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2002 350 Black Scorpion Top-End Overhaul

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Did this last summer... thought I'd share my experiences with the community.

Been a long-time lurker and just recently became active on the forums. Summer of 2012 I purchased a 2002 RLX from my father that he had purchased earlier that year as a project boat. It had started life as a Roc Aqua Jays boat before being sold to the abusive 2nd owner. After countless hours of scrubbing, polishing, glass repair, and gauge repair I took her out on the water. As you can probably guess from the title the engine was not running well and misfiring badly. We tried fresh gas and resetting the timing to no avail so off to the dealer she went. They diagnosed it as a burnt head gasket with compression in the two center cylinders on the left bank at 30 psi of compression. The corner cylinders were in the mid 180's and the opposite bank center cylinders were in the mid 140's. The repair estimate was near as can be 5,000 dollars!!! Needless to say this wasn't in the budget so I dragged her home and order the Merc manuals and got started.

1.)The motor before we started the work.


2.)Starting to take her apart.


3.) Jury-Rigged timing marks. We had trouble setting the time after re-assembly due to Merc sending the wrong manuals. All the torque specs on the engine were the same but I had a different distributor.


4.) Engine post tear-down. You can see the cylinder that was causing the detonation or mis-fire. The piston-top is darkest. No damage to the piston luckily!! Bottom end was in excellent condition with 927 hrs on the short-block at the time of the rebuild.


5.) The head with the low compression was cupped by .007 inches and the opposite side was cupped by .005 inches. This was due to overheating which was supported by the chunks of impeller found in the transmission cooler. Took the head to Mick's High Performance machine shop in Racine, WI to have both heads decked by .007 inches and have some head work done. I was able to re-use all but 2 of the valves. the 2x exhaust valves on the 30-psi cylinders were badly pitted.


6.) The bad head gasket. As you can see she was burned all the way through and the opposite side gasket (visible on the paper pad in the back-ground) exhibited bad scorch marks but was not burned completely through.


7.) Exhaust manifold cleaned up, magna-fluxed, pressure checked, and painted. The exhaust manifolds, exhaust elbows, and intake manifold were given this this treatment. Special attention was paid to the exhaust components due to the pitting on the exhaust valves. All I can guess is that water was backing up the exhaust during the boat's time as a show-ski boat as the manifolds were A-OK.


I have put nearly 60 hours on the boat since the rebuild in August of 2012. No problems thus far and and all compressions are in the hign 190s-low 200s. Total investment with parts and machining was approx $1,100 USD. With the head work that was done and the bump in compression Jim @ Merc Racing in FonDuLac, WI estimates power gain of 10-15 hp. The boat now runs 50 to 51 mph with an Acme 449. His only recommendation was to run 91+ octane instead of the 87 the engine was rated for as I am running the stock programming on the ECM.

Edited by BoatFlyRide
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Adjacent exhaust ports! Both exhaust valves close together create additional heat and stress on the head gasket. I would suggest a head bolt retorque if you have not done it since the rebuild, cheap insurance on new gasket life.

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Start by replacing your impeller. :Tease3:

damn you Boz! Give me a break haha! I never said I didn't have A LOT of learning to do! :blush:

Don't worry, I started with the impeller, thousands of dollars ago.

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