I have a 91 w150 with a 318 and i a cracked my radiator. anyways im buying a new one and theres 2 choices a 18 or a 21 core. whats the diff? 18 is cheaper and im kinda strapped for cash so thats the one im leaning to right now. the dodge dealership said go with the 21 and my step dad whose a mechanic said just go with the 18. will the both fit? i know the 21 will hold more coolant but is it really worth the extra money? what u think? o and whats kind do you think is in my truck now a 18 or 21

 

I assume that is fins per inch. Shouldn't have any thing to do with capacity. If you live where it is hot I would go with the 21.

 

I just bought one from Oreilly`s for my 88. Don`t know any numbers on it, it`s just a stock replacement. Seems to work fine though.

 

Bigger radiator will fit in place of standard but if you have the standard radiator now, the fan shroud will not match up. You will have to fab some tabs to mount your existing shroud to bigger radiator. I had the standard radiator and live in the south and I never had overheating issues. My old radiator quit after 20 years, replaced with the bigger unit. "84 W-150 SWB, 318, 360 heads.