Not sure if this was posted before and if it does please disregard:
Copied from another website credit to Different:
"USGI and commercial manufacture parts are theoretically interchangeable but occasionally the fit is too tight or too loose between such parts. Generally, USGI and commercially manufactured parts are interchangeable with their Chinese counterparts but there are some exceptions.
Barrel - Chinese barrels have metric dimension barrel shank, gas cylinder lock and suppressor nut threads. However, the Chinese barrel shank thread size is very close to the American made receiver barrel ring thread size. Chinese M14 barrels will usually, but not always, thread into an American made M14 type receiver without any trouble. Nonetheless, barrel installation and headspacing should be performed by a reputable M14 gunsmith.
A USGI gas cylinder lock will not fit on a Chinese barrel because of the differing thread sizes. Similarly, the Chinese gas cylinder lock will not fit on a USGI or U. S. commercial manufacture M14 type barrel.
Chinese flash suppressor nut-to-USGI barrel - The Chinese flash suppressor nut will go on without resisting but at a slightly downward angle due to the slight difference in threads.
USGI flash suppressor nut-to-Chinese barrel - The USGI and Chinese flash suppressor nuts are interchangeable on the Chinese barrel.
Bolt - For reasons unknown to Smith Enterprise, Inc., or anyone else the author has contacted, the Chinese did not use equivalent AISI 8620 alloy steel for their bolts as per the USGI drawing for their bolts but elected to make them out of equivalent AISI 4135 alloy steel. Chinese bolts have a surface hardness of 40 to 47 HRC which is too soft. This applies to Chinese bolts exported to the United States prior to September 13, 1994. If the bolts are heat treated to improve the hardness, the hardness increases all the way through the bolt instead of just at the surface. Hardening a Chinese bolt will add only a little more service life to the bolt to the item. In the long term, however, hardening a Chinese bolt is still not desirable. This is because increasing the core hardness of the bolt decreases the core toughness.
Another significant problem plagues Chinese bolts imported into the United States. Besides not being made of a material suitable for long term use, the locking lugs are too narrow. Thus, American and Chinese bolts are not interchangeable except in very rare instances. The owner of a Chinese M14 rifle with a Chinese bolt should not expect a USGI bolt to drop in to the Chinese receiver. Even if a USGI bolt does fit the Chinese receiver, it may headspace less than the safe minimum, 1.630 ", or it may headspace longer than the SAAMI maximum, 1.634 ". Converting a U. S. imported Chinese M14 type rifle to accept an American made bolt should be performed by an experienced M14 gunsmith.
These problems may or may not continue to affect Chinese M14 bolts which were more recently exported. For example, three Norinco M305 rifle bolts shipped to Canada in 2003 were tested for surface hardness. The results ranged from a minimum of 45 HRC to 48.5 HRC, with the average at 47 HRC. More detailed information on the bolt material or the core hardness of these 2003 vintage bolts is presently unavailable.
Gas System - Chinese gas cylinders and gas cylinder plugs are made of molybdenum-chromium alloy steel and gas pistons are chromium plated. Chinese and American gas cylinder plugs are not interchangeable because the Chinese sight parts have metric threads. The Chinese gas piston outside diameter and the gas cylinder inside diameter are slightly larger than USGI gas piston and gas cylinder drawing specifications. A USGI gas piston may be too narrow to function properly inside a Chinese gas cylinder. The inside diameter of a Chinese operating rod spring is about 0.010 " smaller than a USGI spring. Some National Match operating rod spring guides will not fit inside Chinese operating rod springs because the interior diameter is too small.
Operating Rod - Three kinds of operating rods have been identified on Chinese semi-automatic M14 type rifles. One kind has no notch at all for the connector assembly. Since this version cannot be used with a connector assembly, it is likely post-1978 production. Another has an almost straight angle cut to the notch, which does not closely match the USGI connector’s contours. These operating rods have numbers engraved on them. The notch may have been machined away after original manufacture. The third version of Chinese operating rod has a connector notch very similar to the USGI model, but with a slightly more circular shape. The third version was made in 1965 as part of the production of 100,000 complete rifles. In all other important dimensions, the three types of Chinese operating rods are the same. Chinese operating rods are forged and are as hard as USGI operating rods.
