The Hertz HSK 163 is a 3 way component system, but has a 3 inch mid speaker. The factory door speaker is a 4 inch. Would it fit? Would I need to make a ring or something to fit the hole? Hertz states the midrange mounting diameter is 71mm.

 

It should fit...Yea you might have to make a ring....

 

im not sure about the depth tho....... might be a tight squzzed i would deff make a baffle.

 

The bolt pattern is completely different. MINI uses a 3-hole pattern whereas standard 4" speakers use a 4-hole pattern. You'll have enough room in front and behind the speaker, but you will have to cut off one of the 4 metal flanges on the speaker and drill new holes for the other ones to get it to fit under the plastic grille/cover. It's a bit of a pain, but where there's a will, there's a way!

 

And you couldn't just make a mounting ring why???

 

Could probably make one from ABS. I am now considering the DSL C36 speaker system. 6.5", 4" and .8" silk tweeter, 93 db efficiency v Hertz 92 db efficiency. http://www.dls.se/en/car/prod.html?produkt=en_57 There is a review linked if you can read German!
The 4 inch is listed as 2.28 deep and the thread on installing speakers states the factory 4 is 2.25" deep. he did not state how deep the hole was or the max depth available. I am sure the DLS has a larger magnet structure and although close, I think it may fit. What do you guys think. I have not taken the door apart yet.
I bet this is the same 4 inch speaker http://www.dls.se/en/car/prod.html?produkt=en_54
The DLS 6.5 appears to have room to fit.
I have seen them for between $189 and $220.