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Spark Plugs for 74 MPI

hmeislin

New member
I have a 1999 7.4 MPI Mercruis

I have a 1999 7.4 MPI Mercruiser IO. The SELOC manual states I should use NGK BPR6EFS spark plugs. The plug I pulled out was an AC MR43LTS. The replacement plug I was planning to use is a Champion RV12YC. The difference in these plugs is the reach - .406 vs. .708. Does that matter?
 
"merc number is
33-816336


"merc number is
33-816336

AC MR43LTS
Champion
RV12YC
NGK BPR6EFS

the reach should all be the same, might want to call merc on this one"
 
"Why is there not a plastic co

"Why is there not a plastic cover on the motor with the plug, gap, timing, fluid types, and that kind of thing? They have been placing that info on their motors for years.

Yes, the reach matters. Is this the first time the plugs have been changed or is the boat new to you/first time doing the service on it?"
 
CHAMPION RV12YC and NGK BPR6EF

CHAMPION RV12YC and NGK BPR6EFS plugs have a reach of .406

CHAMPION RS12YC and NGK BPR6FS plugs have a reach of .708

This is the first time I've replaced the plugs in this motor and the plugs I took out were AC MR43LTS which has a reach of .708.

Different references I've looked up show both types listed. It would make sense to me to replace with plugs that have the same reach. Your thoughts?
 
JUST-IN-TIME - Never seen the

JUST-IN-TIME - Never seen the wrong parts used in a boat or car? I have and sometimes it gets pretty ugly.
 
Mystery solved. I was working

Mystery solved. I was working on the right side of the engine and found the sticker that stated which plug to use. It said to use AC MR43LTS plugs. Thus the Seloc manual and several sother sources I used were wrong. The correct plug has the longer reach. Thanks to you all for your help. I'm going to post another question now that I've got a corroded plug I can't get out.
 
"A Stainless body plug corrode

"A Stainless body plug corroded that badly, eh? Definitely use anti-seize, as was recommended in the other thread and loosen them in fall or spring if they aren't being replaced. Sounds like they were in there for a while.

Get a thin-wall socket for this. A regular Craftsman or cheapo won't work. Grinding a thick one down will probably make it blow out the side with high torque. The chrome plated ones are a lot harder than impact sockets, too. If you can't find a thin-wall socket, grind a chrome plated one down to fit. You might be able to just knock the outside corner off at the end, which won't cost it very much in durability."
 
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