In addition to the difference in the spark plug gap, the BUHX plugs are a "Booster Gap" plug. The internal electrode inside the ceramic housing has a small gap in it. If you take an OHM meter and measure from the exposed center electrode to the spark plug wire connection, it will read "Open". This is done to insure that the spark energy is sufficient to jump the internal gap to fire the plugs. This technique was used in the early years of Capacitive Discharge ignition systems because sometimes the CD Modules would not consistently produce sufficient voltage to fire the plugs.
BUHW plugs are basically standad plugs. There is no internal gap in the electrode. If you measure resistance from the exposed center electrode to the spark plug wire connection, you will read continuity or a dead short.
If you use BUHW plugs in the early Prestolite ignition systems, you run the risk of not allowing maximum voltage to fire the plugs. It pretty much depends on your particular CD Modules as to whether or not the BUHW plugs work well. The components that make up the Prestolite CD modules are individual capacitors, resistors etc. so there is some variation in the performance of the module. The later model Mercury style Switch Box Thunder Bolt started using solid state integrated circuits. The mid 1990's the Mercury CDM style Thunder Bolt ignition systems went completely solid state. Both the Mercury style Thunder Bolt ignition systems had a faster rise time for the ignition voltage and were much more consistent than the older Prestolite ignition system so there was no need for a "Boost Gap" plug.
If the BUHW don't work well with your Prestolite ignition system, try the BUHX's. I have to say though, I know many folks that use the BUHW plugs with their Prestolite ignition system.
