What's going on with my spark plugs

[Cheap Guy]

I have a 1989 Sundancer, with the 260 and Alpha 1 Gen 1. Last year my drive locked up and apparently shot the porcelain from the rear most starboard cylinder out of the seat of the plug. The threads were still good. Normally I do my own work but due to health issues I had a mechanic replace the stern drive and the plugs but the engine still wasn't running right. Fast forward to today. I had the stern drive replaced by another mechanic and I checked out the plugs and rotors myself. The mechanic used Champion RV91MC. I replaced them all with AC Delco MR43T. Two of the Champion plugs had smashed electrodes as shown in the picture. After replacing the plugs, the engine is back to running smooth like it used to, as would be expected. My question is what could have caused the electrodes to be smashed in like shown? One one on the starboard side as stated, the other was on the port side, second from front.

 

[Bill Curtis]

Different plugs with slightly different dimensions. Those plugs were siting too deep and got kissed by pistons. Personally, I don't use Champs for a few various reasons. Always used AC's and Autolites.

What does the manual spec for replacement plugs.

Champion RV91MC Reach: 11mm (.4375") Note: These also show/cross on some sites as same plug as a CCH942M, which has a reach of .442".
AC Delco MR43T Reach: 10.95mm (.4300") Insulator height: 25mm (.98")

If you have a borescope, look inside the plug hole and see if there is anything floating around inside the cylinder / sitting on the piston head. If insulators broke off, may still be in there.

 

[Cheap Guy]

Different plugs with slightly different dimensions. Those plugs were siting too deep and got kissed by pistons. Personally, I don't use Champs for a few various reasons. Always used AC's and Autolites.

What does the manual spec for replacement plugs.

Champion RV91MC Reach: 11mm (.4375") Note: These also show/cross on some sites as same plug as a CCH942M, which has a reach of .442".
AC Delco MR43T Reach: 10.95mm (.4300") Insulator height: 25mm (.98")

If you have a borescope, look inside the plug hole and see if there is anything floating around inside the cylinder / sitting on the piston head. If insulators broke off, may still be in there.

Thanks! They looked a little long. I have to change the exhaust manifold so I’ll scope it when I do that. Looks like I’ll just let it sit for now so I won’t risk any damage. This is one more lesson to do things for yourself!

 

[Cheap Guy]

I spoke with a local mechanic and he said he actually prefers Champion plugs over AC Delco, but part number RV15YC4. I have to do the manifold so I may switch them then.

 

[techmitch]

I spoke with a local mechanic and he said he actually prefers Champion plugs over AC Delco, but part number RV15YC4. I have to do the manifold so I may switch them then.

Champion plugs are for Chryslers and lawn mowers.

 

[eweise]

AC-Delco plugs for all GM engines.

 

[scoflaw]

NGK are good as well

 

[Cheap Guy]

Lol, don’t make me ask a “best oil” question.

 

[Bill Curtis]

Lol, don’t make me ask a “best oil” question.

Please do not go there.

 

[SCORPIO]

Those Champions were too long a reach.
I think the Champ equivalent for the MR43T is an RV15YC4.

 

[Cheap Guy]

Just a quick follow up. So far so good with the plugs. I was getting a vibration at 3,000 rpm and the engine seemed to balk at going over 3500 rpm WOT with approx 22 mph, but I switched out the four blade 15x16 prop for a three blade 15.25x15 prop and it’s running much smoother - 30 mph at 4200 rpm. The hole shot obviously isn’t as good with the 3 blade but I’ll take the smoothness as the trade off.