Monday, 21 September 2015

Fixer-upper Part 3: TM SIG 552

I can't seem to help myself when it comes to a deal that looks too good to pass (this is your cue to tell me about the bridge you have for sale).

Patient: Tokyo Marui SIG 552
Symptoms: "Shop-tuned to M120 or M150, piston broken IIRC, sat in a closet for 5 or so years"

I got this gun from the very same Facebook for-sale group as the previous two guns in this series. I can't really find that many boneyard guns from shops around here, because (I guess) the number of active airsofters is rather low and shops thus have precious little to offer when it comes to broken guns.

Day 1

Since the previous owner had a feeling the piston had broken, I didn't even bother to try and fire the gun once I got it, but instead dug right in and started taking it apart.

The gun was indeed rather worn and dirty, plus the upper receiver had cracked on both sides just behind the front handguard.

I had heard horror stories about how difficult these guns are to take apart and put together, but I forged on with confidence. My first move, once I got the upper receiver off, was to clean out any dirt and grease, and glue the cracked parts back together as best I could.

I used brake cleaner to get all the grease off and then took a rough file to the inner edges of the cracked parts. I was hoping to make them rougher without altering the look of the outside surface.

This was followed by taping the two sides together with some vinyl tape, which I hoped would both align the parts properly, hold them together while the glue dried, and also prevent any excess glue from seeping on the outside. I was hoping for a very nice and smooth end result.

I applied a rather sticky universal glue (which has worked very well on other plastics in the past) to the inside of the crack by slightly bending the taped-together sides apart and then pressed the two sides back together. I also similarly filed and taped the other side of the receiver, which had the smaller crack right at the edge of the ejection port, and then left the glue to dry.

I continued with the disassembly to get to the gearbox and see what kind of a disaster waited inside. As soon as I got the motor out, I wanted to see if the electrics of the gun were in working order. The motor spun very nicely and without any weird noise, so at least there were no electrical faults.

Once I got the pistol grip and fire selectors off, I found out what a finicky, multi part thing the selector assembly was. It was made all the more fun by the fact that the crescent shaped gears were simply held in place by being stuck between the walls of the gearbox and the receiver, i.e. they fell right off as I didn't have enough sense to hold onto them as I pulled the gearbox out.

There were a number of very small pieces which apparently are all needed in order to make the fire selector work its magic. I'm not saying I could design a better system myself, but it does look kind of over engineered.

I thought I might run into some potential trouble when putting the thing back together, but at least I had plenty of pictures for reference!

Once I took the selector plate off, I also noticed a rather interesting solution to a missing contact surface on the selector. The two contacts had simply been soldered together, so basically the only thing keeping the gun from being fired on safe was the physical piece of metal stopping the trigger from moving. Since I didn't have a spare selector plate, though, I left it the way it was. The block stopping the trigger is, after all, pretty sturdy, so accidental discharge isn't really that likely.

Once I got inside the gearbox, I could not readily tell anything was completely destroyed. The piston teeth had worn down somewhat, but they still had plenty of surface left for the sector gear teeth to catch onto. The piston had one metal tooth and a ported aluminium piston head. I decided to keep using it until it breaks.

I hadn't chronoed the gun before opening it since I was under the assumption there was something broken in the gearbox, so I had no idea about the rate of the main spring. The wire seemed to be rather thick, but as I compared it to an SHS M110 that I had lying around, I noticed the mystery spring was also quite a bit shorter.

So, basically, still had no idea what the rate was. At first I thought I would put the gun together with the mystery spring first, then put in the M110 for actual use, since an M150 especially would be complete overkill for a weapon of this size (which is the rate the previous owner thought the spring might be rated at). In the end, I decided against this idea and opted to test the mystery spring with a gun that requires a bit less work in the reassembly department.

The other bits inside seemed to be fairly high end. All gears were made of steel, although unmarked. Still, definitely not the stock Marui gears that are, I guess, pressed together from leftover porridge and snot.

The gearbox was shimmed pretty ok from the get-go. The gears had very little play with the box screwed together, but apparently they were shimmed a bit too close together, as is apparent from these marks on the sector gear.

