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Discussion Starter #1 (Edited)
I have had a Polytech M14-S that has been patiently waiting for me to give her some attention. She was slightly used when I took her home and the Muzzle Erosion (ME) gauge read about 0.3. As far as I could tell the only change her first owner made was to put her in a USGI Fiberglass stock.

However, he did not change out the Chinese Connector Lock.

The Chinese Connector Lock fits flush with the side of the receiver when it’s closed and the Chinese stocks are cut so the wood is against the receiver at that location and the Connector Lock cannot work out while in the stock. The USGI Connector Locks are longer and do stick out of the receiver when closed, and the USGI stocks have a relief cut for this. So if you put a rifle with the shorter connector lock into a GI stock the lock can work loose. I’m not sure if the previous owner was aware of this or not, but the Operating Rod Spring Guide coming out of its place during shooting may have been the motivation for him selling the rifle so cheaply.

Since I noticed this, the first thing I did to the rifle after purchase was to install a USGI Connector Lock and pin. I found the Polytech Receiver’s Connector Lock hole to be undersized a bit. I could manage to get the Connector Lock into the hole but it was too tight to lock or unlock by finger pressure. I put the Connector Lock into the chuck on my minimill so it was holding the portion that would stick out of the receiver when closed and turned it while applying gentle pressure with some 400 grit wet/dry sandpaper. Taking the parkerizing off that area was sufficient to do the job.

The other things I did to the rifle right off the bat was shim the Gas Cylinder Assembly, switch Flash Hiders and install a Stripper Clip Guide, as it appears that the previous owner removed it to use a scope mount.

Everything I have read on the Chinese rifles says that they did an excellent job on the receivers. They are the only option in the US for a drop forged M14 type receiver that does not have a NFA tax stamp attached to it.

However everything I have read about the bolts says they are junk, at least the ones imported into the US before the import ban. A GI bolt will not generally just drop into a Chinese receiver and close. Material must be removed from the rear of the receiver ring, where the bolt lugs bind. Knowing this, I bought the Polytech with the idea of learning how to perform my own GI bolt modification.

In deciding what bolt to use, I tried two different New Old Stock (NOS) GI bolts to see which one would close more. A Winchester bolt won the contest and therefore was my choice for this conversion.


In the process of doing the GI bolt conversion I removed the barrel from the receiver. One, it’s easier to handle the receiver without the barrel on it. Two, I wanted to deal with the areas of interference from the receiver first and not worry about any possible interference from the barrel itself, at least not yet.

Chinese receivers have a set screw on the receiver ring that must be removed before pulling the barrel or you can bugger up the barrel threads. These set screws are reportedly very soft and can be difficult to get out in one piece. I found a screwdriver bit that fit the thickness of the slot the well, but was too wide to fit into the flathead slot itself, so I modified it with a Dremel and Cut-Off Wheel.



I placed the modified bit into the slot, slid the screwdriver over it, tapped the screwdriver with a hammer to be sure the bit was fully set into the slot and carefully started removing the set screw. I was in luck this time as it came out in one piece.



Next I removed the Connector Lock and pin I had installed, as I didn’t want the pin interfering with my action wrench, and once the pin is out the Connector Lock itself can fall out and disappear in the dark recesses of SS’s Man-Cave.


Note the area where the finish is mostly removed on the Connector Lock, per my polishing for fit.


Ok now to the press to remove the barrel. Brandon1 donated a barrel vise and action wrench to the SS M14 Build Disorder Society and they work great.




I put the receiver in a vise lightly clamping down on the lugs up to the rail, just tight enough to secure the thing so it wouldn’t move while I was grinding with the Dremel.


I tried various attachments and did not take photos of everything I tried, but carefully removed material from the offending areas as smoothly as I could, until the bolt would close. Be careful not to hit the locking lugs! Remember to take your time, check and recheck. It’s hard to put it back once you’ve removed it.



When I got it really close, it seemed that it was stopping on the curved contour between the receiver ring and the shelf that the lug is supposed to stop on. I compared it with other rifles I had around and decided to carefully reshape that curved transition from the receiver ring to the shelf. Blue Dykem helped determine exactly where the offending areas were and soon I had the bolt fully closing.



Once I was satisfied with the bolt’s closed position it was time to rebarrel. Note my $7 framing squares in use with Badger’s Timing Gauges.


Note that I wanted to stick with a Chrome-Lined Barrel for this rifle and had several handy to try in the efforts of determining which barrel would give me the headspace range I was looking for. Obviously there is no way to shorten headspace, and I can only lap a few thousandths or so to lengthen headspace without getting too far into the carburization. I would index the prospective barrel, clean the chamber, insert a 1.630” Go gauge, and note the right bolt lug position to figure which one would be the best for use. None of the barrels I tried would close on the Go gauge, which means the chamber dimensions as is would be too short and I would need to lap the bolt lugs to set back the bolt and effectively lengthen the chamber headspace.

