How do I know the correct cap?

I have a new Radaitor.com Rad installed and the old cap did not fit and was replaced by one from a parts car the installer had. I went and picked one up at Auto Zone, It fits but has 13lbs on cap.. IS that a good cap for a 12a. or does the engine size have nothing to do with finding correct cap.

Thanks in advance .

 

13 lbs is what the manual calls for... so is the right one...

 

keep an eye on your temps for a while though. i learned the hard way that aftermarket caps are not always the best solution.