Motorcraft replacment for 351C with 4V heads

1971 FORD MUSTANG
5.8L • V8
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M1
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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
Feb 18, 2015 at 10:22 PM
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HMAC300
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42's were the correct plug for that car because it's a 351C 4v it's considered a SJ or CJ I forget which. you can run either plug one may be hotter or colder depending on porcelain size in plug at bottom.
Feb 19, 2015 at 8:14 AM
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M1
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Thank you for the quick response. I guess you meant to say 420 not 42s since ARF42 is no longer available through Motorcraft. Iwas leaning in that direction too. Think the SP-420s are the answer.
FYI- The BOSS and CJ (Cobra Jet) heads valve chambers are different configuration than the 351C 4V on my Mach 1.

Thanks again for your help

Regards,
M1
Feb 19, 2015 at 7:14 PM
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HMAC300
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having been at ford dealers and doing heavy repair at that time, that was Ford's hot dog motor for the Mustang and Torinos. they ran pretty well for what they were. but that was a long time ago and I forget with the cj sj designation as I'm not up on that stuff any longer. to much other stuff clogging my memory banks lol.
Feb 20, 2015 at 7:13 AM