How tight should my reed plate screws be?

How much is too much?

Hello Harp Techs, many of you over the years have asked me, “How much torque should be applied to the fasteners (machine screws) that hold the harmonica reed plates together”? As a fabricator for about 60 years, I’ve developed a “feel” for tightening fasteners of all shapes and sizes. 

There was a time, however, I couldn’t trust my sensitivity. This was when I was rebuilding cars and air conditioning compressors. Here I had to use a torque wrench, a tool that allowed me to exactly set the tightness that would be applied to the head of the fastener. The perfect Pound Per Square Inch (lb/sq2) setting was determined in advance by the engineers of that particular machine. This was critical because, with the wrong fastener torque, machines with close tolerance parts would perform poorly and worst yet--blow apart when put under extreme operating conditions. For our humble harmonicas; applying too much torque can permanently warp the reed plate, when not applying enough, there will be sufficient air pressure loss. In either case, this is a bad outcome for the player.

Always helpful, Steve Baker* from Hohner responded to my query regarding recommended torque settings of the Hohner reed plate screws. So for all you gear head geeks out there, the correct setting is 13.22 Newtons. This converts to 2.9719742278 lb/sq2

*included in my top 5 favorite harmonica player list ;o)

In October 2008, I toured the Hohner Factory in Trossingen, Germany, I saw the assemblers at the MS (Modular System) area using rotary powered tools to fasten the reed plates onto the comb.

Reed plate fasteners installed by the factory already cut the threads on the bottom/draw reed plate and are easy to spin back into place after servicing the harmonica. However, when new replacement reed plates and fasteners are installed for the first time, you may notice the fastener being extremely tight when screwing into the bottom plate. This is because the “self tapping” machine screw has not completely opened up the threaded hole. Let’s take a look at the standard practices for inserting reed plate fasteners.

Here is a close up view of the Pozi head machine screws that Hohner uses to assemble their harmonicas

Pozi head machine screws require a special screwdriver. Notice the extra tines, in-between the usual Phillips tip screwdriver

The mating of a Pozi screwdriver with a Pozi machine screw is a beautiful thing. It’s almost as if the screwdriver is magnetized. The screw is straight and secure. For those of you who have “rounded” the inside portion of the machine screw with standard Phillips tip screwdriver, you learned a hard lesson (isn't that right Pete?). It’s time to purchase a Wiha 262/PZ0 x 50 screwdriver https://www.wihatools.com/precision-pozidrivr-0-x-50mm

You’ll thank me in the morning, especially if you are performing a reed plate replacement operation! 

Nest the reed plates back into place on the comb. Loosely insert the machine screws through the clearance holes of the top reed plate and give the screwdriver a couple of turns clockwise (“righty tighty, lefty loosey”) for the threaded shaft of the machine screw to bite into the threads of the bottom draw reed plate. DO NOT TIGHTEN YET !!!

This is a good step-up tightening pattern; starting with machine screw #1, then move up to #2, then #3, etc. Once you reach #6, reverse the pattern. With each cycle you will twist the screwdriver just a little bit more. The idea here is to start in the middle and work towards the edges. Tip: when you feel resistance, apply just a tad more torque for the final pass. Note 1: For the Hohner Special 20, the two upper corner holes do not require machine screws. Note 2: Suzuki harmonica owners will find this pattern very useful, because many of the Suzuki reed plates are bowed inward. 

When done properly, you will see approximately 4 threads of each machine screw passing through the threaded bottom draw reed plate.

You see that, investing into expensive torque calibrating devices was not necessary!

Your Harpsmith, Kinya