Super Ugly Pistons

[ForcedError]

In the process of a complete rebuild on an 87 force 125. But there is a catch. The pistons bores seem to be 3.375... I called a parts supplier and he said someone likely changed the block out to a newer year and he is suggesting a rebuild kit of 91-94. My question is, what caused some of the pistons to melt down this way? I noticed some of the reeds were not closing all the way. I see some daylight though them. They are not chipped but open a bit like a few specs of sand are holding them open. I just bought this turd and couldn't get it to run right so I got aggressive and found that 3 cylinders were 120 psi ish and one was 90. I decided to rip it apart and found that all four pistons has cracked rings and damage to the pistons. Amazed it ran at all. I also found a Crack in the plastic water pump housing. I'm asking what definitively caused it as I don't want to recreate the same issue. thanks in advance!

 

[Nordin]

ForcedError melted/detonation pistons are a result of either lean settings of the air/fuel mixture screws of not right set of the timing. It can also be both of this issues.
As you says the reed valves are not sealing tight and you can see daylight through them.
A little bit of gap is acceptable but no more then about 1/275 of an inch (0,10 millimeter).
Reeds that not sealing properly create lack of fuel to the cylinders and that can also result in melted pistons.

 

[jerryjerry05]

The port cover gaskets probably went bad and allowed more air to get in the system and created a lean condition, loose carb or gaskets somewhere in the intake side could have contributed to the problem??
An open condition on the reeds: a small amount is acceptable.
Your reeds didn't cause that.

 

[Jiggz]

Usually, this is caused as mentioned by lean fuel conditions in the cylinders. Why lean conditions? It reduces lubrication which causes high heat in the cylinder and eventually detonation.

 

[ForcedError]

Ok fellas, thanks for the input. I did find one of the vaccuum lines off on the worst piston. the line was behind the cdi modules. That may have contributed some. Not sure.

 

[racerone]

There are some oil recirculation lines on this motor.

 

[ForcedError]

There are some oil recirculation lines on this motor.

That may be them. There are several rubber 1/8 inch lines on the side of the block.

 

[Jiggz]

Disconnected fuel recirc lines will allow more air into that cylinder and less fuel/oil mixture, i.e. lean condition. The fuel recirc lines do not provide lubrication instead collects unburned fuel/oil mixture from the cylinder cavities/plenum. And the same is recirc to the carb intake.

 

[ForcedError]

Thank you!

 

[jimmbo]

Disconnected fuel recirc lines will allow more air into that cylinder and less fuel/oil mixture, i.e. lean condition. The fuel recirc lines do not provide lubrication instead collects unburned fuel/oil mixture from the cylinder cavities/plenum. And the same is recirc to the carb intake.

Actually a lot of the Recirc systems do send the puddlings to Crankshaft Bearings, especially the upper most, as a means of ensuring Lubrication

 

[jimmbo]

You say all the Pistons had broken rings? Check the Bore to determine if a previous owner did a poorly done rebuild. If so, I suspect that the Rings were never checked for proper gap, just slapped on and in they went.

 

[ForcedError]

I suspect you are correct as well. Having it bored out next week. Do you guys know what the clearance should be on 3.415 wsm pistons between the wall or ring gap?

 

[racerone]

Typically 0.003" / 0.004" piston to bore clearance----Measured in the correct place.-----Even new pistons have a taper to them.