US GI op rods are usually well worn at the tabs, and don't often stick in the Chinese receivers slots. If this was a Sproingfield CASTreceiver, then the slot could wander, change dimensions, and have very rough machining marks, with humps here and there, so budget some extra time for the SA receivers.
NOW, IF YOU USED A NEW OR FAIRLY NEW GI OP ROD, the tab might still be close to original dimensions, and might need to be carefully hand fitted to the slot in the receiver.
A BIG rubber mallet is your friend here, along with some reloading case lubricant, fine files, wet and dry sandpaper in various grits, and a lot of patience. Wherever you see the parkerising wearing off inside tghe slot, this shiny spot is a high spot that needs attention.
You want the op rod tab to stay at maximum size, and the slot to get cleaned up till the op rod falls of its own weight when the action is tilted to 30 degrees.
PS: Valve grinding compound can help a bit, but remember the grinding takes place equally on the tab and the slot, and you want to keep the tab as big as possible, so dont just slop on the grinding compound and beat the op rod back and forth with the rubber mallet. DON'TDODAT!!
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good luck,
BTDT lots of times,
LAZ 1