Best spark plug for 312?


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By 221239ford - 11 Years Ago
I am running Autolite Platinum AP45 spark plugs and it is time to replace.  Any suggestions for a better plug?  My 312 is a '56 with '57 heads, stock teapot carb, mild Holman & Moody cam, pertronex ignition, and Fordomatic trans.  Also, what gap do you suggest?
By charliemccraney - 11 Years Ago
I've been using regular Autolite 45s.  I haven't had any issues.  I noticed a while back that they are made in China, which I just don't support if I can avoid it, so I will try something else once my current ones are used up.  I've heard that the platinums are not made in China but have not confirmed.  I like normal, basic spark plugs.
Motorcraft BF42 (SP420 now) is an alternative which I think is made in Mexico - I haven't had a chance to check.  Edit:  On 3/12/2015 I had a chance to check.  Motorcraft SP420 are made in Mexico.  At least that's North America.
NGK is also generally good.  I haven't seen one yet that was not made in Japan.  I don't know the comparable model to the A45 or BF42.
I don't think any are made in the USA anymore unless you find NOS.

Regarding the gap, .035.  If you are running a hotter coil with the pertronix then try something wider like .040 or .045
By Ted - 11 Years Ago
Autolite #45 spark plugs work well on most Y engines that are in fair to good condition.  For one that’s lower in compression ratio or is using a little oil, then Autolite 46’s tend to be a better choice.  I’ve found the Autolites to be made in both Mexico and China which may simply be the distribution of those plugs that determines which source ends up in the local parts store.  Being in Texas I do see a lot of made in Mexico Autolites but occaisionally come across those made in China.

As Charlie mentions, 0.035” gap works well with the carbureted engines.  Leaner fuel mixtures tend to like a wider gap but those leaner mixtures are usually tough to get with a carburetor versus what a fuel injection setup can do.  Stock point ignitions will like the factory recommendations which is typically 0.035".
By 221239ford - 11 Years Ago
Thanks, Charlie.  I am running the hotter coil.
By 221239ford - 11 Years Ago

Sorry, Charlie.  Sent message to Ted.  As mentioned, I am running the hotter coil with the Pertronex.