Try C & R again.  They offered to make a 4" for me last week.  Jerry
--------------------

I'm curious, did you tell them it was for an airplane (if that's what you
were shopping for)?  Some companies don't care, but I usually won't tell
them.  

>From everything I've read, the number one factor in cooling ability is the
frontal area.  In other words, a large single tube core would be better than
a dual tube core with half the frontal area.  So, it seems that step number
one is picking the largest frontal area that you can fit.  

Assuming you don't have enough space to put a single tube core, then you
have to start add tubes, and core thickness to make up the volume that you
need.  Surprisingly, I keep reading that this isn't so bad as it seems from
an airflow standpoint.  I found some data that said it only took 10% more
pressure to move the same volume of air through a 4" core, compared to a 2"
core.  

I would like to use an oil/water heat exchanger if possible, but that will
take a larger radiator.  Tomorrow, I hope to have my cowling fitted, so I'll
have a much better idea of the area I have to work with.

Thanks to everyone for the interesting ideas, and suggestions.

Cheers,
Rusty