Re: Late '60s 100HP Mercury with knock Retire or Fix
Hi Jimmy,<br /> Is it worth sinking a lot of money into it? That depends. Compared to the cost of a new motor, you could rebuild your motor a couple of times over and have money left over! Will it be a good motor when you're done? Probably.<br /><br /> A knock could be any number of things, including a lower unit going bad. If a gear loses a chunk and that chunk gets pinched between the gears, it can push the driveshaft up into the crankshaft, resulting in a knock that sounds an awful lot like it's coming from the powerhead.<br /><br /> The knock is more likely a bad bearing on one of the connecting rods. If you're lucky, it'll be on the piston end, and you'll only have to come up with a new piston, connecting rod and bearings. However, it's probably on the crankshaft end, in which case there's a good chance it has taken the crankshaft with it. If the crankshaft is shot, you're entering a gray area as to how much time and money you want to put into it. If you're a decent wrench and willing to learn, then it could be a worthwhile project to go through it. If you have to pay someone else to work on your motors, it probably won't be worth it. An easy check to run would be to place a long thin screwdriver blade into the spark plug whole and check for excessive play on each piston. Find top-dead center with each piston and look for one that doesn't move initially when you start turning the flywheel. This will localize the cylinder with a bad bearing.<br /><br /> A third possibility would be a broken piston with a piece rattling around in the cylinder bore. A good first check would be to pull all the spark plugs and look for evidence of aluminum build-up. Also, run a compression check. <br /><br /> By the way, you don't have to remove the cylinder "head". It's only a water jacket cover and doesn't give you access to the cylinder bores...<br />- Scott
