Do you need to change the stock spark plugs after upgrading to a big bore kit on a softail? Thanks in advance...

 

Heck no,use the old oil and fuel too

 

It's nice to know smart asses are everywhere.

 

Harley do have spark plugs available for stock and for modified engines, but I believe that the stock plugs would be fine.

Screamin' Eagle ST Performance High Compression Spark Plug-32186-10


Screamin' Eagle ST Performance Stock Compression Spark Plug-32192-10

 

Years ago when they had points and the amount of dwell time the points remained closed determined the voltage of the spark. (V8s of course had less then straight 6s) They tried to adjust the heat range just to make the plugs not fowl between tune ups. With the ignition now, Harley's standard plug with any where close mixture will last 50K probably. Remember, ideal fuel to air is what makes the most HP. Also, Harley's plug is different in the resistance range as an equivalent NGK plug. They work for me but I stay off it. Not sure about yours but it is the knock sensor on older bikes like mine. Just throw a new set of Harley plugs in it now and ever 10K with just a film of anti-seize (one small line down the threads the size of a toothpick).

 

I would try the stock new ones first,

 

NGK Iridium, sparks for powerful engines.
I always use this sparks in my bikes.
In ebay you can buy it with great price.

 

I use the NGK Iridium plugs in mine as well. No troubles with performance or the spark sensor.