Hey Guys, So I pulled the motor on my Scarab Concept (496 chevy bb) Was in the middle of a cyl head swap and noitced scuffing on the cyl walls. Better safe the sorry. Had 2 slight over heating problems in the spring and have been running the boat all season.
Temp went to 230ish both times and I shut down ASAP both times.
Running times were only about 4 to 5 min. I am thinking that could have caused the rings to scuff the wall's as the pistons are still in good shape. Any ideas?

Post some pictures of the pistons and cylinders if you can.

overheating means the oil might have got too thin....hours?

cant do the pics now. block at the machine shop. They weren't to bad and mostly on the thrust side. The oil was dark because of blow by but only have 5 to 6 hrs on fresh oil. Motor has maybe 40 hrs. I'm leaning (hoping) towards the oil side also. With timing and fuel out of the equation Im not sure where or what else to think. Going to measure the the bores tomorrow and make sure good to hone and with any luck back in the water by Friday.

Just left the machine shop and I was told poss crack in crank on #2 main? The bearing wasen't hammered or discolored nor were the rod bearings. I'm starting to worry about the shop thats doing the work. Motor ran fine and never had a miss. I'm thinking in order to damage a crank I would have had to do alot more damage elsewhere also?

Dark oil after 5 or 6 hours and scuffing I would have to venture a gues you're running to rich and fuel is washing the cylinder walls. How much power are you running and what crank do you have? A lot of reasons to crack a crank, we need more info. If the scuffing is as light as you say a good glaze breaker hone and light oil will take care of that. Where are you located, there might be a good shop near you to go to.

Making around 375 to 400 and its a scatt crank and was balanced, flywheel and all. Carb is a 850 mech second jetted down 74 frt 76 rear. Plugs look great med to drk tan. Boat ran 58 to 60 all day long with 24/14 four blade at a 4650rpm. There are alot of other weaker parts that should have gotten beaten up before a crank gets damaged. It makes no sense to me. I am in Chesapeake City, Md. Any help would be great.

Crank is toast! Going to a gen 5 block 4 bolt and a new scat steel crank, new rods (old gen 4 wont work with new block) and should be good. Here's hoping this all works out!

nonstop,why do you believe mark 4 rods wont fit in a gen 5 block,they will,also,the scat crank is a low buck peice,but good for your hp,im wondering if you had some detonation that contributed to the crack on the journal and helped scuff the cylinder wall,when you overheat the chance of detonation increases a lot,even if only run hot for 5 minutes,if running hard,this probibally caused your damage.

On a fresh engine, I would venture to say the scuff was from not enough piston to wall clearance. Marine engines need a couple extra thousanths to deal with hot pistons and cold coolant temps.
It usually takes a good bit more water temp than 230 to scuff a piston from overheating. Just my 2 cents....