As I took out the gears to degrease and re-grease the gearbox, I kept my eye on where all the shims came from so that when putting it all back together, I could switch shim thicknesses to set the height of the gears in relation to each other properly while still keeping the same overall gear height inside the gearbox.

While the gears soaked in brake cleaner, I moved back to the upper receiver, as I had more plans on how to make the glued parts hold together a bit better.

I sanded down the insides of the front of the receiver with 80 grit paper in order to, again, give the glue a better surface to stick onto.

And now came my masterstroke. I cut out strips from a drinks can and sanded the surface down.

Finally I poked some holes into the strip to allow the glue to both dry faster and also seep through the metal and possibly make an even stronger bond with the plastic surface. All that was left was to press the strip into place and cut out any excess.

I put similar strips on both sides, anticipating the old ABS plastic might be on the frail side and the receiver might crack on the other side in the future. I used a smaller piece on the little crack on the corner of the ejection port, and a larger on the side where the piece had snapped off completely. I left the glue to dry overnight and moved back to the gearbox.

No major hiccups occurred in putting the box back into working shape. I greased it all around rather amply and tested the compression of the cylinder with and without the nozzle (and saw that it was good). I put the box together only so far that I might test fire it outside of the gun, and that is when I noticed the trigger wasn't returning to the front on semi. After some puzzling, it became obvious there was no trigger return spring, and the trigger was simply pushed back into place on full auto by the trigger switch return spring (on semi the cutoff naturally snapped the trigger switch back over the trigger).

So, in the spirit of this blog, I went and did it all again. Unscrew, release main spring, the works. At first I thought I'd simply lost the return spring at some point, but as I went through my pictures, it was apparent there never was one. I racked my brain for a while as to where the get a replacement part, since I did not have one in store.

A quick session of back and forth pacing got my brain working, and I went for my SO's stash of safety pins. I reasoned that if I only found a suitable size, I could bend it to shape and be done with it. Of course there was none large enough, but as luck would have it, I noticed I could just bend it open just enough to slip it onto the trigger post. After that, with some pliers and side cutters, I fashioned a perfect trigger return spring (still in progress pictured below).

With the new spring in place, I set out to reassemble the gearbox. Of course, this time the main spring would resist my attempts until the very last, whereas the first time it had simply slid in without any problems. I had to try three or four times before I got everything in place, but after that I was set. I ran into no more mishaps in putting the gearbox back into the gun and reassembling everything. Unless you count the fact I had not properly run the negative motor cable into a slot on the side of the gearbox, and had to again take out the selectors and all sorts of little bits and pull the gearbox out of the receiver in order to thread the wire through the slot. Wheeee! At least nothing broke. I left the gun with the lower receiver assembled in order to let the glue on the upper dry and then finish the assembly.

Day 2

The following day I returned to attach the upper and lower receiver together, and naturally wanted to test how strongly my glue had set. Some of the edges of the aluminium strips were not completely attached, and as I pulled on them with less than moderate force, the strips simply came off. My glue obviously was not really at all compatible with my materials. In the end I had to sand the surfaces down for a second time, and this time went with the ever trusty cyanoacrylate. Super glue itself is rather poor when it comes to stressing the joint (it tends to crack when bending, for example), but since I now had a large flat area and I was basically making a composite piece of material, I decided to give it a go. Super glue worked very nicely in attaching the two surfaces together, and the only problem was the dried glue on my fingertips afterwards. I just had to cut out the excess material that covered the holes in the plastic.

In the end, the seam was left rather visible, but the joint is extremely sturdy now.

I finished the project off by first installing a fuse box and a fuse since the gun did not have a box, let alone a fuse, when it came to me.

With the gun all in one piece, I hooked up a battery and dry fired the gun a few times. Everything sounded great, the gun fired as intended and all was well. Apart from the fact I had not a single magazine, because the gun came with none. The SIG 552 uses proprietary magazines (of course), so I had to improvise. Digging through my selection of mags, I found out the MP5 ones were slim enough to fit the magwell. They did not lock into place, obviously, but I had enough hands that I could set up a chrono and shoot for a bit to see how the old girl performed. And I must say, I was pretty pleased with the results.

Now, if I could only get my hands on some magazines, I could get rocking with this thing.

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