Gauge or light? Do you know what year the engine is? The OP 21x fits up to 78 in my book. OP 92X fits 79-87 (as far as my other book goes) fits all 6 cyl with light. OP21x is 3/8 18 thread. No thread size for the OP 92x but it looks the same and not 1/8" PT. The connectors are different between the 2. The 21x has the button and the terminal slides on. The 92x has 2 male spade terminals.