I need a index spring for my 36-220 compound

Exploded diagram for 36-220 is here.
Operators manual is here.

Once you have ID'd the index spring on the exploded diagram, you should be able to order it from Delta, assuming it's still a part they stock. If they no longer stock it, you may be able to find someone who does still stock it by a google search of the part number.

I've got a bad feeling this part may be very difficult to acquire though.

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