I have a 1970 Pontiac LeMans. The previous owner replaced the engine, but I don't know which Pontiac engine he installed. It's a 5736cc, 400hp engine, engine number 235690-B601630. That's all I know. I bought spark plugs for a 1970 Pontiac engine (R45S), but they don't fit. The diameter is 20mm (0.78 inches). I need 14mm (0.55 inches) spark plugs. The thread is correct, and the length is good. Do you have the correct spark plugs for me?

Welcome to the site. Need to figure out what you have 1st. We have members that can help with that.

any numbers on the center exhaust ports of the heads?

Or any numbers off the heads? That will answer the spark plug question.

Any numbers off the block? Some numbers are on the passenger side front pad near the water pump. Latter Pontiac blocks had their displacement cast into the center of the block along with the cast number. Date code is near the distributor hole.

Picktures are appreciated too!