how to modify a commutator motor to make a generator ??

Are there any modifications that i could /should make to a small dc commutator motor ??..
by small i am talking about anything from a automotive winsheild wiper motor down to a 9V hobby motor.??
ie add a diode to one of the outputs outputs ?
add a full wave rectifier to the outputs??

small dc motors are suitable for use as generators without any modifications.
make a test: take two small motors, connect their shafts, power one of them and see what you
can do with the power generated by other motor. the comutator will work as rectifier so no electronics is needed.
if you intend to use this to power some electronics, add capacitor to filter the noise and ripple.

Exactly! - a DC motor is simply a dynamo, and just like a dynamo gives out DC (or at least DC pulses), as Panic Mode says, the commutator rectifies the output.

An alternator is a different device, it provides an AC output (no commutator), and requires rectifiers to give DC.

And a British inventor "Trevor Baylis" used this age old principle for his wind-up radio.

http://www.leeselect.com/baygen/baygenamfmsw.htm

Trevor Baylis used a spring with gears and a small electric generator to power his radio..

That is what I meant, a small driven DC electric motor as a generator.

The is a moment where the comm plates will actually short each other out, where part of the energy that is produced by one web is charged into the proceding arm web. So to speak depending on rpm this gradually decreases to micro seconds.

The more poles the cleaner the wave form pulsing off the motor.

Also 3 pole modification or dual phase motors utilize a brush directly over the comm plate for every neutral degree movement, say it is a 23 degree motor so every 23 degrees the brush is centered over the comm plate and the brushes are set so that the comm plates never short, this allows for circuit manipulation of the motor for a electronic adjustible offset.

2 pole uses a degree offset adjustible by the endbell (mechanical) for the amount of double plate contact at the 2 poles, the more positive the more torque less RPM, the moter negative the less torque but higher RPMs