Are you planning on recording a jet engine?

If not, I wouldn't worry about max SPL for the 421...it can take it. One of the more rugged microphones out there.

Sorry I can't throw a number at you...

Divo,
Checking the Rycote Microphone Encyclopedia for the Sennheiser MD 421 II, there are no max spl figures listed. Here are the specifications that are listed.
Frequency Response 30Hz. – 17kHz.
Output Sensitivity 2 mV/Pa
Output Impedance 200 ohm
Recommended Load >0.2k ohm
I can say that they have a very high spl ability. The chances of your mic pre overloading before this mic "tops out' is much more likely. I will look around and see if I can find a spec. Kurt

Don't worry, you can record a gunshot of a battleship with a 421 :D