The automobile / engine manufacturer's gap spec is the one that should be followed, not the spark plug manufacturer (they have no idea what engine their plug goes in, and it could be hundreds of different engines with different specs).
As for gapping the plugs: gapping techniques can damage the plug. Be careful or be sorry. If you visit different plug manufacturer's websites and read their FAQ, you will see that they will tell you that it's a good idea to check it, but not always a good idea to adjust it.
For example...here's what Bosch has to say:
Do I need to gap my plugs?
Maybe, but likely not. Many plugs are pre-gapped for their most popular applications, however, the gap can be altered if the plug is dropped or mishandled in packing/shipping. Therefore even if the plugs are supposedly preset for your motor, it is always wise to check the gap on each plug prior to installing. For information on how to adjust the gap see Proper Gapping.
It is not recommended that you adjust the gap on multi-ground plugs.
Bosch Ir Fusion, +4, +2 Spark plugs are never to be gapped for any application. The gaps on these plugs are not adjustable.
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Last edited by Mogman; 04-20-2011 at 04:31 AM.