piston scoring, pics included! What to do?

[chrishayes]

This is a 74 merc 1500 o/b serial number 3753595. Well, after rebuilding everything on the motor I was putting everything back together and decided that while I was at it I would go ahead and replace the exhaust cover gaskets just to be thorough. THis is what I found! Now this motor ran, had some surging and bogging issues but it would occassionally get up and go. My question is, does this look like a total deal breaker? Does this need rebuilt ASAP? I have no problem taking the proper steps to ensure that this motor lasts me for as long as my soon to be restored boat does.

Do regular machine shops do boat motors? I assume that this would need to be honed at the very least and get 2 new pistons.

If it needs to be overbored are pistons available?

I have a parts motor that is a 115hp of the same vintage. Would the pistons from it work?

The other 4 pistons are perfectly fine and almost look new...just the #1 and #2 pistons are scored. The scoring is about a fingernail deep...

Thoughts?????????

 

[Yepblaze]

Re: piston scoring, pics included! What to do?

Well, after rebuilding everything on the motor

Thoughts?????????

I guess you really didn't get around to "rebuilding everything on the motor" and it may be time to do that.

While you are at it make sure it's getting enough water and fuel to the top of the engine so it doesn't happen again.

 

[chrishayes]

Re: piston scoring, pics included! What to do?

you are correct, thanks, I needed to feel worse about the situation

I would really like to know if a rebuild can be done by a local machine shop. This is a new to me motor. This damage was done long ago. I didnt realize this damage was even there until yesterday. There was a slight compression problem with the #1 cyl, which was 130# not 150# like the rest. THats it. And when running, a little water would leak out the #1 spark plug hole so I replaced the water jacket gasket. I thought that was my only issue. THe bogging and surging I beleived was from the carbs and or the electrical system, hence the rebuild of those systems.

 

[Jim662]

Re: piston scoring, pics included! What to do?

I'm new to this too, but I'm going through the same thing with my 1150.

The way the block is constructed, there is no head. That affects the way the boring can be done. It is a blind cut. You'll need to call around and find a local machine shop that works with 2 stroke motors. Normally its just a through cut because the cyliner is open. It requires a little more finesse. You want to make sure the person who is doing it knows what they're doing. All you can do is call around.

Even with piston scoring, don't assume that you need to bore the cylinders over. The pistons are soft aluminum. They get hot and begin to melt and leave deposits on the piston walls. The piston walls are steel and are much harder. On my motor, the #1 piston melted because the motor was run with a bad impeller. The other pistons were scored because the motor ran lean. The machine shop/rebuilder wanted to overbore each cylinder by .015 (at $50 per hole). Finally he agreed just to hone out the bottom 5 cylinders (at $6 each) and they cleaned up just fine -- no need to overbore. My point is to try the honing first. It could save you a bit. You may still need to replace the pistons, but you'll save the cost of the boring.

Of course, you 'll have to pull the crank and pistons out to take it into the machine shop. I haven't put mine back in yet, so I can't help with getting them back in.

You can buy standard sized, .015 over and .030 over pistons. They are supposedly weighted the same as the originals so that you don't have a misbalance issue. I don't know if the pistons are the same size off the 115 as the 150 (I kind of doubt it) but you can do some sluthing on the the Mercury part numbers and then take a look at the Sierra catalog to try to figure it out.

I also got advised to change the lower bearing on the crank as well.

Good Luck.

 

[chrishayes]

Re: piston scoring, pics included! What to do?

thank you very much for that. I too was a little confused about how the cyls can be bored as they are not a pass through design...I hope I can find somewhere that does it. I live near Columbus Ohio so there should be somewhere that can do it. I am not scared of doing this, just hope my clymers can get me through it. It has been fine for all the other things ive done. I assume the bottom 4 cyls are still perfectly fine, I will of course re ring them while im in there. Will they need to be honed? The other 2 we will have to see once I get the crankcase off. Wish me luck...Are you going to replace all the bearings and reeds while you are in there? I am kinda stuck on what all needs to be R and R. I sure wish there was a rebuild kit like for a vehicle engine. That way you just replace everything.

 

[Chris1956]

Re: piston scoring, pics included! What to do?

If you have 130PSI on the bad cylinders, you within 15% and are fine. Put it back together, clean and rebuild all the carbs with new inlet needles and gaskets and perform the link and synch procedure. A bad cylinder will have 30PSI or less, and that is when you need to bore and install oversize pistons and rings.

 

[MikDee]

Re: piston scoring, pics included! What to do?

thank you very much for that. I too was a little confused about how the cyls can be bored as they are not a pass through design...I hope I can find somewhere that does it. I live near Columbus Ohio so there should be somewhere that can do it. I am not scared of doing this, just hope my clymers can get me through it. It has been fine for all the other things ive done. I assume the bottom 4 cyls are still perfectly fine, I will of course re ring them while im in there. Will they need to be honed? The other 2 we will have to see once I get the crankcase off. Wish me luck...Are you going to replace all the bearings and reeds while you are in there? I am kinda stuck on what all needs to be R and R. I sure wish there was a rebuild kit like for a vehicle engine. That way you just replace everything.

Once you remove the rings on all the cylinders, you must break the glaze, and rehone them all in a crosshatch pattern, before putting new rings in. Hopefully, you can get away with just a rehoning. I had an old 6cyl inline 70hp Merc. outboard that I rebuilt back around 1970 all the spark plug holes were stripped, you can imagine my surprise when I took the head off, and found it was just a water jacket cover! It was a real "Bear" to rebuild! Good Luck, Mike

 

[chrishayes]

Re: piston scoring, pics included! What to do?

