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I often read of people doing this because the action on the USA vibrato unit is 'smoother'.

Surely you'd only need to replace the spring – isn't that where the smoothness (or lack of) comes from?

I think the string spacing is different too. It's considered "correct" on the US one from our friends over on the other forum. Not sure as to the truth of it though.

There are several things about the CIJ trem that might affect the 'smoothness': the spring, the shape of the 'blade' edge that the tailpiece pushes against, the connection between the arm and the collet in the socket, the string spacing (I wouldn't have thought it affects the action of the trem much; it's just, the string holes are closer together on the CIJ tailpiece so it's more of an aesthetic thing). I recall a clunking/creaking in my old CIJ trem, but never did pinpoint where the noise was coming from.

The CIJ trems work okay, but I found the US replacement was better in terms of the arm fitting better and the string spacing matching that of the bridge.

It's the brass doot this bloke has 'machined' for smoothness:

http://www.offsetguitars.com/forums/vie ... =8&t=67566