Measuring Rims

Measuring Rims
Here’s a Lawson B23G705 rim. Its inner diameter is either .705″ or .683″, depending on where you measure it.

We’re often asked to compare the diameters of rims from different makers. Whether a rim measures 17.5 or 18 mm depends on where it’s measured. There’s no “right place” to measure a rim and unless the maker tells you where a rim has been measured it’s impossible to compare.

The Lawson rim shown above was measured at a point .025″ down from its high point (crown). That’s where the “705” number comes from. We think that’s a less-than-ideal place to take a measurement because it’s in the middle of the curve.

Osmun mouthpieces are measured .050″ down from the crown because at that point the rim blends into the cup. So, in our measuring system the diameter would be .683″.  That’s .022″ (.56mm) different.

Other makers may use different measuring points and unless they tell you where they’re measuring, it’s impossible to compare.  So, take rim measurements with a large grain of salt.

 

 

 

This entry was posted in All Brass, Equipment, Horn, Mouthpiece by Bob Osmun. Bookmark the permalink.

About Bob Osmun

We run a small music shop specializing in brass instruments, especially horns. We sell Engelbert Schmid, Finke, Hoyer and Alexander horns and have a world famous repair shop staffed by three of the best technicians in the trade. We regularly perform valve rebuilds, screw bell conversions and manufacture our own line of trumpet and horn mouthpieces. Jim Becker, our trumpet specialist builds custom trumpets and does all sorts of trumpet modifications.

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