Mercury 402 with small hole burned in cylinder

I just bought a boat that came with a free 1978 Mercury 402 40hp 2-cylinder 2-stroke motor. (Or at least that's what I'm telling myself.) Anyway, I got a manual, played around with it a bit, and found that there was zero compression on the top cylinder.<br /><br />I removed the cylinder cover and found that on the top cylinder, next to the hole for the spark plug, there's a tiny 2 or 3 millimeter hole burned through to the inside. On the cylinder cover, there's a little piece of protruding metal that's burned from fire escaping this hole.<br /><br />If I put my fingers over the small hole and the sparkplug hole and turn the flywheel, I can feel compression in the cylinder.<br /><br />To me, it looks like this thing burned through intentionally to prevent something worse happening. If that's the case, what was it preventing, and is it repairable?<br /><br />Also, who votes I drill the hole out, thread it, put a screw in, and forget about it? (If anyone thinks that's going to make the engine explode in a hail of shrapnel, please let me know, because I'm seriously going to consider this if the only alternative is to get a whole new block.)<br /><br />

Re: Mercury 402 with small hole burned in cylinder

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Re: Mercury 402 with small hole burned in cylinder

I can't see the cylinder walls, but I can tell you it moves freely and appears to keep compression in that cylinder with my fingers over the holes. Of course I can't gauge it accurately without plugging the small hole, so who knows? From what I've read (and I haven't verified this yet either, BTW) the cylinder dome is supposed to be made of cast iron, but the cylinder cover has some little metal droplets on it from where the exhaust from the hole hit it. Cast iron melts at about 2100 degrees Farenheit, so I doubt those droplets are part of the cylinder dome, as it would have to be at the top end of yellow hot to melt.<br /><br />Anyway, here's a crazy theory: The people who made this motor put a plug in this hole with a relatively low melting point compared to cast iron -- say 1000 to 1500 degrees. A few years ago, the fellow who sold me the boat fired up the motor and left it on without a coolant water hookup while he went to smoke some crack, and when the cylinder dome heated up, the plug did what it was supposed to do. It melted, blew out, and stopped the motor before the fragile cylinder walls were marred. Evidence for this theory would include the fact that the burn mark and beads of metal are there -- if the water jacket was full of water when this happened, the water would have impeded their trajectory and there would be no mark.<br /><br />Dum dum dum....

Re: Mercury 402 with small hole burned in cylinder

The hole was caused by a water pump that lost prime at high speed and when pulled back to idle and the hull settled in the water reprimed, as the top of the cyl was very hot it formed a steam dome and melted the hole, its the nature of a 402. Never tryed to fix one but we have a welder in our area that welds the tops of snowmoble pistions and they live so I think it might be worth going to a good welding shop in your area--Bob

Re: Mercury 402 with small hole burned in cylinder

I'm pretty sure that motor has a porthole for checking on the pistons. If so it would be on the starboard side of the engine halfway down the cylinder. I'd take a look at the pistons before going any further.<br />Eric

Re: Mercury 402 with small hole burned in cylinder

It appears there is very small crack running from about the 3 o'clock position on the burn hole and extending toward the spark plug hole. Maybe its just the picture, but it sure looks like a crack.

Re: Mercury 402 with small hole burned in cylinder

I saw that too, but I cleaned up the hole a little bit today and that triangle jutting to the 3:00 position is just a fleck of corrosion that came off the lip of the hole.

Re: Mercury 402 with small hole burned in cylinder

But there still appears to be a faint line extending from the point of the white fleck much farther to the right toward the spark plug hole. Again, maybe just the picture but it sure looks like a crack.

Re: Mercury 402 with small hole burned in cylinder

The problem with fixing it with a plug is that it's bound to create a hot spot which can eventually cause pre-ignition, etc. I would replace the block. It's not that hard in the grand scheme. Lots of those motors out there to act as donors. Most marinas would probably give them away to get rid of them...<br /><br />FYI - The cylinder dome is cast aluminum - not iron. <br /><br />- Scott

Re: Mercury 402 with small hole burned in cylinder

Just like the small-inch 3-cyl 650's and 700's -- when they overheat the block cracks and the engine's toast. <br /><br />Seen weld repairs attempted on these and they don't ever seem to last due to the alum alloy used. Best to tear down and see if you've got anything inside that's salvageable before spending a lot of $$$ on it. eBay would be a likely source of parts for this motor.<br /><br />HTH........ed

Re: Mercury 402 with small hole burned in cylinder

ecorliss... did you have any luck fixing the hole in the block of your engine? i too have the same problem.sold the boat yesterday and the guy phoned me today to tell me so i gave him his money back, but i would still like to keep it and fix it up for the family.thanks

Re: Mercury 402 with small hole burned in cylinder

Toss the block as previously posted. Years ago a buddy of mine had an old 650-3 with a hole in the block like that, it was caused by water pump failure and instant overheating. Those 3's are just about guaranteed to 'hole' the block if they lose the water pump at speed.

He had a welder (who was probably the best welder in the entire area) weld up the block and it worked just fine for one season. Unfortunately, when the weld finally broke there was plenty of water pressure and lots of water was pumped into the engine's innards, ruining the rest of it!!!

The welder had said that the alloy on the block was some pretty strange stuff, real hard to weld. Probably quite durable but once broken might as well forget it.

You'd be far better off expending your efforts on something like a 4-cyl 50HP Merc. The 40 HP twins were never well-thought-of.

But if you're determined to fix it, best to disassemble and evaluate the internals. If good, I expect you'll find plenty of parts from dead 40's on eBay to build another!

HTH & G'luck...........ed