Rear Sight Assembly - Chinese and American rear sight knobs and bases are not interchangeable because the Chinese sight parts have metric threads. However, a complete Chinese manufacture rear sight assembly will fit on an American made receiver and vice versa.
Stock Assembly - Chinese connector locks were standard length in 1965 production M14 rifles but shortened to sit flush with the receiver in post-1978 production rifles. USGI stocks require some filler material to prevent lateral drift of the shortened Chinese connector lock.
Chinese butt plates and butt plate flappers are both made of steel. The butt plate is a casting. The flapper-to-butt plate joint is built up by welding then ground flat. Two types of Chinese M14 butt plate flappers have been observed: Type 1) the flapper has two raised indentations on the right side of the hinge and Type 2) the flapper has one hole on the left side of the hinge as the muzzle is pointed down range.
Miscellaneous - Chinese M14 firing pins have a gray color phosphate coating. Chinese hammers and triggers are sometimes a little soft, but this can be corrected with appropriate nitrocarburizing treatment. Chinese flash suppressors and rear sight bases are castings. The Chinese trigger housings are fully machined into final form.
Chinese Parts Identification
Chinese M14 parts differ in their markings from U. S. parts as follows:
Table 27: Chinese M14 Rifle Part Markings
Chinese Rifle Part / Identifying Marks
barrel / no markings except for KFS and CAI imported Poly Technologies and some CAI imported Norinco rifles
bolt / may have numbers electro-penciled on the top or bottom
flash suppressor / electro-penciled number
hammer / no markings
hand guard / silver painted bottom that may have the letters DT and a number or a number by itself
operating rod / may have electro-penciled numbers and the connector notch shape is different from USGI design
trigger housing / five or six digit number
windage knob / counterfeit W C E
The bolts and operating rods of Chinese M14 rifles imported in May 2006 into Canada have etched numbering on the bolts and trigger housings and W C E is marked on the elevation knobs.
Copied from another website credit to Different:
"USGI and commercial manufacture parts are theoretically interchangeable but occasionally the fit is too tight or too loose between such parts. Generally, USGI and commercially manufactured parts are interchangeable with their Chinese counterparts but there are some exceptions.
Barrel - Chinese barrels have metric dimension barrel shank, gas cylinder lock and suppressor nut threads. However, the Chinese barrel shank thread size is very close to the American made receiver barrel ring thread size. Chinese M14 barrels will usually, but not always, thread into an American made M14 type receiver without any trouble. Nonetheless, barrel installation and headspacing should be performed by a reputable M14 gunsmith.
A USGI gas cylinder lock will not fit on a Chinese barrel because of the differing thread sizes. Similarly, the Chinese gas cylinder lock will not fit on a USGI or U. S. commercial manufacture M14 type barrel.
Chinese flash suppressor nut-to-USGI barrel - The Chinese flash suppressor nut will go on without resisting but at a slightly downward angle due to the slight difference in threads.
USGI flash suppressor nut-to-Chinese barrel - The USGI and Chinese flash suppressor nuts are interchangeable on the Chinese barrel.
Bolt - For reasons unknown to Smith Enterprise, Inc., or anyone else the author has contacted, the Chinese did not use equivalent AISI 8620 alloy steel for their bolts as per the USGI drawing for their bolts but elected to make them out of equivalent AISI 4135 alloy steel. Chinese bolts have a surface hardness of 40 to 47 HRC which is too soft. This applies to Chinese bolts exported to the United States prior to September 13, 1994. If the bolts are heat treated to improve the hardness, the hardness increases all the way through the bolt instead of just at the surface. Hardening a Chinese bolt will add only a little more service life to the bolt to the item. In the long term, however, hardening a Chinese bolt is still not desirable. This is because increasing the core hardness of the bolt decreases the core toughness.
