I keep seeing incredible deals on e-Bay for parts like oil coolers, radiators, evaporators, alternators, etc.

However, most are vehicle specific, with no or few actual specs on the parts (size, etc.)

How do providers (manufacturers) of these items know what fits which vehicles?

Is there some secret catalog, download or website that provide actual physical dimensions or specs?

Case in point: I had decided to use 2 evap cores in my RV-4, but now again looking at using a radiator under the engine (more space for ducting in the O-360 lower RV-4 cowl for something like 13" wide, and up to 20" long, 2.5 to 3" thick with no cap). Yes, I can get something like that from Summit racing (universal or drag racing radiator), but for $375 or more. Yet I see stuff on eBay for $100 or less, but no physical dimensions.

Is the only solution to go visit a junk yard, other than knowing what others have used?

BTW, what happens when you lay down (at a 20 degree) angle a radiator that is designed to be upright?

Finn