Spark plug type - quick question

[ktbarrentine]

Recently purchased a 20' runabout with a 1990 Mercruiser 5.0l and Alpha 1 outdrive. Noticed the engine manual calls for MR43T plugs and I see that alot in the forums. I pulled my plugs to inspect/change and they are MR44T, so that is what I put back in. Boat runs good. Mostly run it in central AZ. Question is: what is the diff in 44T to 43T. I imagine it has something to do with heat range. I believe in: "if it aint broke, dont fix it", but want to make sure I am running the right plugs.

 

[Bondo]

Re: Spark plug type - quick question

but want to make sure I am running the right plugs.

Ayuh,.... Welcome Aboard,.... Donno why you've got the 44s in it, But,....

The AC Delco, MR43T are the Right plugs...

 

[Struc]

Re: Spark plug type - quick question

Yes, the difference is the heat range.

M=Marine
R=Resistor
4=14mm Thread
3=Heat Range
T=Tapered Seat Shell Design

So, it's still a Marine plug, but with a different heat range. Not positive if it's cooler or hotter, though.

EDIT: Just googled it and it is a hotter plug. The larger the number, the hotter the plug with AC Delco.

 

[55evinrude]

Re: Spark plug type - quick question

whatever the engine manual calls for is what you should run for best performance

 

[ktbarrentine]

Re: Spark plug type - quick question

Thanks. I imagined was a heat rating. Don't know how long the PO had the MR44Ts in it (possibly up to 400 hours), but I am changing back to 43s. Just wondering why he might have gone to a higher heat... I'll see if there is any difference in the way it runs. Any long term effects on pistons/valves from using a higher heat plug?

 

[ktbarrentine]

Re: Spark plug type - quick question

Wow...Just learned alot about plugs from taking myself to wikipedia. Sounds like theoretically (in older, pre-electronic engines like mine) a hotter plug would be good for alot of low RPM ops (no wake zones, etc) to keep from fouling, but is not advised for WOT due to chances of preignition related damage. Sounds like the 43Ts are the good all-around plugs specified by the manual.