Rotor button and distributor cap replacement

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Hey guys,

          On a regular basis I scrape the scale off my rotor buttons and the contacts in the distributor cap. But I was thinking this probably increases the gap between the rotor and the contacts making the spark less effective?

 

I'm just wondering how often I should replace the rotor and cap?

 

Both my cars run the electronic ignition.

[PZG302]

I genearlly replace mine when they need it, so far I have had the cap break down and crack due to the high voltage from the coil, but it only seems to need replacing after a couple of years of serious racing.