merc 4.3 plug size to use

[sharkin95]

I purchased a rebuilt 4.3 block (#14099090) with vortec heads (#12556140) what size plugs should I use. my original parts are from a 1996 4.3 with a thunderbolt 4 distributor, no timing sensor ,4brl weber carb. I was using mr43T@.035....do I match the plugs with the heads and if so what gap?

 

[Don S]

Re: merc 4.3 plug size to use

Look at the threads in the heads, are they long or short?

MR43T is for short threads, MR43LTS is long threads.

 

[sharkin95]

Re: merc 4.3 plug size to use

long ..so as a rule you match the plugs to the head and it doesn't matter that it is a thunderbolt 4 or 5, what gap .045 or trial and error

 

[Don S]

Re: merc 4.3 plug size to use

Merc has always used .045 with the MR43LTS plugs in the V6's

 

[sharkin95]

Re: merc 4.3 plug size to use

I'm looking for a more technical answer as to why, I have a early 1996 4.3 merc v6 with a 12 bolt manifold non vortec heads and I replace the heads with a set of 1999 vortec heads and a 8 bolt manifold... what plugs do I use and at what gap and why?

 

[Don S]

Re: merc 4.3 plug size to use

I'm looking for a more technical answer as to why,

Because the Mercruiser and GM engineers determined that plug and gap combination worked best with that head design and ignition system. The spark from a spark plug is not just an explosion inside the combustion chamber, it's more of a controlled burn. Spark plug type and gap is what gives them the control.