I am interested in buying a boat but the motors have been winterized for at least 6 years. Should I assume that the motors need to be replaced or is there a chance they are ok with rebuild/fresh? The motors are Mercruiser 420s. Thank you in advance for the input!

really depends on how good they were winterized...They will need some attention. pull the plugs and look at the cylinders with a scope, any corrosion?? does it spin freely?? change the oil, pull the distributor and prime the oil system, maybe check the valve springs, Provided they were in decent shape and winterized/stored correctly, they shouldnt need too much.

definitely scope the bores and make sure they still have lubrication. If you go through the steps to make sure the engine isn't frozen (hand rolling over the engine) and do proceed with the purchase. Flush the fuel system completely (count on carb rebuilds) and pull the distributors and prime the oil pumps. If the engines will fire, call it good for then and put new springs and seals in the heads - 6 years sitting under compression will not do anything for the valve train even if they were done prior to being put to bed.

Good Luck, they were a popular engine that was straight forward to work on.

I bought a twin boat years ago that sat outside shrunk wrapped for probably 7 or 8 years. I bought with the intent of new power however I pulled the distributors and primed both, changed fuel filters, added some fresh fuel and both started up effortlessly and ran fine. Changed engine oil and beat them badly for a couple years. 454/330’s, Been down the same road different times especially old cars etc that sat.

As as long as the blocks aren’t cracked run em.

This pretty much says it all. Odds are good that you can dewinterize them. Make sure they'll turn by hand. Add new fuel and fire them up.

When I bought my cruiser it had sat for 5 yrs shrink wrapped and winterized in Michigan. On the way to inspect the boat I bought a battery and a can of starting fluid. I asked the salesman if we could roll the engines over for a second. He said he didn't have the keys. I told him I'll just hot wire if for a second. I pulled the ignitions, hit them with some starting fluid, touched the wires together and they fired. I bought the boat and it ran flawlessly for 6 yrs.

Some would probably want to pump or remove old fuel but I didn't. Wasn't much in tanks to drain. I just kept a couple spare fuel filters in the boat. I wouldn't load it much until your comfortable most all the old fuel out.

I’m NOT a mechanic. I can’t comment on 6 years but I had a little experience with our boat that couldn’t be used for 2 seasons. It was winterized cylinders fogged etc. when it came time to start it up again I pulled the plugs and put a couple of ounces of oil in each cylinder. Put the plugs back in. Then I disconnected the coil wire. I attached the water hose to the outdrive and cranked the engine over. Oh before I forgot the fuel tank was left empty during storage. I put in about 20 gallons of fresh fuel. I reattached the coil wire. Cranked it and in short order it started. Oil pressure was good smoked like heck for a few minutes. Ran pretty good. I changed the fuel
filters and had a new impeller out in. Never had any issues

3pointstar

Make sure you prelude the motors if the cams are dry they won't last long and you'll be doing motors , like everyone has chimed in , fresh fuel, fresh oil ( with zink additive), prime the oil pump, a splash of oil in the bore , roll it over by hand a couple of times, spin it over without plugs , check the oil pressure, new plugs , fire it off , check for leaks. If your curious cut the oil filter open . Then beat them like they owe you money

I would agree with most of what has been said, hopefully they were fogged well and stored somewhere decent and moisture shouldn't be an issue. Plan on draining fuel, replacing filters and likely going through the carbs. I would not worry about the valve train. Sitting compressed is not as bad for the springs as you think, what really caused fatigue is cycling and heat. Motors have sat much longer than 6 years and ran fine!