Please ensure that you select the correct type of machine heads for your guitar.Most acoustic guitars (and some electrics such as the Les Paul) have a symetrical headstock with the machine heads mounted 3 left and 3 right. But most electric guitars such as the Strat have all 6 heads mounted on one side. For these guitars you will need 6 left.
Machine head bushes are fitted over the spindles on the face of the headstock. Some have screw-in bushes and in this case the drilling through the headstock would normally be 10mm. diameter.
Some have press-in bushes which are a tight push fit. The drilling in this case is usually 6mm. diameter straight through - but then opened to approximately half the depth of the hole to accept the diameter of the bush.
Generally, screw-in bushes are preferable as they are more secure and when replacing machine heads, most guitarists would choose this type. When upgrading to these it will usually be necessary to drill out the holes in the headstock to 10mm. diameter.
