Precision Measurement Supply - Fixtures, Tooling And Measuring Devices For Professional Engine Builders

Flycutter Setting Fixture Instructions

The fixture is easy to use if you will keep two things in mind.  First, when the micrometer tip is in contact with the calibrating gauge and it is set on zero, the fixture is preset at 1.00” diameter (.500” radius) …. So be sure you ADD .500” to any micrometer reading.  Second, the setting fixture reads cutter radius not diameter.  


Assume you want to set flycutter for a 2.02” diameter valve relief …. Use the following procedure:

A.  2.02” divided by 2 equals 1.01” which is the valve radius. 

B.  1.01” minus .500” (radius of calibrating gauge) equals .510”  

NOTE:Subtracting gauge radius from valve radius is the same as adding gauge radius to micrometer reading, so don’t let it confuse you.

C.  Set micrometer at .510” (.500” plus .510” equals 1.01 which is the valve radius.

D.  Lock thumbscrew so setting won’t accidentally change.

E.  Insert flycutter holder where calibrating gauge was.

F.  Slide cutting tool out until it contacts the center of the micrometer tip.    (Shank of holder may extend through bottom of base.) 

G.  Lock cutting tool in place with sets screws and you are reading to go!


If micrometer does not read zero when in contact with calibrating gauge it can be adjusted by holding knurled portion and turning spindle with spanner wrench (provided).  Note: For accurate setting, hold calibrating gauge in fixture by pressing down on the top of it to prevent the micrometer from rocking it.  Also, if you will use the ratchet screw on the end of the mike, it will assure better repeat readings.  Remove calibrating gauge when turning with wrench so you won’t force tip into gauge.

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