on other sites

The links don't work as it is. They force you to download a webp content... Strange.

 

Are you planning to produce these and sell them? My 870 developed an internal link recently, so I'm very interested.

Share this post


Link to post
Share on other sites

Now I have 10 pieces on stock. Send me a PM if you're interested.

 

I have a photo album in the TM M870 FaceBook group, here you are: CLICK.

Somewhy I can not place photo to this topic, maybe Google Photos is banned?

 

15250697_1464443830251881_77847511548543

 

15259202_1464443763585221_63315347220440

Edited by Batmause

Share this post


Link to post
Share on other sites

Hey while we're on the topic of the valve there... I'm having some issues fixing mine.

 

I have a weird problem where whenever the gas tank is inserted into the stock, ALL the gas goes out the front of the gas cylinder.

 

The weird thing is that if I were to disassemble it so that it's just the cylinder and blow into where the stock attaches the receiver, the thing doesn't leak. I can't blow air into it so I KNOW it's airtight before inserting gas. I suspect it's because the o-ring is too soft and is flexing when the propane is inserted.

 

The problem now is that no matter where i switch the o-rings and what I do with the front and rear valve it still shoots out all the gas without stopping. I've even removed the "firing pin" to try and isolate the problem.

 

I hate to be a bother but could someone please post some reassembly pictures without the cylinder on? I'm stumped on this one.

Edited by Rainyeyes

Share this post


Link to post
Share on other sites

You can't blow into it because the valve is reversed. Instead of gas pressure holding the valve tight, the only thing keeping gas in is the force of its spring. I don't think your lungs can generate enough pressure to force the valve open, but 12kg gas does especially when it's hot out. How I fix them is I stretch and retemper the spring (about 130% original length) so it has a higher pre-loaded tension against the gas.

  • Like 1

Share this post


Link to post
Share on other sites

Thanks for the tip. Apparently this problem persisted with the previous owner where it began leaking before the o-rings were changed. I should have known it was another factor.

 

The strange thing is that I actually tried shimming the spring with an o-ring to increase its strength but the it started to leak where the firing pin engages the valve at the rear of the cylinder when I blew into it. I took it out and the leak stopped.

 

I will see what I can do about stretching the spring 130%. You could be right that it lost tension over time... Sourcing a replacement would be rare at this time.

Share this post


Link to post
Share on other sites

I finally found some time today to install Batmause's part, but the cylinder is still leaking. I took all the o-rings off, soaked them in silicon oil and reinstalled, but as soon as I put propane into the Angry Gun tube it leaks at the front... :(

Share this post


Link to post
Share on other sites

You want to be really careful with the AG tube that when you thread it back in you're not cutting the O-ring. You can get away with a tiny bit bigger than the original but not much.

 

I also give the threads a wrap of teflon tape.

Share this post


Link to post
Share on other sites

The tube and grip support part are ok. It's the damn cylinder that's leaking. I disassembled everything again and now the cylinder doesn't leak from the front, but it does from the back... I even stretched the spring in the valve but the stupid thing won't stay closed when I put just a little gas in.

Share this post


Link to post
Share on other sites

Aaaand just now disassembling again I noticed that one of the long screws was broken, therefore the massive leak from the back. Now to hunt some decent 60mm screws...

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use and the use of session cookies.