piston melt down

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I'm hoping I have the correct photo attached to this post. First time with this. Forgive me if I get it wrong. This piston is out of a 1986 Force 125 that I bought for parts. The owner said that he lost spark in one cylinder, diagnosed a bad stator, replaced it with a new stator, got spark and then discovered there was no compression in cylinder #2. I tore down the motor to indeed find a new stator and this piston in cylinder #2. Would a bad stator cause this type of damage or are there other parts I should be looking at for this type of failure? Maybe the trigger? Surprisingly the cylinder doesn't look to bad. All the other piston and cylinders look fine. Thinking this was a parts motor I didn't check overall compression before i took it apart.

Re: piston melt down

Try again with the photo. However, On Force engines, Pistons melted are almost ALWAYS a result of lean running, although advanced timing or wrong plugs can also cause this. Definitely check the setting of the carbs AND check them for clogging and debris. When you do rebuild, be certain to install a cheap disposable filter BETWEEN the fuel pump and the carbs. If you have little experience with these engines read the stickys at the top of the page.

Do not assume anything. Check all systems and DO ASSUME that anything that can be out of adjustment is. Anything that needs replacement periodically should be replaced as a matter of course (water pump impeller, fuel pump diaphragm, lower unit oil, etc).

Re: piston melt down

I believe the only thing that can cause scoring is stuck piston rings and or clogged carb/improper fuel mix or bad reeds. Even if there was no spark the cylinder should still draw the air/fuel mix into the combustion chamber to keep the piston and parts oiled....that is if the reeds were good and the carbs were working correctly. Lack of spark cannot cause scoring by itself.

Re: piston melt down

Let's see if I have the photo this time? Thanks Frank and Jason, your comments make good sense. I've been banging my knuckles most of my life, mostly antique cars. This is my first venture into 2 cycle outboards. I suspect this motor had the wrong plugs in it. I'll look into that this weekend.

Re: piston melt down

Definitely lean run/detonation damage. This is classic damage. The lean burning extra hot exhaust gases overpressure and first melt the ring lands, then the rings break. Finally, the hot exhaust gases blow past the now broken rings and melt the piston where it is thinnest--right in the skirt.

That amount of damage happened in only a couple of seconds at full throttle. If you had been running the engine, by the time you heard it, it would have been too late to react.

The cylinder may not look bad but there is a 100% chance that it needs to be rebored.

As I said, when rebuilding you need to disassemble and clean both carbs then set low speed needles correctly. Even though they rarely cause this, check reeds to be certain there are no broken petals

Re: piston melt down

Frank, You are the best. You should know that I was given a 72 checkmate mx-16 a few years ago. After hours of research on the the best way to power it with a fixed budget, it was information on this forum that pushed me in the Force direction. That and the fact that they are plentiful and I have been able to barter upholstery services for parts. I've been in and out of the auto/marine upholstery trade over the past 25 years but this is my first boat.
As I said in my other thread, I now have 3 125's (86,88 and 89), all with issues. I decided to dismantle the 86 first as this one has seen salt water and appeared to be in the roughest shape. I will be sure to dismantle and clean the carbs. I think I remember reading that automotive carb cleaner is to harsh. Do you have a recommendation?
As I suspected, the plugs seem to be wrong for this application (champion L77JC4 with projecting electrodes). The reed blocks are not matched. They appear different in design and materials. As a musician and harmonica player I know that reeds do go bad (@#^&*). Is there a test for this?
I have ordered a bore gage to assess wear and damage. I'll include a pic of cylinder #2 .

Re: piston melt down

About the only way stock steel reeds go bad is to break a petal. They are allowed to stand open at rest a maximum of .010. So, the only test you need to to look through the back of the vee block to see if light is showing. If light shows, then estimate .010. If the petal is significantly bent open, then replace it. Your engine appears to have the stock reeds on one block and much later vee blocks and reeds on the other. I THINK I have seen those five petal square tipped reeds without stop plates on a late 90s force 40 but they could also be aftermarket.

At any rate, while they probably will do no harm mixed like that, they offend my aesthetic sense and I would have all four the same.

In the photo, it is obvious that the cylinder needs reboring, the only question is how much oversized does it need to go.