Oddly enough, out of all the barrels I tried, the Chinese barrel that came on the rifle was the one that would require the least amount of lapping. I estimated that I could lap to achieve my desired chamber headspace without going more than about .003” into the carburization of the bolt and receiver. I had an excellent muzzle and a visual inspection of the throat showed no chipping or flaking at the lands, so I was good with using the original Chinese barrel.

After rebarreling, I checked the bolt to see if it would still close on empty. It would almost, but just not quite close on empty. Using a flashlight, Blue Dykem and dental floss, I pinpointed the interference at the top of the bolt. The shroud or hood of the bolt was impacting the barrel itself.


If my headspace was already where I wanted it I would have relieved the barrel at that location until the bolt closed. But knowing that I still had a few thousandths to go by lapping the bolt, and knowing that the bolt would move to the rear when I did so, I presumed that it would make the necessary clearance.

So I lapped until I had the desired chamber headspace, and it did indeed make the necessary clearance at the barrel.

Here’s some photos of the homemade bolt lapping tool I use. Sized, cut, sanded brass case, hammerspring and nylon screw from Lowe’s with the threads taken off by a belt sander (so the shaft of the bolt will fit inside the hammerspring). The nylon screw just happens to be the perfect size to fit the bolt face. These photos are from lapping my SAI after correcting the bolt roller impact defect. The brass case is easy to remove if you size and polish or sand it before using it. If you don’t size it, you may need a broken shell extractor to get it out when you’re done lapping.








Before performing this conversion job I have lapped a TRW bolt into a LRB receiver, a TRW bolt into a Fulton receiver (both for new builds using chrome-lined GI barrels), and lapped a TRW bolt into a SAI receiver to lengthen the chamber headspace after fixing the bolt roller impact defect (which shortened the chamber headspace).

I believe this Winchester bolt and Chinese receiver was harder than any of the others I had lapped before. It took noticeably longer to remove material than what I had experienced before. I noticed that the Winchester bolt had a lighter dimple (from heat treat test) compared to any of my TRW bolts. Perhaps this dimple is a visual indication to a prospective bolt buyer of how hard the bolt is? It would be interesting to know whether this is so, and if so, how that affects the depth of the carburization. If the bolt is harder does that generally mean the carburization is deeper? If so, those who prefer chrome-lined barrels may wish to use harder bolts to allow for a greater measure of safety when lapping bolt lugs to achieve chamber headspace?

I reassembled her and dressed her in something a bit more fitting, a nice birch stock I picked up from CMP-South…..



A little refinishing of the stock, bedding, a websling and some park and she'll be ready to go out on the town....or at least to the range.
 

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Nice post! Thanks.

Quote:
"...Chinese... receivers. They are the only option in the US for a drop forged M14 type receiver that does not have a NFA tax stamp attached to it."

Not exactly....GI2
 

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Range report and no excuses!DI4

Oh did I read you need to repark. Sigh :)
 

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Thanks for the excellent write up with the great pictures. I just did the same thing with my second Poly Tech M-14/S. I've replaced everything except the barrel, op rod and receiver with US GI parts. I got lucky this time in that I met a gun smith that had a box of 25 US GI M-14 bolts (Springfield Armory and H&R). I was able to sit down and try each one in my receiver. I found several that would easily drop into battery, with the bolt lugs going completely into battery. I chose the nicest one from the batch of Springfield bolts but I had to pay a premium for it, $260.00... The last TRW bolt I bought at a gun show back in the 80s was $35.00 but it was too long to fit this rifle and would only just start to close.

My first Poly Tech was built with GI TRW parts. I took the barreled receiver to a large gun show and I checked every bolt that I could find until I found one that almost dropped into battery. I had to polish the bolt lugs with valve grinding compound until the bolt would drop into battery with the lugs fully engaged which was a laborious process. I did not want to go through that mess again and gladly paid the tariff for the Springfield bolt on this latest project. The first project turned out head spacing at 1.630 and I stopped right there. The Chinese bolt that I replaced on that rifle head spaced at 1.643 which is ok for use with milsurp 7.62 NATO ammo.

7th
 

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Nylon Screw

I'm not being critical of your use of the nylon screw, but it should be noted that squaring the bolt to the chamber centerline by lapping could be compromised by the flex in the nylon screw if the receiver lug recesses cant (left to right yaw) the bolt during the lapping process. This can result in one lug bearing the majority of the recoil when the rifle is fired. Typically the Chinese receivers have bolt lug recesses that are not cut squarely with the geometry of a USGI bolt. Part of the process of converting a Chinese receiver is squaring up this discrepancy by lapping by utilizing a method that forces the bolt to remain square with the bore centerline.

