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fronts are 4in and rears are 4 but can fit a 6.5 and the sub is a 10in

 

My car is a 1998 SC400 with the Nakamichi system. Since I couldn't find the exact size anywhere, I'll show them here, hopefully help someone like me out. I have the fronts here needing new surrounds, Nakamichi S400F04, The cutout is 104mm (@4 1/8) mounting holes center to center diag. is 123mm (@4 7/8) overall OD 114mm (4 1/2") the largest OD of the speaker frame is 130mm (@5 1/8")
So, the fronts are more like a 4 1/2" but after-market sizes are all over the place. I got a pair of Infinity Kappa 42.9i "4.0" inch that are Way too small: 100mm (3 15/16") overall OD. I don't consider these "cheap" speakers, but they sound like crap compared to the stock Nakamichi, using the stock amplifier. I know the stock drivers don't look like much, but in my opinion, they sound great, very clear mids that blend well with the tweeters and set a nice "sound stage". I'm repairing the Nakamichi fronts and re-installing them, they just sound too good to replace. All they usually need is the foam surrounds replaced which is easy, the hard part is finding foams in the correct size, you'll need foams with an ID of 72mm (@2 13/16") and an OD 101mm (4") of course they can be close to that, the actual cone is 79mm OD, so size the ID of your replacement foams at close to the 72mm or less, so there is good overlap. The OD of the new foams need to be at least 97mm (3 13/16") best would be 101mm (4") if larger they could be trimmed.
The small rear deck speakers are the same dimensions as the fronts without the mid frequency "whizzer cones" but I put 6.5 inch back there with little trouble, these are not as important as the fronts, the rears just do some mid bass fill in.
Summing this craziness up, don't go throwing your stock Nakamichi speakers out, thinking you can just put anything back in there, you'll loose so much. Maybe with after-market amps, it would be a different story. I have read elsewhere, that the stock amp only works well with the stock speakers.
I did add a Kenwood KAC-8104D to the stock 10" sub-woofer. I just like a little more sub bass than the stock system could offer. This amp takes speaker level inputs, and has nice equalizer controls for tuning your sub. Installation couldn't be much easier; since it is a digital amp, the factory power wire is sufficient, unless you want to listen to rap or something really demanding, cut the wires at the sub, plug them into the amp, run new wires to the sub, boom. The stock sub handles this with ease, I have been very impressed.
If your just into "blasting it" no, the stock system won't do that too well. I'm speaking of the "Quality " of the sound and the staging at normal volumes, this old stock system is pretty hard to beat, without spending a bunch of money. Sorry for rambling, just hope I can help someone out.

 

^^ This. The 6.5" will fit in the rear by mounting directly to the deck. 4" will be a bolt-in replacement to the plastic mounts IF you dremel off a couple of small nubs (extremely easy to do) for a flush fit. 5.25" or any other sizes will require custom mounting solutions and aren't bolt-in.

 

If you're going to customize the door speaker mounts you might as well go with 5 1/4" in the doors and as mentioned 6.5" in the rear.