Another significant problem plagues Chinese bolts imported into the United States. Besides not being made of a material suitable for long term use, the locking lugs are too narrow. Thus, American and Chinese bolts are not interchangeable except in very rare instances. The owner of a Chinese M14 rifle with a Chinese bolt should not expect a USGI bolt to drop in to the Chinese receiver. Even if a USGI bolt does fit the Chinese receiver, it may headspace less than the safe minimum, 1.630 ", or it may headspace longer than the SAAMI maximum, 1.634 ". Converting a U. S. imported Chinese M14 type rifle to accept an American made bolt should be performed by an experienced M14 gunsmith.
These problems may or may not continue to affect Chinese M14 bolts which were more recently exported. For example, three Norinco M305 rifle bolts shipped to Canada in 2003 were tested for surface hardness. The results ranged from a minimum of 45 HRC to 48.5 HRC, with the average at 47 HRC. More detailed information on the bolt material or the core hardness of these 2003 vintage bolts is presently unavailable.
Gas System - Chinese gas cylinders and gas cylinder plugs are made of molybdenum-chromium alloy steel and gas pistons are chromium plated. Chinese and American gas cylinder plugs are not interchangeable because the Chinese sight parts have metric threads. The Chinese gas piston outside diameter and the gas cylinder inside diameter are slightly larger than USGI gas piston and gas cylinder drawing specifications. A USGI gas piston may be too narrow to function properly inside a Chinese gas cylinder. The inside diameter of a Chinese operating rod spring is about 0.010 " smaller than a USGI spring. Some National Match operating rod spring guides will not fit inside Chinese operating rod springs because the interior diameter is too small.
Operating Rod - Three kinds of operating rods have been identified on Chinese semi-automatic M14 type rifles. One kind has no notch at all for the connector assembly. Since this version cannot be used with a connector assembly, it is likely post-1978 production. Another has an almost straight angle cut to the notch, which does not closely match the USGI connector’s contours. These operating rods have numbers engraved on them. The notch may have been machined away after original manufacture. The third version of Chinese operating rod has a connector notch very similar to the USGI model, but with a slightly more circular shape. The third version was made in 1965 as part of the production of 100,000 complete rifles. In all other important dimensions, the three types of Chinese operating rods are the same. Chinese operating rods are forged and are as hard as USGI operating rods.
Rear Sight Assembly - Chinese and American rear sight knobs and bases are not interchangeable because the Chinese sight parts have metric threads. However, a complete Chinese manufacture rear sight assembly will fit on an American made receiver and vice versa.
Stock Assembly - Chinese connector locks were standard length in 1965 production M14 rifles but shortened to sit flush with the receiver in post-1978 production rifles. USGI stocks require some filler material to prevent lateral drift of the shortened Chinese connector lock.
Chinese butt plates and butt plate flappers are both made of steel. The butt plate is a casting. The flapper-to-butt plate joint is built up by welding then ground flat. Two types of Chinese M14 butt plate flappers have been observed: Type 1) the flapper has two raised indentations on the right side of the hinge and Type 2) the flapper has one hole on the left side of the hinge as the muzzle is pointed down range.
Miscellaneous - Chinese M14 firing pins have a gray color phosphate coating. Chinese hammers and triggers are sometimes a little soft, but this can be corrected with appropriate nitrocarburizing treatment. Chinese flash suppressors and rear sight bases are castings. The Chinese trigger housings are fully machined into final form.
Chinese Parts Identification
Chinese M14 parts differ in their markings from U. S. parts as follows:
Table 27: Chinese M14 Rifle Part Markings
Chinese Rifle Part / Identifying Marks
barrel / no markings except for KFS and CAI imported Poly Technologies and some CAI imported Norinco rifles
bolt / may have numbers electro-penciled on the top or bottom
flash suppressor / electro-penciled number
hammer / no markings
hand guard / silver painted bottom that may have the letters DT and a number or a number by itself
operating rod / may have electro-penciled numbers and the connector notch shape is different from USGI design
trigger housing / five or six digit number
windage knob / counterfeit W C E
The bolts and operating rods of Chinese M14 rifles imported in May 2006 into Canada have etched numbering on the bolts and trigger housings and W C E is marked on the elevation knobs.
