83 Nautique 2001 - Mercruiser Chev 350 260 issue

Hello everyone,

My engine recently developed a slight knocking sound, didn't notice any major performance issues but did have a lot more people on the boat that usual so I was revving higher for normal speeds.

Anyway to the point, I did a quick and nasty compression test, I can't guarantee the accuracy of results but they are roughly as follows. I just did a dry test,

1. 135
2. 145
3. 115
4. 130
5. 95
6. 125
7. reads +/-50 if i zero and try again needle doesn't budge
8. 150

So other than the fact that there is quite a bit of inconsistency, cylinder 5 and 7 and clearly a problem, 5 specifically. Also I'm not a mechanic, but am quite technical and hands on, if I just have broken ring/s could this be a quick fix or am I in for a full rebuild? in your opinions?

Now this engine no longer has the serial plate on, does anyone know if there are major differences between the 78-82 Mercruiser 350 260hp and the 82-86 Mercruiser 350 260hp engines in terms of internals, as being an '83 model I assume it could have either range installed (almost 100% sure its still factory engine after discussions with Art)

100%, thanks for the reply
Will have to find a buddy with a nice big garage that I can abuse :D
not ideal timing with a trip planned for weekend after next

Do these marinized motors require different gaskets as apposed to standard engines?

I agree with Gottaski on the next steps. Minimal time/effort to determine where to go after that.

There are technically marine gaskets but they really aren't needed. Many have used standard auto gaskets without issue. I personally used a marine head gasket for my rebuild and then used standard gaskets for the rest.. Some others might disagree.
One exception is if you're using it in saltwater. Then definitely want to use marine gaskets.

As for your original question about any changes through the years listed for the SBC. They are minimal but may/may not cause issues.

79 and prior had the dipstick on the port side with two piece rear main seal.
80-85 moved the dipstick to the starboard side with two piece RMS.
86 still had the dipstick on the starboard side but changed to a one piece RMS which changed the crank flange and oil pan. '86 cranks and oil pans will not fit on 85 and older blocks. As such 85 and older cranks and oil pans will not fit on 86 blocks. All the rest of the internals should interchange.

If that is truly an 83 and the factory motor, I gotta believe it will be an 83 block. I just can't imagine a ski boat manufacturer putting an older overstock engine into their boat without some potential consumer fraud backlash.. No way in hell a factory 83 boat would have an 84 or newer engine unless it was changed out somewhere along the way.

backfoot100 wrote: As for your original question about any changes through the years listed for the SBC. They are minimal but may/may not cause issues. If that is truly an 83 and the factory motor, I gotta believe it will be an 83 block. I just can't imagine a ski boat manufacturer putting an older overstock engine into their boat without some potential consumer fraud backlash..

Eddie, that may be the case now, but was common on older boats. My brothers and I bought a Century Resorter in 1981 and it has a Mercruiser 255 in it which according to the Merc serial number is a 1977 engine. My BIL's 1988 Malubu Skier has a 1987 PCM 240 hp engine based on the engine serial number. The boat is an '88 according to its serial number. Correct crafts, especially older ones might vary several years from when they were built, when the motor was built and what the bill of sale stated was the age.

I was in my early 20's when we bought that Century and I was pretty surprised when I found out that it had a 1977 engine, especially as all the '81s at the time were advertised as 260's and ours was a 255. I asked the dealer about it and was told that was "normal" . Today, you could probably make a stink about that.

thanks everyone, now to decide if I want to take on pulling the heads myself or pay someone else to do it.

I have worked on bikes before but never more than single cylinder work

Thanks everyone for any info.

Pulled the head today, actually pulled exhaust manifold and everything off, no stripped bolts thank goodness. (Actually one was pretty loose come to think of it)

Attached a photo of my head and gasket, the problem cylinder (7) you can notice a different colour to the valve (far right valve), also the valve is smooth and does not have that recess that the others do, is this normal?

No noticeable blow through on the head gasket between cylinders, there was some water going through by the head bolts though. previous owner seemed to patch this with some silicon in the bolt hole . No water in the oil however.



Cylinders seem pretty clean, no major wear on the walls that I can tell with my untrained eye, very slight scoring but can only just feel it with bare fingers. No noticeable play in pistons.

I will take the head through to my local mechanic if I get a chance and get his opinion but any thoughts from anyone here as to what my problem could be?

my mechanic is going to swing by today and have a better look for me, might be pulling the engine soon and dropping it with him to work on right in peak season here

Time wise felpro is not an option right now, as for me it will be 3 weeks just to get the gaskets in. Shipping is also about the same price if not more than the gaskets themselves.

Mechanic confirmed middle looked suspect but also not the reason number 7 has no compression, He suggested I do a quick valve test with some solvent to see if I have a valve leak on that cylinder.

Okay engineer says valves are biggest spec already and the head is in pretty bad shape.

Are these just standard chev 350 heads? aka I can find one off a car and slap it in

The idea was to do both heads but to get this one on asap to see if it's a bigger problem before I spend unnecessary money.

Unfortunately with living this side of the world parts are a lot more expensive and a lot harder to come by than that side of the pond.

Busy trying to find a set of replacement heads now and even that is posing to be a bit of a problem.

Take a Mic and measure the thickness of the head gasket at all the hot spots, normally the hot spots are close to the exhaust valves. It does indeed look like your gasket failed between the center cylinders.
If the gasket measures smaller in certain areas the engine was overheated at some point and crushed the head gasket. You are looking for a difference in thickness of .003 -.004 normally from crush. This crushed area will fail later after the overheat. Could be 1 day or 2 months but once crushed the gasket will fail. The head bolts in the same area are usually low on torque when you remove them again from the crushed gasket.
The gasket can't rebound from a crush. Make sure your head is still flat with a straight edge before assembling again or have a shop rebuild the heads.
A leaking valve can quickly be found by pouring solvent in the ports. If it escapes past the valve it is not sealed.
When you have hot spots or localized overheating the block and head expand, this is what crushes the gasket. Once the block and head cools the gasket remains crushed but the block and head go back to original size leaving the head bolts in that area loose and the gasket will blow.

So the head is being rebuilt today, Engineer says he will get it right but because the head is so worn I should look for another set to replace these with.

New gaskets etc and torqued back to spec, hoping I get it back this afternoon then I can get cracking at assembling this evening, hopefully test and soon find out if that's related to my compression issue, holding thumbs.

The burn looks very normal to me, but I just don't understand this 'head is worn' thing....

No cracks, seats, guides, valve grinds, lap, resurface, thats about it.

But from very afar, the head gasket appears odd at 3 oclock on # 7