This is an area in transition. Our research shows there is no standardization of style in stating of measurements in American publications.
For the Journal, use English units for common measurements such as length of a grand piano or height of an upright, tool and materials dimensions, distance from your shop to a customer's home, etc. If an author expresses these measurements with metric units, add an English conversion to the text where it will aid understanding for an American audience. Example: The piano in question was a 168-centimeter (5' 6") Kawai.
Use metric or English measures where both are currently in common use now by American technicians, such as scale lengths and action regulating dimensions. Use editorial judgment. If your author states initially that key dip is 3/8", edit for use of English throughout. If your author states key dip is 10 mm, edit for metric throughout, when possible. Avoid switching back and forth without good reason.
We will probably need to revisit this issue in future years, planning for an eventual move to an all-metric style.