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Merc 30 Blown Head Gasket

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mjennings

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"Hey guys, thanks for your hel

"Hey guys, thanks for your help in advance. My question is do you think a wrong propeller size 14X21 on an '87 17 ft. Sunbird with a 3.0 merc. and alpha 1 could blow the head gasket? I have not been able to get my rpm's past 3800 with a speed of about 30. Also, while out 2 weeks ago I was idleing in shallow water and hit a rock causing the motor to stop and bend one blade on prop. I wouldn't think this would be the cause. I think wot rpm's should be between 4000 and 4500. The temp. has looked ok, don't think it overheated. No water in oil. Thanks again."
 
"21" sounds like a lot of

"21" sounds like a lot of pitch on any 3 L. Is this a new prop, or what you have always run? Not likely the cause of BHG.

Hitting a rock would not likely cause BHG either, but I would be more concerned about possible bent propshaft or stress cracking in the drive gears. How much power was she making at the time of the strike?

Rod"
 
"Thanks for the response, Rod.

"Thanks for the response, Rod. This is a new prop but there were no legible markings on old prop. I went by advice from different people but will replace it before running again. I believe I need a 14X17? The boat was idleing and really wasn't even moving when I hit the rock so I wouldn't think that would have caused any damage? I had also just re-torqued all head bolts to 90 ft. lbs."
 
Will your dealer let you try 2

Will your dealer let you try 2 o 3 props out before buying? Most will do that. 17" sounds like a good place to start. You want one that will allow the engine to hit max rpm range at WOT.

I agree; at idle you did not likely do any drive damage. If it was an aluminum prop they bend quite easily.

You can likely match up the pitch closely to the old prop by putting them side by side on a flat surface. You will be able to see how close the blade angles are to each other.

Rod
 
"how have you determined the h

"how have you determined the head gasket has blown? Have you removed the head?
Why were you torquing? Was the head recently installed?

neither event would cause just a head gasket to fail. The lugging can definitely ruin an engine though..cupping valves, melting pistons"
 
"When I got the boat in Januar

"When I got the boat in January I replaced all pistons due to the front two being burnt away down to the top piston ring and previous blown head gasket. It was overheating which I'm guessing caused the burnt pistons and head gasket. I've since replaced the thermostat and impeller. After rebuilding I've taken the boat out 8 or ten times. Then re-torqued as per manual. The last two times I took the boat out I could hear air escaping that sounded like a header gasket that was blown. I thought maybe a manifold gasket but after removing manifold I can see stainless steel and a gap between the head and block at the second from rear cylinder that looks like right in the middle of where the cylinder is. This boat is on it's last leg and I can't spend a lot on it due to my son starting college and too many other hobbies. But I must admit this is the type of boat for a first timer to get if you don't mind getting dirty, it's fairly easy to work on and all the input from this site really helps. Thanks again."
 
I don't think the original

I don't think the original problem was overheating...overheating generally causes pistons to seize on the lower part of the skirt. The top is usually fine after an overheat. I suspect your original melted piston problem might have been due to lugging or a timing issue or something... good you are correcting the prop pitch.
Sounds like the head is warped or the block's deck is not true now.
Just for kicks...I would suggest taking the plate off the back of your water circulating pump and make sure it's ok (circ pump ensures water flow around all cylinders.)
Tough call on spending money on these old boats.
Last summer I sold a complete carb to pan 2.5L 120 HP engine with manifold and all that maybe had 75 hours on it since new... and was well looked after.... I got a whopping $650 for it... I guess the buyer was still taking a chance but he lucked out... if you could find something like that locally....
 
"I'm starting to think tha

"I'm starting to think that re-torquing the head bolts was a mistake but was recommended in manual. After re-torquing, the first outing was when I hit the rock, started to lose power and started to hear air escaping. I don't think the rock incident caused any problems, just a coincidence. Do you think that possibly over torquing and wrong pitch on prop together causing lugging has caused blown gasket?
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"Retorquing would not cause th

"Retorquing would not cause that, and not likely the prop strike.
There is a mechanical issue.

Rod"
 
that's weird I've neve

that's weird I've never seen a head gasket do that... if the bolts held the torque during the retorque I agree no issue there. I suspect the head or block was not flat before reassembly. warped surfaces not clamping the gasket across it's entire area the way it should.
 
"Depending on the guy putting

"Depending on the guy putting this engine together, improper torque sequence could do this. Instead of doing the criss-cross thing, he may have started at one end of the head and went straight across to the other end. Or started in the middle and went around in a big circle(??). I too have never seen a headgasket do this. Was this a quality brand name of gasket?

