sennheiser md421 transplant

PREFACE (skim or skip at your leisure)
bought another md421 on impulse after spending too much time on the "feelings on an md441" thread.  it arrived today, and i knew it was going to need some love.  the grille had been through a few too many drum sessions (see attached) and the clip was superglued on.  after a couple hours worth of cotton swabs, acetone, and elbow grease, i finally managed to get enough of the clip off to open the body.  once i had everything disassembled, the interior looked fantastic save for a scant bit of dust.  it was pretty conclusive, however, that the body was beyond any realistic attempt at repair.
so that means i need a new body.  sure, i could give sennheiser usa a call.  BORRRRRING.  i hate the body anyway. it's ugly, a pain in the ass to place, and people have been gaff taping (or supergluing  ) their loose/broken clips for longer than i've been alive.  instead, i'm gonna take a whack at transplanting the goods into something cheap and chinese.  the body will have to mimic the LF chamber/port structure of the 421, which might be tough.  nothing wagered, nothing gained!  if i fail miserably, i can always just buy a new body and move on.  here goes nothing!

discussion can be referenced to jakob's generous hosting of the tech drawing here:http://gyraf.dk/schematics/Senheisser_MD421.gif
so here we go.  my main concern is finding a body that will allow me to preserve the mic's bass response.  in car audio, it's common wisdom (though not necessarily correct, so please chime in if you disagree) that the volume of the box tends to trump any specific dimensional characteristics.  porting is a whole 'nother story, but i haven't gotten there yet.  need to know volume first and foremost.
since the mic is ported, i couldn't just fill the chamber with water to get my volume.  quick trip to the pantry took care of that (see below).  the added bulk of cellophane folds was pretty negligible, maybe 3-5mL.  end result was a V1 volume of almost exactly 400mL without subtracting the metal rod that holds the whole thing together.
if the "volume > shape" wisdom holds true, an mxl 990 is almost perfect for the task.  what might get me in trouble is the port length/shape, the 421's rear porting is longer than the body cavity of the 990.  these are baffled with thick felt on the 421 though, so i'm curious as to the true extent of their contribution.  measuring and reproducing them as they are will be difficult w/o a CJ-style hack job.  i may leave them out at first and see what happens.  if it's anything like typical "sealed box" behavior it should just reduce LF response.

question: on the tech drawing, it says volume Vn with Cn (n= 1,2).  V is obviously volume, is C supposed to be a flow coefficient?  if they went all fluid dynamics on this, the 990 might not be so great after all.

i also need to figure out either a stand-in or replacement for the metal rod, as it will no longer be structurally significant in the housing but seems to be both structurally and acoustically significant in capsule volume V2.  not cutting this till i can be sure it's expendable, just in case i need to wuss out and get a "real" body.

EDIT: the threaded end of the metal rod is just 4 turns of M5 followed by another 1.5ish mm of the minor diameter.  from what i can tell of the internal threading in the dynamic element, that last 1.5mm should be able to stay threaded with no problems.  easily ground off if i get picky.  the real trick will be the next portion where it steps up to a 6mm diameter for about 19.5mm of length... this is when grant needs a lathe.  with some dumb luck i'll find the cure at the hardware store and be beading silicone tomorrow.

Great project!

Jakob E.

This is good stuff! Proper DIY!

Grant, is it possible to keep the plastic rear housing of your beat up mic, and put that and the capsule inside your new body?
I'm pretty sure that would work and keep the frequency response close to the original.

Also, have you checked that the capsule actually works, before you get too far down the road.

Keep us posted

Stewart