Jump to content
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

It appears that the TM M870 and KSG Owner's Group has gotten the zucc, so I'll be asking my questions in here.

 

I chopped together a "Cliff Notes" version of everything relevant to my interest from the original review thread as well as this thread. Let me know if anything is incorrect.

Quote

TM M870 notes

 

Goal

- My ultimate end goal of a HPA-tapped M870 pushing 6x 0.45-0.48g at 1.5J out to 125 feet is probably not realistic

   - Modifying this goal to 3x 0.45-0.48g OR 6x 0.30-0.32g (with similar range for both modes) might be feasible

 

Overall

- Feeding system seems to be similar to the TM M3 or SPAS; this contributes to difficulty disassembling and reassembling

- Most performance issues seem to be related to the loading system and gas system

   - Gas system issues can be largely avoided by using HPA

   - Loading system issues stem from tri-nozzle, loading arm/plate and spring, and (to a lesser extent) shell choice and ammo brand

   - Breaking in the loading system can help with feeding issues; remove ammo and gas, then slam-fire about 100 times

- 6-shot mode seems to use about twice as much gas as 3-shot mode

   - Muzzle velocity is nearly the same between 3-shot and 6-shot modes while using Abbey Brut Sniper gas; other gases should yield similar results

   - Range is halved when using 6 shot mode

   - Lighter bbs can be used to regain some range in 6-shot mode: not “light” as in 0.20g, just lighter than the bbs used for optimum performance in 3 shot mode

 

Performance on HPA

- Per eagle258’s setup: WiiTech 380mm inner barrels, SAT valve

   - At 95 psi, 330 fps w/0.20g, 150 ft range w/0.28g

   - At 115 psi, will function (no performance data)

 

Inner barrels

- Groupings with stock inner barrels can be tightened by taping ends together (instead of using spacer)

- WiiTech 6.03 380mm long inner barrels are highly desirable

   - Slightly tighter groups at range

   - Replaces flat nut (used to secure barrel bracket) with threaded inner barrel brace; significantly increases ease of reassembly

   - Reduces/eliminates barrel wobble

 

Hop up

- Fixed hopup

- Factory hop setting seems to be inconsistent from gun to gun

- Seems to generally perform well with 0.25g when used with standard airsoft gases

- No information available on WiiTech hopup chamber or SAT hopup rubbers

 

Valve

- Stock valve yields limited power

- Feedback on SAT valve is inconclusive: some say it’s junk, others have no trouble

   - Upgraded valve is likely the only way to overcome power limitations seemingly imposed by stock valve

   - Will yield 400 fps (presumably with 0.20g) at 100 psi

   - Needs to be lubed and tightened down HARD when installed

   - Valve spring needs to be stronger in order to hold green gas-equivalent pressure; included spring can be “plussed-up” by adding 4 coils from stock TM valve spring to SAT valve spring on SAT valve

- No information available regarding Pro Arms valve’s performance

 

Trigger group

- Hammer spring can be shimmed by placing an o-ring underneath it; should provide about a 30 fps boost

- Contact surfaces on hammer can be polished to smooth out action

 

3-/6-shot selector switch

- Seemingly easy to knock selector switch spring out of position, fixing the gun in either 3-shot or 6-shot mode

- If properly installed, the stock selector switch spring is too weak to keep the switch in 6-shot mode. Blackcat spring set fixes this issue, as the springs included with this set are much stiffer than stock.

 

Shells

ACM shells don’t always feed reliably; much like with AEG magazines, TM shotshells are the only choice if reliability is desired

TM-compatible shells are about 2mm wider than real 12ga shells; real polymer “pinch-type” side saddles are a tight fit, real aluminum side saddles outright do not work

 

Tri-nozzle

- Stock plastic tri-nozzle is a weak point; breakages seem to be fairly common, especially under heavy use

- Two options for reinforcing this part of the system

   - Reinforce undamaged stock nozzle with 4mm brass tube inserts (performance and results unverified)

   - Install Nebula aluminum nozzle (smooth out edges, if desired)

- Nebula “reinforced” plastic nozzle is prone to breakage and a waste of money

- No information available on WiiTech "power up" nozzle or Pro Arms nozzle

 

Loading arm/plate

- Stock loading plate spring seems to struggle with bbs heavier than 0.28g

   - One of the Blackcat spring sets *might* include a stiffer loading plate spring

- SAT loading plate can fix misfeed and rough feeding issues

   - Threaded loading plate pins will need to be loctited/JB Welded onto plate

   - SAT loading plate may break; this is not an “invincible” part

- No data on Nebula loading plate performance

 

Externals

- Outer barrel will end up wobbly after heavy use; WiiTech 380mm inner barrel set may prevent this

- G&P RIS pump fits much better than the WiiTech RIS pump

   - Laylax forearm removal tool is very useful, G&P forearm removal tool is less useful

Edited by ardrummer292

Share this post


Link to post
Share on other sites
On 2/28/2019 at 6:46 AM, ardrummer292 said:

It appears that the TM M870 and KSG Owner's Group has gotten the zucc, so I'll be asking my questions in here.

 

I chopped together a "Cliff Notes" version of everything relevant to my interest from the original review thread as well as this thread. Let me know if anything is incorrect.

Still up for me- where does this link take you?

Share this post


Link to post
Share on other sites

Has anyone managed to fit a real stock to their 870? How about a stock designed for the G&P springer series? Gas storage is not a concern, as I'll be running a remote line for HPA.

Share this post


Link to post
Share on other sites

OPrvHWQ4PlBwk00sTUzDPvLfjhVv_VSEiJcAp3Wb2u2wBeTaLI_hqq6Q-MGBGSVNwWHORCk7prFKjMVRGZLn87IoEo_S7lNHWTfXdkDqyEpnNSYtvYugQ3PpjXb5g5NyraAP7OSN

 

Does anyone know what purpose the springs indicated in the picture serve? My guess is that they provide some sort of automatic throttling function, regulating gas flow into each barrel.

Share this post


Link to post
Share on other sites
On 5/9/2019 at 6:41 PM, ardrummer292 said:

OPrvHWQ4PlBwk00sTUzDPvLfjhVv_VSEiJcAp3Wb2u2wBeTaLI_hqq6Q-MGBGSVNwWHORCk7prFKjMVRGZLn87IoEo_S7lNHWTfXdkDqyEpnNSYtvYugQ3PpjXb5g5NyraAP7OSN

 

Does anyone know what purpose the springs indicated in the picture serve? My guess is that they provide some sort of automatic throttling function, regulating gas flow into each barrel.

Late reply but I think it's to provide consistent output in case of 1 or 2 barrels not being loaded. The valves will shut allowing sufficient pressure to build up in the barrels that are chambered. 

Share this post


Link to post
Share on other sites

Quick question: are the charging arms for the tactical and breacher models interchangeable? I need to order a pair for a breacher but all I can find are labeled specifically for the tactical model.

 

edit:

NVM. A quick look at exploded diagrams and the breacher is totally different. Here's hoping I can just bodge the tactical ones in.

Edited by renegadecow

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

×

Important Information

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