Reed Slot Sizing Overview

In previous articles I have touched upon different subject around optimizing harmonicas that I think are useful to know about. These topics areĀ tuning, changing comb and reed gapping. This week we look at reed slot sizing and the benfits of it.

Reed slot sizing explained

As the harmonica is a free reed instrument it is quite easy to understand that the reeds of the instrument need to be able to vibrate freely to generate sound. The holes punched in the reed plate in which the reed vibrates is the reed slot. If it is too tight, the reed will get stuck. If it is too wide a lot of air is wasted and more effort is needed to play.

reed slot sizing example

A reed plate placed on a light table will tell you how much space the reed has to move.

When a harmonica is brand new the reed slot is often a bit to wide. To make it narrower customisers do reed slot sizing, sometimes referred to as embossing. The basic idea is to push material from the side of the reed slot down inte the reed slot. This will make the fit tighter.


When I first came in contact with reed slot sizing, or embossing, the tool of choice was the back end of a pitch fork. By repeatedly pressing the little sphere over the reed slot repeatedly material is pushed into the slot. This method does not cannot treat the entire slot without first removing the reed (the Ultimate Sizing Tool shown to the left in the picture below can treat the entire slot). The last few years I have seen a number of different tools been developed that offer more precision but also require more skill. Reed slot sizing does take a bit of practice so if you want to try it then don’t try on your favorite harp first.

reed slot sizing tools

Two possible tools for reed slot sizing.


If you feel your harmonicas are leaky it may very well be that they need a bit of reed slot sizing. If you are unsure of how to do it then get the help of a good harmonica service technician.

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