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After I bought my '71 with its 454" I had a lot of trouble with AC Delco, Champion, NGK, and Autolite spark plugs as their gaps increased about .001" per 1000 miles driven. As it had an HEI distributor I had gapped my plugs at .045" as most people had recommended but after just 10,000 miles of driving their gaps had opened to .055" at which time they quit firing. So I bought a set of 5305 DENSO iridium spark plugs and that solved the problem as after about 40,000 miles of driving their gaps have only increased .001" to .002" or next to nothing. Although costing two to three times as much as conventional spark plugs the iridium spark plugs actually cost a lot less in the long run.
 
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