Nikon FM focus screen replacement

[siman]

I've searched around the net to find out if the focus screen on the FM can be replaced, what I've found out is, I know the FE, FM2, FM2n and FM3a have user replaceable screens and the original FM does not.
But there is some info stating there are 2 forms of the original FM, the older with serial numbers starting with 2 and a knurled lock ring around the shutter button and the top of the camera has to be removed to remove the screen, and the later version with a smooth ring, non locking around the shutter button with serial numbers starting with 3, this later model is the one I have, and there is a tiny screw above the mirror cushion which when removed will allow the focus screen to drop out?
My question is, has anyone here ever changed or removed to clean the focus screen on the FM?
I also have found that the screen on the FG is not user replaceable but some have done this also, and have used a screen from an FE in place after removing the tab, does anyone know if the FM and FG screens are the same?
My FM is very near mint condition but the focus screen is dirty, if possible I would like to replace it with the B3 screen.

 

[siman]

Well I managed to sort this out, I successfully modified a B3 screen to fit in my old FM, just had to file off the tab and shim the screen, the old screen is 2.1mm, the new B3 is 1.6mm. What a world of difference, the B3 screen is lovely and bright with amazing clarity and contrast compared to the old FM screen, so easy to focus and everything just snaps into focus now, definitely worth the effort.

 

[Ranchu]

Thanks for the update!

 

[kaiwasoyokaze]

good on you for modifying yourself, however there might be a problem with the metering cells though, as there was some information on how the brighter screen could cause the picture to come out overexposed or something.

 

[siman]

Update: After having a good go with the new screen the focus was off, it would appear the screen does not need shimming after all, although the edges of the screens are the thicknesses I mentioned above, the old screen is perfectly flat but the B3 is slightly raised on the polished side. Because the screen sits right on top of the prism now I had to file out a small notch on one side as the meter gauge sticks out into the viewfinder. All is now good, just been out giving it a good going over, perfect!

 

[siman]

good on you for modifying yourself, however there might be a problem with the metering cells though, as there was some information on how the brighter screen could cause the picture to come out overexposed or something.

You are correct, the B3 is one full stop brighter than the old FM screen, I just adjust the ISO dial and its fine.