Hammer handpieces! boy I bet this produces an array of replies.
Personally I have used the foredom brand for 25 years and here are
my answers to your questrions.
Criticisms: The hand piece with the duplex spring tends to kink and
break the spring when used at too steep an angle or too high a speed.
If used for extended periods the hand piece tends to heat up.
Problems. overheating and binding of duplex spring
Modifications. I don’t use the hand piece with the duplex spring.
Availability: It is a foredom and everyone carries and repairs them
or ships them back to foredom or you can contact foredom direct
Comments: I keep the old worn out had pieces for parts in an
emergency and usually return them to foredom for repair when it is
convenient. Most of the parts are interchangeable between the foredom
models. Frank Goss