[99 Mariah 258]

Anyone know the correct plug gap for a 99 mercruiser 7.4 mpi. Thanks. Can't find anything in the forums and google search is all over the place...Thanks

[Hudson River Steve]

According to the service manual it is 0.045".

[ShabahZ280]

And just an FYI, expect to change the plugs about once a season, or two at max, if you want to keep the engine running optimally. Also good to change out the rotor button and distributor cap at the same time, if you use the boat a lot. For about $50 it's worth it.

[99 Mariah 258]

Thanks guys. I did a tune up a couple months ago ...new quicksilver cap, rotor, wires, plugs and even a coil..but the wires were some off brand and plugs were ngk's so just to be safe I bought quicksilver wires and plugs and installed them. I noticed a slight stumble at idle that I never felt before and knew it had to be the plug's. Sure enough they came gapped to .030 so I pulled them all gapped to .045 and she's all better now...also got down in there and sprayed some rust converter and re sprayed the motor.