Spark Plugs

[electricaldoctor]

I have a 1988 390 EC with twin 454 engines. I am experiencing intermittent spark plug breakdown from my port engine. I want to change all of them on both engines. What spark plugs should I be using for replacement? I have a Mercruiser part number of 33-59571, but I would like to use some other brand name. NKG, Champion, etc. Any suggestions??

 

[fc3]

Probably AC MR43T, Champion RV8C or NGK BR6FS.

I'm running MR43Ts in Just Ducky. They are the easiest to get and work fine.

Best regard,
Frank

 

[rondds]

electricaldoctor
we have the same engines and i'm on my second set of autolite 25's. they perform flawlessly and are available in walmart in blisterpacks of two (pretty inexpensive).

FYI the gap is .035" and if you prefer champion, the plug is RV15YC4 (that's what i took out and what a friend uses in his '90 350DA with same engines).

i dont think frank's equivalent is the same as ours as he is fuel injected.

 

[fc3]

Actually, I knew the MR43T number off the top of my head. The other numbers come from Mercruiser's 1987 Service Manual. I can't remember everything. The manual recommends Autolite 144s, too. Missed that on the first revision.

Best regards,
Frank

 

[rondds]

frank, if i were a hard a$$, that'd cost ya' a green ball! :smt018:grin:

 

[fc3]

Green balls? Whatever. This is like talking to my wife. All of a sudden she gets mad and I'm wondering, "Now what the heck did I say?" Not to say you guys are like a bunch of women. No, not at all.

I just posted Mercruiser's spark plug recommendations from the old shop manual that I have. I bought it around 1987 when bought my previous boat. So the recommendations should be pretty darned close to what edoc needs for his 1988.

Best regards,

 

[osd9]

My 260HP 5.7 in my 1983 225SXL used AC MR43T spark plugs as did my 330HP carbed 7.4s in my 1993 330DA as did my 310HP MPI 7.4s in my 1998 330DA. I think I actually still have a set laying around in my garage. Don't know what the 370HP 8.1s in 2002 380DA used.

 

[rondds]

ok, now i'm thoroughly confused. using the champion website cross reference, the BR6FS produces an RV9YC, the MR43T produces the RV8C and the autolite 25 produces the RV17YC. on the autolite site, the MR43T and the RV8C produces the 145. point being i dont use ANY of these plugs. am i totally screwed up? the boat starts and runs flawlessly with the autolite 25s, and the plugs i took out from the boat, which was always yard maintained by the previous owner, were RV15YC4s.

it's now raining in holmdel.

 

[fc3]

OK, Quick lesson in spark plugs. There's more to them that just sparks. First of all there are the mechanical considerations. The hole diameter, threads, and seal type. These are pretty simple, if the plug fits, it's OK. Seal types are either gasket or flare. GM engines use flare seals. Then there are the other mechanical considerations. Chief among these are reach. The plug has to reach far enough though the head into the combustion chamber so it can light the fire. It can't reach too far or the piston will hit it. That's a bad thing.

The electrical considerations are pretty simple. Resistor or non-resistor.

The most complex is heat range. The ceramic conducts heat from the electrodes to the plug body. Different ceramic sizes allow the tip to run hotter or cooler depending on how much of the ceramic is in contact with the plug's body.

So if the plug fits and it is working OK, then you just need to worry that it is the right heat range. If the plugs foul, are coated with black gunk, and have deposits on the tip and ceramic, then the heat range is too cold because the plug is not getting hot enough to burn off the deposits. On the other hand, if the tip looks burned or melted, and the ceramic looks blistered, then the plug is too hot. The heat range is right if the electrodes look OK and the ceramic has an even light tan color.

Best regards,
Frank

 

[rondds]

pulled the hatch today to check fluids and noticed, right on top of the engine, there it was, SPARK PLUG MR43T, and GAP 0.035". that about ends all discussion on this matter. thanks frank!

 

[electricaldoctor]

Thanx for all of your good advice guys. I will pick up some MR43T plugs. If the periodic miss is still there, then I will move on to wires, rotor, cap, etc. I guess that I will then need advice on what to use in that case as well.

~ Ken