Is there suppose to be an o-ring at the canister end of oil cooler line on 1996 Chevy pick-up with 350 engine?

There's no universally applicable answer to whether an O-ring should be at the canister end of an oil cooler line on a 1996 Chevy pickup with a 350 engine. The presence or absence of an O-ring depends entirely on the specific design of the oil cooler and its lines. Some systems use O-rings, some use crush washers, and some rely on a precisely machined surface for a leak-free seal.

To determine if YOUR specific vehicle needs an O-ring there:

1. Consult a repair manual: A factory service manual (FSM) for your exact year, make, and model truck is the definitive source. It will show exploded diagrams of the oil cooler system and specify all necessary seals.

2. Check the parts diagram: Many online auto parts stores (like AutoZone, Advance Auto Parts, etc.) have parts diagrams. Enter your vehicle's details and find the oil cooler and lines. The diagram should show if an O-ring is part of the assembly.

3. Examine the old line (if you have it): If you're replacing the line, carefully examine the old one. If an O-ring was present, there will likely be evidence (either the O-ring itself, or its imprint on the fitting).

Without this information, it's impossible to say for certain whether an O-ring is needed. Installing an O-ring where one isn't supposed to be can be detrimental, leading to leaks or damage. Always refer to the correct documentation or consult a qualified mechanic.