If you have 130PSI on the bad cylinders, you within 15% and are fine. Put it back together, clean and rebuild all the carbs with new inlet needles and gaskets and perform the link and synch procedure. A bad cylinder will have 30PSI or less, and that is when you need to bore and install oversize pistons and rings.

So Chris, the scoring that is in this cyl is not just on the pston, the scoring goes through the ring too. You really think this isnt anything to worry about? WOuldnt this cause a lot of friction=heat? I would love it if this were the case, but I do have it all ripped down, minus the lower end, along with the boat being stripped for a new deck/stringer job. So, if this needs to be done, no better time than the present...I certainly will listen to you, you seem to have a vast knowledge of the inner workings of these motors. Thanks again for sharing that.

 

[Chris1956]

Re: piston scoring, pics included! What to do?

If the rings are scored, replace them. Use some emery on the pistons if necessary to clean 'em up a bit.

I would not think that the compression would be that good, if the scoring wasn;t anything other than very light. Is the scoring in the cylinders below the ports? If so, it is of no consequence.

 

[Sandhammaren05]

Re: piston scoring, pics included! What to do?

Can't tell anything from your photos, would be hard with photos even if you pulled out the pistons and took photos from crank side. Normal machine shops generally can rebore outboard, I had reborong jobs done on 50 c.i. motors for years. However, the motor should then rebuilt by a competent mechanic, someone familiar with Mercury, after reboring and then honing.
the 115, as I recall, is a 100 c.i. motor and Mercury made many 100 c.i. motors of different hp, but you'll need to look at a Mercury parts manual to see if the numbers match. As to wheterh reboring is necessary? I have no idea from your photos.

This is a 74 merc 1500 o/b serial number 3753595. Well, after rebuilding everything on the motor I was putting everything back together and decided that while I was at it I would go ahead and replace the exhaust cover gaskets just to be thorough. THis is what I found! Now this motor ran, had some surging and bogging issues but it would occassionally get up and go. My question is, does this look like a total deal breaker? Does this need rebuilt ASAP? I have no problem taking the proper steps to ensure that this motor lasts me for as long as my soon to be restored boat does.

Do regular machine shops do boat motors? I assume that this would need to be honed at the very least and get 2 new pistons.

If it needs to be overbored are pistons available?

I have a parts motor that is a 115hp of the same vintage. Would the pistons from it work?

The other 4 pistons are perfectly fine and almost look new...just the #1 and #2 pistons are scored. The scoring is about a fingernail deep...

Thoughts?????????

 

[chrishayes]

Re: piston scoring, pics included! What to do?

If the rings are scored, replace them. Use some emery on the pistons if necessary to clean 'em up a bit.

I would not think that the compression would be that good, if the scoring wasn;t anything other than very light. Is the scoring in the cylinders below the ports? If so, it is of no consequence.

I cannot see scoring in the cyl itself, just on the piston. Those scores go through the rings. What do you mean by "it is of no consequence"?

 

[Jim662]

Re: piston scoring, pics included! What to do?

He means on the cylinder walls as opposed to the pistons. If its below the exhaust port, the scoring, if it exists, wouldn't affect compression. I don't think you can tell until you pull the pistons out.

 

[chrishayes]

Re: piston scoring, pics included! What to do?

so would you pull the pistons or run it seeing as though the compression is just approximately 15ish percent off. I want to be clear, when I first tested compression I hadnt run the motor in a year. The first test I did was dry, nothing added to the cyl and the motor not warm. I got 30ish psi on #1 and around 90 on the others. Then I sprayed the syls down with fogging oil and let soak. Then I tested the comp again without running. The numbers were 80psi on #1 and around 100 on the others. THen I got some help from laddies and hooked up the garden hose to the water tube since I had the LU off to rebuild WP. I then ran the motor and did the best decarb that I could without having the motor in the lake. After this I tested comp again and got 130psi on #1 and somewhere around 150-160 on the others.

I had just assumed since running the engine on the hose I saw water seeping out from the #1 spark plug in the jacket, that this was the cause of the compression being low. Therefore I changed that gasket along with running a bead of rtv around the spark plug portion of the WJ cover.

I rebuilt the carbs completely including replacing the floats. I also rebuilt the fuel pump, and replaced all the LU seals and WP. I replaced the stator and the wiring between here and the rectifier(new). That is when I was about to put everything back together I decided that to be thourough I would replace the exhaust covergaskets since I was this far already. That is when I saw this scoring.
I dont know if this helps or not, but I figured I would "spill the beans" so to speak.

 

[Chris1956]

Re: piston scoring, pics included! What to do?

Chris, I suspect that after that motor is run a while, and all the extra oil you added burns off, the compression will drop to unhealthy levels, and you will need to bore oversize and replace piston. On the off chance that I am wrong, I would figure a good way to run it for 10 minutes or so on the flusher and then recheck compression. Use a standard 50::1 gas- oil mix. if the compression drops below 100PSI, you may want to consider a rebuild.

 

[chrishayes]

Re: piston scoring, pics included! What to do?

THanks for the help guys, but I am going to rebuild. I found a shop in my area that all the local stores reccomend highly. They seem like really good people and extremely knowledgeable. I will be tearing the bottom end out on Sunday. Anything I need to know before I start? Again thanks for the help and I will continue to update this thread to hopefully help some others later.