Rust in #7 cylinder exhaust port

David Kane · Post by **David Kane** » Fri Aug 15, 2008 1:40 pm

Distinguished forum members, your thoughts, please. In tearing down my starboard engine, a Mercruiser 255 (GM 350), there was water & rust in the bottom of the exhaust port for my #7 cylinder. The other cylinders in the bank, 1-3-5, are completely dry & no rust. Do you think I have a problem to the point of replacing parts/pieces, & what parts/pieces do you suggest if so? Or, as several local boaters & the mechanic at the marine think, it's just some water that got backwashed in. Nothing to worry about.

The reason for tearing down the engine is that it hasn't run right all season long (my 1st with this boat). A long litany of problems, both human error (marina mechanic) and just worn out too depressing to list. After fixing all the rest, I did a compression check & found zero compression on #1 cylinder, and a little low compression on #3 cylinder. All other cylinders on the starboard & port engines OK. With help from a non-marina mechanic, we took off the one head (1-3-5-7, or outboard side of the starboard engine) & found a piece broke off the #1 cylinder exaust valve. Looks like some blow out between #3 & #5. I'm thankful the problems seem confined to the head area & the compression rings are probably OK. We'll pull off the other head tonight. along with the transmission (bad bearing or bushing?; won't know until it gets torn apart), so I'm dockqueening at the moment. For 33 year old engines, they are in remarkably good shape. Only ~400 hrs on them, so they weren't used much. Hardly any wear on the cylinder walls, telling me the engines were well cared for overall.

Stripermann2 · Post by **Stripermann2** » Fri Aug 15, 2008 4:25 pm

Good luck with the repair.

Lack of use and low hours are probably what helped the damage along.

The valve stems were probably rusted into the guides at one time or another due to lack of lubrication of oil and the condensation from over the years of low operating temperatures. this may have been a performance issue with you as valves would stick. Possible stuck or damaged rings though...maybe you get lucky.

You hopefully are limited to the cylinder repair, (O/H) new head gaskets, fresh parts and away you go...

Let us know how you make out.

LandVF36 · Post by **LandVF36** » Sat Aug 16, 2008 2:58 am

Check your exhaust manifolds. Last boat I had was powered by a pair of 283s with the log style exhaust manifolds. They were natorious for getting fracture cracks between the exhaust side and the water jacket. I had a problem with the #7 where once the engine was up to temp, a crack would open up, providing a mist on the exhaust valves.

To check it out, you need to put a blank plate on the exhaust side and presurize with a garden hose. Then heat each exhaust port with a torch. When I did this, #7 started to squirt.

If you don't want to test just get net exhaust manifolds. Its cheap insurance against a future rebuild.

Mitch

David Kane · Post by **David Kane** » Tue Aug 19, 2008 10:29 am

I'm going ahead with replacing the exhaust manifolds and risers, and also the spacer plates if available. I feel it is a good proactive measure to take. Will sleep better not having to worry about the issue. The tranny didn't get pulled out last week-thge guy helping me promised me he'd be there tonight. Will drain & replace the oil & filters on both engines.

If all goes well, I should be back up & running next week. I want to rework the heads and the exhaust on the port engine (minus that tranny as it is working fine) over the winter so both engines are brought up to the same level. Hope that will give me years of worry-free cruising pleasure!

I'll keep you posted on how the rebuild goes. Thanks for the advise!

David Kane · Post by **David Kane** » Tue Aug 19, 2008 10:29 am

I'm going ahead with replacing the exhaust manifolds and risers, and also the spacer plates if available. I feel it is a good proactive measure to take. Will sleep better not having to worry about the issue. The tranny didn't get pulled out last week-thge guy helping me promised me he'd be there tonight. Will drain & replace the oil & filters on both engines.

If all goes well, I should be back up & running next week. I want to rework the heads and the exhaust on the port engine (minus that tranny as it is working fine) over the winter so both engines are brought up to the same level. Hope that will give me years of worry-free cruising pleasure!

I'll keep you posted on how the rebuild goes. Thanks for the advise!

David Kane · Post by **David Kane** » Thu Aug 21, 2008 8:17 am

I should have all the parts/pieces in today. In talking around about the transmission, a certified mechanic thinks it is OK. I only hear the clunking in gear at idle-it goes away with speed. He's saying it is gear lash due to the rough running engine. The machine shop says my old heads were too rotted out to rework, so I am getting a new set. Hopefully (prayerfully?), the mechanic heling me will show up tonight-he's bailed in the past so I had a talk with him last night about the need to get this done. Although I can run around a bit on one engine, I ran into a problem last Saturday that I sure don't want to repeat. Took the boat over to the gas dock for a pump out. As we live aboard all week long, we need to pump out usually 1/week. My boat is berthed in a channel formed by 2 piers, with boats paralled parked on either side like cars parked along a street. I back down the channel. If I get up enough speed, I can use my rudders for steerage. The problem last Saturday is that the wind, although blowing light, was working against me and I could not get up enough speed to get work out of the rudders without ramming into another boat. So here I was, scalloping along down the channel, guys hopping on boats to push me away. When I finally got to the cross channel, I could get up enough speed without hitting anything to gain control with the rudders.

David Kane · Post by **David Kane** » Fri Aug 22, 2008 9:18 am

The mechanic showed up at 6pm last night and we worked 'till midnight. New heads look great. Beefier than the old ones and the new rocker arms are shaped different than the old ones to prevent sideways movement which can cause the push rods to elongate the holes. The engine fired right up and very little tuning was required. The noise I heard in the transmission went away like another mechanic said it would-the noise was caused by gear lash from the engine running rough. A problem I had with the old heads was that the rpm's would drop off to almost zero between idle out of gear and idle in gear. Now, the rpms drop only slightly. Can't wait to take her out on sea trials!

I put the old exhausts back on. I'm short 2 parts for the new exhaust manifolds/risers-the caps for the manifolds on the end under the risers, and the spacer blocks between the manifolds and the risers (the blocks with 2 hose fittings on them. Will get with the parts supplier today & order, if availalbe.