Rear Brake Cylinders: How to Tell the Difference?

[dibbons]

Nice to find someone swapped in a "489" case 8 3/4 differential (10" brake drum) into our '65 Plymouth Formula "S" which I am sure came originally with a different differential, most probably a 7 1/4.

I need to replace the rear brake cylinders and I purchased two sets: one is listed for 1965-1969 Valiant and the other is listed for 1970-1975 Valiant/Duster. Comparing them both together (still in the transparent plastic wrap) they look identical to me but have different stock numbers (AC Delco 18E584 for 1965 vs 18E80 for 1970).

How do I figure out which pair to use, since I am working with a non factory original setup? Thank you.

 

[Dana67Dart]

Look up the specs on each and see what the ID is for each bore.

 

[chryslerfat]

When I swapped my car to 8 3/4 I reused the same cylinders from the 7 1/4 without any trouble from fitment or function. My car had the 10 inch brakes already from factory.

 

[dibbons]

If you can believe specs posted on EBay, there is no verifiable difference between the two, both listed a having 15/16 inch bore size. Only "difference" I can see in the specs is that one weighs a "hair" less than the other.