I began with a used pre-2gx G36C purchased on craigslist from a local. It is one with the jumbo handguard. It had been rebuilt at the KWA shop and had the "do not remove" sticker on it.
Chrono'd at a very consistent 360FPS 17 ROF with a 9.6v. Shot very straight. This G36C would have been just fine as is and may have lasted a long long time. Hindsight is priceless.
Originally I wanted a higher ROF and should have just stuck an 11.1v lipo in and let her rip. But no I had to open it up, adjust AOE and put a stronger spring in to counter possible overspin as several posts here recommended. I put some sorbo on the CH and ground the second tooth from the pickup tooth from the piston and put it together with a FET and Deans. It stripped the piston after about 100 cycles. An expensive lesson.
Now my new goal for the rebuild would become ~25 RPS with somewhere around 330- 350 FPS. The field I frequent is indoor where the long shots are less than 120'. The field has a 400FPS limit for full auto and 410 for semi only. Personally I don't want to hurt anyone and use .20's because most of my engagements are under 50 foot. For more on bb weight vs engagement distance take a look at the Airsoft Trajectory Project here
Another goal is to spend the least amount necessary to get this gun fieldable again.
Jumping ahead to the end of my July 2012 build here is what the result was.
360-411 FPS depending on the hop-up position. Adjusting the hop up for a flat trip out to 120' the FPS is 391 or so. Very stable FPS once the hop-up is set.
Rate of Fire is 23.75 rounds per second.
Feeds beautifully, shoots straight, hop up very effective.
This gearbox will not cycle with an 8.4VDC NiMH battery
What was done to the gun:
New piston to replace the old ruined one
Lighten piston to allow it to cycle faster
Short stroke piston to allow it to cycle faster
Remove second tooth from pickup side of piston and half of 3rd tooth to correct AOE and preclude PE
Remove rubber pad from CH and install sorbo and put rubber pad on top of sorbo to correct for AOE
Glue the cylinder head to the cylinder using Permatex Anaerobic gasket maker.
Replace heavy non vented aluminum ball bearing piston head with--
New vented POM piston head to allow lighter piston and head to cycle back more quickly
Lower ratio 13:1 gears - make gun cycle faster
Reuse bevel from KWA
Install SHS plastic sector clip
Remove sector tooth to match short stroked piston
Replace 9mm & 8mm ball bearings with bushings for reliability
Grind down outside of bushing to prevent interferance with selector plate
Remove two loops from tappet spring to make it cycle faster
Cut inspection holes in GB to see the pinion engaging the bevel.
Shim up the new non-self shimming gears and bushings starting with bevel to pinion.
Heavier spring to counter overcycling and premature engagement
New torque motor to pull new speed gears and heavier spring
Rewire with : 3034 FET, 24 amp PTC, 14 AWG wire and terminate with Deans connector
Add hole in back of GB to insert/remove screwdriver with shells closed.
What was not done to the gun
Never removed hop-up/barrel to eliminate possibilities of causing air seal problems too.
reuse KWA ported cylinder, plastic single o-ring cylinder head and o-ring nozzle to preserve good airseal.
11.1V LiPO not used, this gun has been tested only with a 9.6VDC 1600mah NiMH battery.
Re-use spring guide
SRC piston short stroked by one tooth and semi-swiss cheesed. This is a piston I had lying around.
APS POM vented piston head and O-ring- total weight of piston and head is 20 grams
SHS gen2 13:1 speed gear set Spur and Sector - first tooth on the pickup side of sector is ground off
SHS blue plastic sector chip.
KWA bevel gear
Lonesx A2 short, Torque Motor and pinion.
SHS M120 spring
3034 FET with two parallel 12amp PTC protection devices and 14AWG wire and Deans Battery connector.
Reused the motor tabs on new wire.
Scatterplot 3/16th 70D sorbo washer
In the begining this is what the parts looked like before I stripped the piston. This is the pre 2GX gearbox.
Cylinder head before installing the 70d 3/16th piece of sorbo. The rubber was removed from CH and glued onto the sorbo to protect the sorbothane from the piston head. The CH was glued to the Cylinder with Permatex Anaerobic gasket maker. To apply it I pushed the CH in about a quarter of an inch, put the sealer on the CH then pushed it out which prevents the sealer from getting inside of the piston side of the cylinder. After applying the sealer the cylinder was installed in the GB and pushed all the way forward while the sealer set up for 24 hours. Use this stuff and forget about putting teflon on your CH. Teflon is for taking up space in tapered plumbing fittings not your AEG.
This picture shows the geometry of the KWA piston to the gears with the sorbo on the cylinder head. It is hard to see the AOE with the KWA piston This is the piston that stripped.
Pictures of how the AOE was adjusted and the process of watching it come up and miss the ground off teeth can be found here. http://kwausa.com/fo...indpost&p=82550
Next issue was using the right bevel gear with the SHS 13:1 speed gears. Some discussion covered the why here.
In the end the KWA bevel was used. With the bushings there was plenty of room to shim the KWA bevel to the pinion.
I found these solid oiless bushings to replace the ball bearing bushings. The grinding down of them is discussed here. http://kwausa.com/fo...indpost&p=82575
More on why I did not use the LiPO to test this build is here.
Inspection holes drilled at bottom of GB so the pinion to bevel interface can be visually set. Other side of the GB has a similar hole drilled. This is the finished gearbox too.
Blow up of the gear in the inspection hole. When shimming the bevel to pinion I measured the distance of the top of the gear to the outside of the GB. This helped assure that the shim used was the right size. Once this shim was in place I put a feeler gage into the other side to find that shim size. There was plenty or room on both sides of the gear to shim up the KWA self shimming gear, the ones with the shims moulded into the gears, using the 5KU bushings.
One last thing. Before snapping the gearbox back together it is important to get the sector timing right. If it is not just right the tappet plate may be pushed up by the sector cam (to advanced) or the cutoff leaver will not be down all the way (too retarded). This picture shows the sector timing just right.
Where do I go from here?
This gun is just a tad hot for me so I would like to reduce the FPS some. Also I would like to be able to run the 11.1V LiPO without fear of stripping the piston. I have a couple of 3 metal tooth SHS pistons coming so I could short stroke it by two once those get here. With the LiPO this gun could get pretty close to a ROF of 30 and then the feeding problems can rear their ugly heads.
Any help on getting this LiPO ready would be appreciated.
Edited by philsaudio, 31 July 2012 - 07:14 AM.