I have ARF42 Motorcraft plugs currently installed on a 1971 351C 4V. Standard replacment plugs from Ford call for SP-420 or SP-450. My engine is NOT a BOSS 351. The dimensions shown for both alternates are the same and Gap on both are.035. BOSS CJ OEM plugs from 1970-1973 were Motorcraft AF-32.
Question: Which alternate is correct. Motorcraft SP-420 or SP-450?
Question: Why are there 2 part numbers. Maybe 2V and 4V heads?
Thanks in advance if you can shed some light on problem
Regards, M1
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Wednesday, February 18th, 2015 AT 10:22 PM