In other words, forcing the bolt to lap into the receiver while the bolt face is perpendicular to the chamber with as close to 90 degree perpendicularity as possible is necessary.

I can't say if the nylon screw can maintain the required force to keep the bolt from yawing during the lapping process.

To test for squareness, you can close a stripped bolt on a loaded round, in the chamber and hold a light under the receiver in a dark room. If you can see daylight on either side of the bolt lugs at the rear, your bolt may be bearing on one lug more than the other. You can add tension by putting a piece(s) of tape on the bolt face until you feel pressure when you close the bolt completely. Use thin Scotch tape and cut it in a circular pattern so that it doesn't bind on either side of the cartridge. Adding too much tension will offset the cant, so just go for light pressure.

Very few Chinese receivers have square lug recesses as it relates to a USGI bolt.

If it's not done correctly, the end result is one bolt lug bearing the brunt of the recoil as opposed to both lugs bearing the brunt evenly.

I noticed you also removed material from the receiver on the front side of the bolt lug recess via griding/sanding. You could also lap some of this out by applying forward tension on the bolt and lap both the bolt and receiver; it usually doesn't take much when you lap both the bolt and receiver. There should be clearance of both sides and lapping both the bolt and receiver keeps you in the "safe zone" by not removing too much from either. Often times, the barrel face will protrude and break the plane of the receiver lug recesses. It may or may not contact the bolt, but often does.

EDIT TO ADD: The nylon screw may allow the bolt to yaw enough to show that lapping is taking material off of both sides, when the desired outcome is to take it off the side that is longer first. This exacerbates the problem of the bolt being out of square. More than likely, more needs to be removed from the left side, as you look at the receiver from the top side. This has been my experience anyway.

A slightly blown out rear portion of a fired case will provide adequate tension for squaring the bolt with the receiver.

Thank you for the excellent post!
 

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Discussion Starter #7
Sorry for the delayed response. That snow storm hit my internet source.

jywolfe-

Thanks for taking the time to bring that possibility to attention! Incorrect alignment of the bolt would be a costly mistake.

Since reading your post I have compared the nylon bolt and the end of a fired cartridge on the hammerspring in the cut case and observed.

Honestly there appears to be less likelyhood of movement or yawing with this particular nylon screw than with the end of a case. Maybe I'm not approximating properly, but this is what I'm seeing:

The case head will wobble on the end of the hammerspring where the nylon bolt does not move independently of the spring when the nylon bolt is inserted all the way in the spring.

The nylon bolt's shaft keeps the hammerspring from bowing under compression, sort of like the op rod spring guide & op rod spring work together.

The nylon bolt head fits the bolt face perfectly.

The end of the hammerspring makes contact in almost a complete circle around the slanted or countersunk part of the bolt.

I cannot perceive any flex in this nylon bolt from the point where the hammerspring makes contact with the back of the nylon bolt head to the face where it would contact the bolt.

The outside edge of the nylon bolt head has not shown any signs of wearing or bending and has been used for lapping 4 rifles so far.

I performed the darkroom flashlight test with the stripped bolt and a carefully cut piece of electrical tape on the head of the live round and could not see light from between either side's contact area between the rear of the lugs and forward facing locking recesses.

I could not find any scotch tape around at the time and we had almost a foot of snow here in global-warming ravaged Alabama so I didn't just run to the store to get some. GI6

Concerned that I had too much pressure with the thicker electrical type of tape, I removed it and used just the DAG M80 round and still could not see any light from between the lugs and recesses. I tried to wiggle the bolt around, up & down, left & right, forward and aft, with the round in it and still didn't see any light peeking through at the mated surfaces.

I did write down in my notes that I thought I removed about a thousandth more off the left lug than I did the right, based on dial caliper measurements before, during and after the lapping process.

Based on all of this I think I'm good. But clearly lapping the bolt so the bolt face is perpendicular to the boreline is of critical importance.

Thanks again for bringing this to mind. I much rather learn from others' experience than my own mistakes. I surely don't want to influence others to perform improper techniques.

All- Range report will come in a month or so, after I mod the FH and replace the gas cylinder assembly. I don't have a FH alignment tool and this gas cylinder has it's barrel holes milled off center.

Brandon - Yea it'll need park'n. Can I put in a request for supper?

Regards,
SS
 

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Thank you so much. I've looked all over for the answer to that question. You should really publish it. I have a really nice M!A complete, I bought all the parts to build one so I could learn all the mechanics and everything is fine but I find that this is the most important and most difficult step. I will continue the build but this part is going to be done by a pro.
 
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