Carb cleaner is cautioned against because it may dissolve the sealer on the inlet nipples and the large top welsh plug. I use WD 40 and am satisfied. I suppose any pentrating oil would do.

Re: piston melt down

Thought maybe I could show you pics from the 89 while we are on the topic. I'm thinking I am also looking at lean mixture deterioration in the early stages. The previous owner told me that he rebuilt the carbs, replaced a CD and then only used it once...said it started hard but ran OK. I found the compression to measure 85,127,150 and 90 (1-4). I pulled the head and found the three down cylinders all have this puckering on the top/exhaust side. I'm hoping maybe I can get away honing on this one. As before, your input is greatly appreciated.

Re: piston melt down

That is not lean running damage. Most likely the top rings have broken and flipped out of the groove. That is what causes the scalloped edge. If your 89 has the semi-keystone top ring, they are prone to this. In fact, I can just see in one photo the dents on top of the piston baffle caused by a (ring piece) bouncing around in the cylinder.

The one bore that I can see does not look too bad and may indeed clean up with only a honing. The pistons are now crap and should be thrown away. Save any good bottom rings though. Try to replace them with pistons with two square rings--they almost never flip a ring.

Unless the last engine is in decent shape, you may not get one working engine from three. Unless you decide to buy new pistons, you will need to salvage some from junker engines.

Your 125 is a 3.3125 standard bore. If you mike them and they are not oversized from a rebore, you can use pistons out of an early Chrysler 75, 85, 90, 105, 125, Force 85, 90 (84-89), 120 (90), 125, and even the early 150 (89-90). Just be careful on the Force engines because they did change to a 3.375 bore in the 1990s.

Re: piston melt down

Thanks Frank, I am learning a lot. Are just the top rings different (semi-keystone) or are the ring groove also different?
I would like to ask about the last of my triumvirate (the 1988) if I could. This motor has 120lbs of compression in three cylinders and 90lbs in #3. As you can see by the photos all the wear appears on the top and bottom of the bore. This is my last example on this topic. Please let me know what you think. I acquired these motors separately knowing I was probably in for a rebuild. Having worked on old stuff in the past I wanted spare parts. I only have $300 in cash invested so far. Thanks for your time and knowledge.

Re: piston melt down

That example is aluminum wipe-off from the piston. Probably from lean running or lack of oil. The damaged piston has stuck the ring in its groove and that is why compression is down. There is a possibility that you can scrape off the aluminum from the bore, hone it, and insert a good piston. With the amount of aluminum I see there, it is a good bet that the piston is not usable. However, sometimes if you are good, you can scrape the aluminum off the ring(s) and free them without breaking them. If the piston is not too baldo worn on the skirt, it can be reused but that is not a good idea and reserved for a Hail Mary.

Not all engines had the semi-keystone top ring and quite frankly (no pun intended) I never paid attention to which ones did. However, on all engines, the second ring was a regular rectangular one and they rarely break. It is common for these engines to continue to run even after breaking a top ring and they will run at almost peak output on the one good ring.

The semi-keystone ring is triangular in cross section and the groove is the same. Some pistons had the groove right at the top and some had it about 1/8 inch down. The reasoning with these rings was that combustion pressure could more easily get behind them, forcing them against the cylinder wall. Thus, they would leak less when running. Unfortunately, they seem to catch more frequently in the ports.

Re: piston melt down

Thanks once again Frank, I'm amazed at your knowledge and astounded at your commitment to helping others. The wind is blowing pretty cold through the cracks in my barn these days. I have some time to think about which motor I will invest my time in first.

Re: piston melt down

It's warm enough to get back in my barn so I finally got all of my motors torn down. Here are pics of the 89. As you said Frank, it has the tapered top rings, three of which are broken. I'm thinking this block is the best candidate for rebuilt. I'm ok with having it bored and investing in new pistons. My question is... looking at how the exhaust ports got beat up as the rings let go, if the over boring doesn't completely clean this up, I'm I asking for trouble, and/or, will it run properly? Any input is appreciated here. Thanks

Re: piston melt down

If you intend to bore out that block you should have no problem. You can thin and streamline the ports to clean them up but don't go too far. On these engines there is a 30 degree open overlap and cylinders 3 and 2 fire right after each other 1, 3, 2, 4 and exhaust can bleed back into the cylinders. Thus: Increasing port area actually reduces power output. You can, however square the bypass ports and still gain an increase in power.

