i bought fender string guides to replace the tree on my mim strat, but noticed they have a pin on them.the question is do i put said pin in the existing screwhole and re-drill a new hole for the screw or other way round? cheers-dan

Depends where you want to put the string tree.
Closer to the nut will get you more down pressure behind the nut. Making for a cleaner open note. At the expense of string life and nut life.
The further away it is from the nut, the less it does.

It isn't really a guide to keep a string in position.

You could just file the locator pin off and screw it into the existing screw hole.

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No no and no

+1
that's what I have done in the past. this way you have the option to turn it around if you want to change the break angle.

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i didnt relish the thought of drilling another hole,sounds like a plan thanks guys

I hadn't even considered that. It's a very valid point and a great tip.

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No no and no