JKG8R

Hello everyone. This is my first post on here because the abundance of nice boats and wealth of knowledge intimdates me. : )

First some quick background: Last year I bought my first boat, a 1990 Checkmate Convincor with a 518 NA motor (Eagle 4.5" crank, H-beam rods, BRC pistons). The boat ran great on several day trips and just playing around over the winter and spring. A couple of weeks ago we were running down the St. Johns River and I thought I could hear it start pinging so I backed off the throttle for a couple of minutes, checked all the guages (water temp was low 100's with a crossover system - no tstat, oil temp was ~220) and didn't hear it anymore so I was getting back into it and at around 3,500 RPM it went BANG and shut off cold. Tried to crank it to see if I could tell what it was and make sure it didn't lock up and sounded like a mechanical failure so I conceded defeat and got someone to pull me to the nearest ramp and got her home. I should mention that when I lifted the engine hatch the intake was so hot I couldn't put my hand on it so my water temp gauge doesn't work, the water pump may have failed plus any number of other things.

I got home and went to do a compression check and had water in #6 and #8 cylinders and the plugs were burned up so I pulled the engine out and tore it down. I have / had 088 heads and the starboard head was cracked and will now make a fine paperweight. From looking at the head gaskets it doesn't appear the gaskets failed but I can't tell for sure.

The cylinder bores are all good with no ridge at the top, all the bearings look brand new, the pistons look okay with the exception of #6 which took a beating from a piece of the plug which also scored that cylinder wall. I took the block to the machine shop this morning to get two of the suspect cylinders sleeved and bored and I'll have others honed so I can throw some new rings in it.
Finally to my question: The #6 piston measures small across the skirts so it appears it "mushroomed" a little when it was hot and hammering the end of the plug. The guy at the machine shop said he's seen a lot of guys with forged pistons push the skirts back out to size and they run fine but that's not something they do there. The grooves are all in good shape and the top of the piston cleaned up so if I can get it to size I could save the $250 a single piston will cost. Has anyone ever heard of doing this and maybe have some advice? Worst case I screw it up and buy a new piston but I'd like to give it a shot if I can save some $$$. I am definitly a boater on a budget and my budget is almost gone becuase of the cylinder head. I'm getting a set of oval ports in my garage ready to go until I can afford to get another 088 or some aluminum ones down the road.

I'm going to drill and tap the rear water ports in the intake manifold and put a dump out the side to make sure I have water flowing in the back of the block, put a new impellar in my water pump, fatten up my carburetor to make sure it wasn't running lean and figure out why the water temp gauge didn't read properly. Any advice on the piston or how to make sure it doesn't happen again is greatly appreciated.

Turbojack

Buy the piston. Will be way cheaper in the end. Heard the story pay me now or pay me more later.

So the water got in the cylinders via the head? I would have the rods checked out on cylinders that had water. Most times when there is detonation it is #5 and or #7.

MILD THUNDER

iTrader: (3)

I agree. Replace the piston. Sounds like all your issues were pretty much from the fact the engine got hot. From pinging, to hurting the valves, to cracking the head.

JKG8R

Thanks guys. I'll call BRC and get them to make me a new one. I checked the rods before I tore it apart and they are all straight so despite everything I think I got off pretty easy with this one. I don't know exactly how / when the water got into the cylinders. If it did while running I would think the rods would have bent. It wasn't a spoonful, it had quite a bit in it. It has cast aluminum jacketed exhaust (Gil or Hardin?) and when I saw water come out the plugs I pulled the risers and it had water sitting in the exhaust. As I said, the head gaskets looked okay to me so I'm not sure how the water got in.

articfriends

iTrader: (1)

You need to positively figure out where the water came from BEFORE you run it again other wise the same thing may happen and your budget will REALLY be shot!!

MILD THUNDER

iTrader: (3)

Didnt you say the head cracked? Id prob look at that as a source of water intrusion?

rer233

had a similar prob on my n/a 454 after installing a stainless marine preheat crossover system- stuck and bent an exhaust valve (#8 i think) at idle. engine temp never got hot and i already had the back of the intake overboard dumps. engine builder said merc uses a circulating pump to prevent localized overheating 'cause you NEED one, but me being the hard-headed SOB i am figured there had to be a better way... long story short, i drilled and tapped the back of the new cyl heads (one of the old ones was cracked) for temp sensors, installed a second temp gauge at the helm, wired the sensors to a toggle switch on the dash, and now i can check the temp at the back of the heads instead of just the front.
also, just to be sure i didn't repeat the problem, i nstalled thin brass restrictors in the cooling passages in the front part of the head gaskets to force more water toward the back of the engine. ni had the engine builder loosen up the exhaust valve stem clearance and i don't run exhaust valve stem seals (oil's a lot cheaper than engine parts and a DAMN sight easier to install!)

JKG8R

Just out of curiosity, how do your temps at the back of the heads compare to the front? Mine had a crossover setup on it when I bought it and the way the power steering pump is mounted I would need a new pump (mine is a crank driven pump) and brackets to add a circ pump among other things. I'm going to run what I got if I can. Thanks again for the responses.

rer233

Actually, i never went back to the circ. pump- temps run 140 front, 120 rear. I've run the current setup for 8-9 years with no problems. Would also recommend running water and fuel pressure gauges if you're not already.

FIXX

iTrader: (2)

make sure you pressure test your manifolds,,sounds like you may have a hole in one of them..