Re: piston melt down

HI GUys

Not meaning to hijack a thread, but my 1988/125 was rebuilt ( not by me) with 1 cylinder +40thou. If the repairer used 'stock' pistons, when can I expect them to throw a ring? Will they throw a ring 'hap hazzardly' or will running rich and doing the right things in regards to maintenance and tuning resolve this issue? I don't want to be on the water, expecting a 'BANG'.....

Re: piston melt down

I don't think this is a hijack. I'm not the expert here (still learning) but I would do a compression check. If the numbers don't look good I would pull the head (about 5 minutes) and take a look. In my experience the problems are quickly apparent and with the help of the guys on this forum I am learning the reasons why they occur. Sounds to me that you're not really sure about your rebuild. I think it might be worth the time to do some investigating before you get to far from shore. May the Force be with you. Sorry, I'm sure that pun must have been used before.

Frank, When you say "bypass" ports are you referring to the intake ports? I've read your threads on porting and I think I remember you saying that you felt it was more successful on the 3 cylinder motors. I understand your comment on staying conservative on the exhaust side but I'm still trying to get my head around the 30 degree open overlap. I've studied the photos you've posted on squaring intake ports. Not having done this type of work before I'm trying to decide whether to go for it. My main fear is that the nicks on the exhaust side will catch and break rings. In dealing with the overlap issue, would it be most important to stay away from the top side of the port? I'm hoping to make my 16' boat scoot, but I would like it to be reliable. Thanks for your help.

Re: piston melt down



I will address two issues here. First: The semi-keystone rings are more PRONE to catching and breaking--it does NOT mean that they defintely will do so. Many engines with these rings are still running well. The rebuilt engine may last for your life. Then again, it may not. Run it too lean and it definitely will not last.

Second: 30 degree overlap and porting. The ports on the opposite side of the exhaust ports are called "Bypass ports." This is because they are serviced by the bypass channel. Fuel/air is bypassed from the crankcase, around the piston skirt and up to the cylinder. Carbs and reeds are intake system.Since these ports do not share a common cavity, they can be enlarged with no penalty.

Port timing is measured in crankshaft degrees. If you look at the exhaust ports, they take up about 1/3 of the total stroke length or 120 degrees. Since the crankpins on a four cylinder Chrysler or Force are 90 degrees apart, the exhaust ports from #3 are still open when #2 starts to dump exhaust. Thus: a 30 degree overlap with exhaust refluxing back into #3. If you enlarge the exhaust ports, you only exaggerate this with a resulting loss of power. Couple this with a common exhaust chamber where all four cylinders exhaust and backpressure will cause reflux into all cylinders.

Three cylinder engines respond better because there is less exhaust passing into the chamber or chest and with crankpins 120 degrees apart each cylinder is closed before the next one exhausts ---allowing peak pressures to dissipate before the next cylinder opens.

Chrysler DID enlarge and square the ports on the racing 135 and advertised it BUT: They also had tuned stacks separating the top two cylinders from the bottom. A while back, a set of those tuned stacks went for $1600 on ebay.

Since ports can be about 1/3 of the total bore diameter and since boring a cylinder will clean the edges of a nick, once you bore the cylinder and SLIGHTLY chamfer the tops of the ports, there is no danger of catching a ring. Here are some photos of really big exhaust ports.

Raising the top of the exhaust ports :advances: port timing and while it may increase horsepower that increase is always at a higher RPM. So, in a non racing application yes, you want to stay away from raising the port tops,

Re: piston melt down

Thanks Frank, That clears things up a lot for me. I don't think I'll be buying those pricy tuned stacks but I am becoming more confident about squaring up the "bypass" side and SLIGHTLY chamfering the tops of the exhaust ports. If you ever decide to do a weekend seminar, sign me up. I'll bring the coffee and doughnuts.

Re: piston melt down

Frank Acampora said:

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The ports on the opposite side of the exhaust ports are called "Bypass ports." This is because they are serviced by the bypass channel.
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Click to expand...

These are also known as "transfer ports" as they transfer the charge from the crankcase into the cylinder.

Peter