Question about Piston Slap

[cywelchjr]

Hey Cy,

Simple test, surprised someone didn't already suggest it! DISCONNECT The vacuum hose going to the advance unit, just plug it so you don't have a vacuum leak, and then ride it to see/hear if you hear the noise!

If you don't, then you've discovered the cause of the noise, but if you still hear the noise, then it won't be the vac. adv. unit, and it could be piston slap?
T.C.

Your right, that's a good idea. I happen to have an extra rubber cap since I had to buy a set when I synced my carbs because the ones that were on there pretty much came apart when I removed them. They came in a package of like 4 or something so I have a few spares. I'll just pop one on the carb fitting and leave the hose hanging in there and give it a try as soon as things dry out here (commute is going fine, but I need to get it out on the road to see how it does).

Maybe if the weather is decent on Sat I can do the test after I finish installing the new fork seals and front brake pads (I've had the pads sitting waiting till I fixed the leak before I install them).

[mack]

question

not trying to hijack this thread but, if one was to look at the sleeves while the head is off, piston slap would score them and be quite evident wouldn't it. My head is off and that is something I hadn't looked for yet, but since my compression numbers were pretty low, this might be a good idea?

[trbig]

In the ones I've seen, any markings will be much more pronounced on the piston skirt since it's much softer than the steel of the cylinder. You will, however, be able to see a mark where it was slapping the cylinder in the same spot over and over... though it looks more like a discoloration than scoring. You'll need to MIC the cylinders front to back very carefully as they get elongated before trying to re-use them as well as the pistons and skirts.


Tod

[Ivan]

In my experience what you describe is probably loose wrist pins. Not a big deal but annoying none the less. Almost sounds like a low end knock, but isn't in the low end. Most of the time piston slap comes from FOD (Foreign Object Damage) when someone is changing plugs and drops a screw or some other small object that gets lodged in the side of the piston, and eventually bounces out in the exhaust. The piston will cock sideways from the object not compressing and actually squishes the skirt in. Then every time the cylinder fires, the piston wobbles a bit. It sounds different to a trained ear, but unless you have run some gravel through your engine, I would likely say it isn't truly piston slap.

[trbig]

While I won't argue about what his sound is, I will say I have had two motors with piston slap. I just went out to the garage to see if I had any of the cylinders with the marks in them, but they are too rusted to see now. There was a definate mark there in the cylinder that was an outline of the shape of the bottom of the piston skirt. I've never had a loose wrist pin in any of them before, but Kat has! lol. I hear it's tough to push a bike up an off ramp?

I did find one of the old pistons and just for grins mic'd it. Now this is in inches, but the piston above the top ring was still round at roughly 2.790". Measuring down below the oil ring on the skirt, I was expecting, with the grinding force of the piston on the wall, (The rod is at a pretty good angle and shoves the piston forward against the front cylinder wall as it goes up.. this is what eventually makes the cylinders egg-shaped) that the piston would be smaller front to back and wider on the sides. Turns out it's just the OPPOSITE!? I measured 2.810" front to back and 2.800" at the sides under the pin hole.


Tod