I always wondered why, if most everyone insists that the geometry of a Polytech rcvr is exactly GI, then why does it need to be modified to take a GI bolt?  If a modification is needed what is it??

Opinions vary but some experts will tell you that the problem is not with the receiver but with the bolt. Check out the M-14 info at Smitrh enterprises site and on Fulton Armory's site. Take it with a grain of salt because a lot of Chi-Com owners shoot their weapons as they come out of the box.  

It's really very simple.  The Chinese barrel is out of spec.  If you change both the barrel and the bolt, you won't have any problem fitting the parts onto a Chinese reciever.  I have a Polytech, and rebuilt the whole thing with USGI parts and had not a problem with any of it fitting together.  

I tried various combinations of Chinese and USGI parts while I was doing the build (just to find out myself) and there wasn't much between the two (USGI and Chicom) that would fit together.  Some did, but not anything really important.

Mount a Chincom barrel into a Springfield M-1A reciever and the USGI bolt isn't going to fit there either.  The problem's not with receiver geometry.  The problem is trying to get two parts that have VERY little chance of working together to fit.  

The answer seems to elude many, but it's the same as asking why a 1/2 inch socket doesn't quite fit a 13mm bolt head.  It doens't fit because it's the worng size.  Simple as that.

People want a cheap way out, so companies provide that service to make a buck.  They'll modify your USGI dimensioned receiver to accept a USGI bolt headspaced to an out of spec Chicom barrel.  It costs less, so people do it.

If you use a USGI dimensioned receiver (Chinese forged) and a USGI bolt, and a USGI barrel the most amazing thing happens.  It all fits!  Unbelievable!!

The best thing to do is make sure the Chicom gun is really out of headspace.  Many people don't check it correctly for a military rifle and what they think is out of headspace is actually fine.  If it's good, shoot the crap out of it.  If it's not good , the best thing to do is have a new USGI barrel and bolt fitted.  

Having your receiver modified to take the worng combination of parts is not the route to take IMO.  But, it's just my opinion and I don't mind what anyone does to their own gun.

Above all think of total cost.  If it's going to cost you as much to fix a broken Chicom gun as it is to just buy a US made gun, the choice is obvious.  I wanted a forged receiver, and I wanted to do the build myself, so I went the route I did.  I also own a match modified M-1A, so the Chicom is the "blaster".  

My experience in the Polytech build almost mirrored exactly the info on the Fulton website just FYI.

Ross

My Poly has the bolt conversion by Ron Smith. I'm not sure where the "U.S.G.I. parts don't fit" info is coming from. It also has the folowing U.S.G.I. parts; op-rod, op rod spring and guide, connector lock, gas cylinder, gas piston, gas cylinder plug, rear sight, front sight assy. (minus castle nut), trigger group, front barrel band, stock and hadguard. Only the bolt required fitting.

Quoted: Only the bolt required fitting.

How do they fit an M14 bolt? I was thinking the bolt / barrel were a 'matched' pair?

A barrel once installed and indexed properly requires no fitting. The head space is obtained by a proper fitting bolt. Given that the receiver and barrel are in spec, it may require trying a few different bolts to get the proper headspace. They don't all fit the same. Ron Smith believes that the camming surfaces on the Chinese receivers can be slightly out of spec. He modifies the receiver to fit a U.S.G.I. bolt. By doing so, any G.I. bolt can be fitted to the receiver, most simply by lapping it to within the proper head space. Bolts aren't generally speaking, a drop in item. Normally minor fitting is required. Fulton Armory on the other hand, modifies the bolt to fit the  Chinese receiver. Creating the need to modify any bolt that would be put in the rifle. This is how I understand the differences between the Smith Ent. and Futlon Armory bolt conversions. Who's right? Who's wrong? Who Knows? I imagine that like most mechanical endevors, gunsmithing is as much art as science.

The barrel is the problem then. That makes sense.  I have a Polytech and have no need to change anything as it headspaces and shoots great, just wanted to know why a GI bolt could not just be put in.  Thanks Joe

Quoted: The barrel is the problem then. That makes sense.  I have a Polytech and have no need to change anything as it headspaces and shoots great, just wanted to know why a GI bolt could not just be put in.  Thanks Joe

If it headspaces fine, just dog it out with thousands of rounds of 7.62 NATO and have fun with it.  

Ross