Also, running that fuel mixture too lean will raise combustion temps way up a cause this to melt. What about spark plug heat range?

I agree that the 21 prop is too big and a good place to start would be at a 17. I have a 17 on my old 16 ft. ski boat with a 115 hp O.B., and it works great, but a 19 " is too much.

Wrench"
 
"Thanks to all for your respon

"Thanks to all for your responses, it's greatly appreciated. I put the head on after replacing pistons and rings. I checked cylinders for scoring but they looked ok. I'll have to admit that I can't and won't invest a lot on this boat only because it is old and was not taken very good care of before me. I torqued the head bolts according to Clymer manual both pattern [page 157] and torque specs [page 175] which say 90 ft. lbs. Also tightened each bolt about a half turn at a time. The manual also states to retorque after running motor 20 hrs. This seems to be when my problems started. After talking to my neighbor who is a drive train engineer and has rebuilt 4 cylinders before and watching high performance auto builders, 90 ft. lbs. sounds kind of high? I just made the gasket from an old tire inner tube! HAHA I don't care who you are, that's funny! Seriously though, I think it was a Quick Silver but I don't remember. I did have to replace plugs due to a carbon problem and used same ones recommended on valve cover. The motor ran pretty good after that. Also something worth mentioning, I haven't been able to get my RPM's, only about 3800, but thought it was due to too much pitch. I really am having fun working on this boat and reading all the different issues people are having with theirs. I almost hate to have to trade up because new boats sound like a lot of headaches. Well thanks again and hope to hear from someone that knows more than me, that shouldn't be hard!"
 
hmm... I don't see anythin

hmm... I don't see anything in your posts about checking the block deck and head with a straight edge. I suspect something was warped during the first failure.
 
"Well from what I have read yo

"Well from what I have read you have really done a lot of work on this motor? correct?

Replacing pistons???????????

Did you remove the ridge? Did you hone the cyclinders to get a nice cross hatch pattern for the rings to break in on?

Where did you get the pistons? Rings?

I am a bit confused!!

I think you need to start your topic over and START FROM THE BEGINING and tell us EVERYTHING you have done write down to the smallest detail!!

After that is done then ask questions!!

No one can answer a genuine question whith so many variables not known.

I am glad to see that you are enjoying working on this motor BUT there would appear to be a lack of experience here and just for the record, Go to the marina and ORDER the MERCURY service manual for that motor. There will be NO question as to whether a tourque spec or anything else is correct or not.

Just my opinion..........."
 
mjennings-
I am amazed to fin


mjennings-
I am amazed to find that you have the same exact problem i do. I thought my 3.0 mercruiser had a simple exhaust leak at the manifold. I purchased a new exhaust manifold gasket and after a little work was able to get it done. Now I am not a "real" mechanic but I am knowledgable enough to replace a simple gasket. After getting it done i started it back up and it sounded the same. What a waste of time. At idle I can feel an exhaust leak just under the exhaust manifold. It was kinda dark and late when I replaced it so I didn't do a full inspection of the area. I thought it was just the exhaust manifold gasket right. After talking to a couple mechanics that work on these engines I am going to do a compression test on each cylinder.
HERE'S THE FUNNY PART------
This all started right after I was idling by an unfamilier dock and the prop hit some big rocks-in reverse. I checked it all out and only found minor damage to the prop so I continued on. When I throttled up there was a loud hissing type of noise. Its strange how hitting a rock could have caused the head gasket to blow. After reading what you wrote I am almost possitive this is what happened. This may not be a coincidence. The boat was running fine the weekend before and I hit the rock just after launching. AHHHHHH!!
 
"Nothing like a submerged BFR

"Nothing like a submerged BFR to ruin your prop.
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Were the head exhaust and manifold surfaces clean?"
 
"Hey Guy and caseycoyne. I thi

"Hey Guy and caseycoyne. I think you are right. I only wiped off the head and block with a clean rag but should have used a solvent when reassembling. I think that the gasket just slid out of position. I'm just a backyard mechanic and this is my first boat. I replaced the 14.3 X 21 prop with a 14.5 X 17, damn what a difference. I get on plane much faster and RPMs are over 4100. I've only been out once since replacing gasket and prop but hoping to go out this weekend. Also, someone else had replaced my sparkplugs with R43TS instead of R43T which looks shorter. Does anyone think that would have caused any damage or a problem? Thanks to all.
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gasket
 
"Did you put a straightedge on

"Did you put a straightedge on the top of the block and the head?
That gasket blew out because it was not properly compressed, and if the head or block is warped, it will happen again. It's only a matter of time.

Rod"
 
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