Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

I bought a 1988 wellcraft 190 clasic last year, 150 hrs, boat is mint. I went to put new plugs in it, pulled them out and seen that they were AC MR44t, the maintenance manual calls for AC R42TS. The question i have is why did the owner place AC MR44T in lieu of AC R42TS? The boat did run very well, with no cold start issues with the MR44T. I understand MR is marine rating, is this the replacement plug for AC R42ts? Thank you.

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

the MRxx TS , the xx is the heat range number of a spark plug, TS referrs to "tapered seat", the M is supposed to designate Marine use. More corrosion resistant. Here`s a list of everything http://www.angelfire.com/extreme4/skidoolympique/SPARK_PLUG_CROSS_REFERENCE.htm

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

The AC MR44T has a tapered seat, can I use the MR44t in lieu of the R42ts? And why do u think he used the MR44t? Also the MR44t appears to have a shorter electrode than the AC R42ts. Also is the MR44t a hotter plug than the R42ts?

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

The MR42TS is the correct plug. Why the previous owner changed them is an unknown no one but that person can answer. Could be the 44 was as close as he could find that day.

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

Don S said:

The MR42TS is the correct plug. Why the previous owner changed them is an unknown no one but that person can answer. Could be the 44 was as close as he could find that day.

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I called the previous owner, he told me that parma marine sold him the AC MR44T in lieu of the AC R42TS, they told him that is the replacement plug. Can you comfirm that is the true replacement plug?
Also there is no AC MR42TS they do not make them or they are obsolete, only AC R42TS are available.

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

he told me that parma marine sold him the AC MR44T in lieu of the AC R42TS, they told him that is the replacement plug.

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They were wrong. Are they a certified Merc dealer?

Can you comfirm that is the true replacement plug?

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No, I can't confirm it, I posted the correct plug according to the Merc parts program.

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

They are a certified MERC dealer, there is no MR42TS, only AC R42TS. Maybe I will call ivansic marine and double check with them. The boat ran very well with no cold start problems at all with the AC MR44T in it, but i want the correct marine plug. The AC R42TS is a little longer (electrode) and I did not want to install it and hit a piston. Thank you for your input.

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

Then use the R42TS, same plug, without the shiny coating on the metal part of the plug.
It's what the manual calls for, why would it hit the piston ??????

Re: Correct spark plug for my 1988 mercrusier 3.7l 165 horse?

Thank you for the info, I over think things at times. I am not a boat repair person but could not understand why they would run a different plug. Sometimes there are upgrades or changes with merc, example (alternator conversion kit to replace stater, newer style heat exchanger to replace old one, exhaust bellows to tube, etc.) I thought they maybe went to a different plug because of idle issues. I am 38 years old and like to keep my boats running and looking the best they can, i do not abuse my boat i treat it like my son. This is a mint condition boat, i wish it had a v6 or another motor in it other than the 3.7l (470). I know the issues of this motor (cam seals leaking in time, stator problems, heat exchanger), but like all thing u need to maintain them. Thank you