Multi-hole tapping a pattern that has more than one thread pitch is sometimes necessary.  The photo below is a square four bolt pattern that has two holes to be tapped 1/4-28 and the other two holes to be tapped 3/8-24.  If a standard multi-spindle head, that has all the output spindle turning at the same speed, is applied, the 1/4-28 tap will feed slower than the 3/8-24 tap.  The will cause tap breakage.  Solutions to this unbalanced feed are available.

For our example problem, a solution is available.  The internal gearing can be adjusted to set the RPM of the 1/4-28 tap spindles to turn at 7/6 the rpm of the 3/8-24 spindles.  This gearing will allow the spindles for the 28-pitch tap to turn seven revolutions for every six revolutions of the 24-pitch tap's spindles.  The feed will then be equal and the holes are tapped with the mult-spindle tapping head without tap breakage.

For minor differences in pitch there may not be exact gear ratios available for the multiple spindle tapping head to solve the problem.  An example is when a metric pitch of 1.25, which in our common inch system has a feed of .0492125"/revolution has to be mated with a 20 pitch tap.  The difference in pitch is less than 2 percent.  For this type of problem, there are available floating tap collets that have axial float built into the collet.  This solution may only work for shallow depth tapping applications.