Spark Plug info needed

[34ford]

Have a ZZ4 crate engine. Came with AC marine plugs MR43LS. These are tapered seat plugs. The tech sheet that came with the engine says there are optional plugs which are listed on the sheet as AC FR5LS and Rapid Fire #5.  Went to the parts store and these two and a NGK crossover all have a gasket ring on them which has a slight taper to it. My question is can these be used on the alumniun heads. Went to the ac site and couldn't figure out anything.

Or does anyone have a tech line # I could call and ask. Don't want to screw anything up.

thanks,

bob

[purplepickup]

Those heads will accept either type plug without any problems.  I use the tapered plugs but have used the ones with rings on them also.

[Crosley.In.AZ]

depending on the headers used , usually smaller plugs are better.

[phat rat]

depending on the headers used , usually smaller plugs are better.

Tony, you're right on that. Did you know that there are two different lenghts of the MR43LS? It's the plug I use in the cpe and I built the headers using the shorter ones and with the longer ones I have some interference problems. Earlier this year I was replacing them and had a hard time finding the shorter ones. When I found them I bought two sets and put the second set in inventory.

[34ford]

thanks guys.  Did get the ones with the rings in a hotter range.

bob