Need advice for buying a flashlight.

protocol217 said:

because there are 17 mm and 20 mm, ant if it is 20 mm how will i insert my 17 mm driver

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Its smaller, it will fit in either one.

Yes, it can fit, but it is best if it is the right size.
But if the led star is smaller then i cant fit in my led, the led star needs to be the right size as the housing because the housing needs to take the heat from the led.