One of the easiest ways to tell is to look at the top of the keg. If the rubber handles are full circles they are Firestone/Spartanburg. If the rubber is notched in the post area then it is a Cornelius type. Take a look at the picture below. If your keg looks like the keg on the right it is a Firestone/Spartanburg. If it looks like the one on the left it is a Cornelius.
The other way you can tell is to take off one of the posts and measure the threads.
If the thread size is 9/16 – 18 on both posts then the keg is a Firestone/Spartanburg.
If the thread size is 19/32 – 18 on both posts then the keg is a Cornelius.
How can you check the thread?
Go to Home Depot, Lowes or a hardware store and get a nut with the 9/16 – 18 threads. (You will probably not be able to find one with 19/32 – 18 threads but if you can get one of those too) Check both the gas and liquid post to determine which one you have.
Again 9/16 – 18 = Firestone/Spartanburg, 19/32 – 18 = Cornelius.
While the above will hold true for probably 95% of the pin lock kegs out there one of the things to keep in mind is that there were many variations of the different brands of kegs manufactured so it is possible that you have kegs that don’t fit either of those two descriptions.
Here is a listing of some of the most common kegs and the thread sizes:
Cornelius Spartan & Super Champion: Gas 19/32” – 18 Liquid 19/32” – 18
Cornelius R (pin lock): Gas 19/32” – 18 (2 pin) Liquid 19/32” – 18 (3 pin)
Firestone V Challenger, Firestone VI Challenger, Firestone Super Challenger: Gas 9/16” – 18 Liquid 5/8” – 18
Firestone A, Firestone R, John Wood RA, John Wood RC (pin lock): Gas 9/16” – 18 (2-pin) Liquid 9/16” – 18 (3 pin)
John Wood 85, Firestone Challenger: Gas 11/16” -18 Liquid 3/4’’ – 18
So although there are some kegs like the John Wood 85 and Firestone Challenger that have a “bastard” thread size, the vast majority of pin lock kegs out there carry either the 9/16 – 18 thread or the 19/32 – 18 thread and can be converted.