Piston pics- what do you think

Hey guys, Just pulled my head this afternoon to check the condition of everything. I have been chasing a hard starting issue and figured I would open it up and have a look while everything was disassembled.

One thing I would like to ask. With the piston at TDC I can rotate it slightly. I can also push it back and forth in the cylinder, on the rings. Is this normal?

Cylinder is new from US Chrome. I have never noticed this before and it has me wondering.

Please feel free to comment on how it looks. Thanks, George.

2011-05-03124546.jpg

2011-05-03125252.jpg

Burn pattern looks average. You can see the scavenging isn't completely symmetrical by this. Seldom is. Nice clean head. Rotating the piston more than about a degree not so good, but it is additive: both ends of rod plus crank slop. I would put rod as reason. You'll have to pull cylinder to find out if both or one end. Don't let it get too sloppy, or you'll really be throwing parts at it.

Top of piston is looser in bore than bottom, as it expands with heat and then fits tighter. You could warm it up to operating temperature with a torch and then touch it.....:thumbsup:

With the piston at TDC I can rotate it slightly. I can also push it back and forth in the cylinder, on the rings. Is this normal?

Cylinder is new from US Chrome. I have never noticed this before and it has me wondering.

Please feel free to comment on how it looks. Thanks, George.

Got your PM George and couple of things. Your piston/head look fine.

The piston movement you're describing is normal and there really isn't

an agreed upon spec/method for checking rock/rotation. You have to actually

measure the piston/cyl. and rod play. That said, I like to check for rod play at approx.

the 45/315 positions (TDC = 0) with a dial gauge in the plug hole while doing a service

mid-point between a top (20hr.). Basically you're looking for any stationary needle while you're

slowly reversing/rocking the crank slightly at those positions. (pin force ~ CL of rod)

Keep in mind, this is not a perfect method for checking the rod/crank, but meant to catch an

abnormal wear/clearance situation at an early time in the crank/rod's life.

There is a slight possibility a dial indicator could hang up, to which checking again

slightly outside the 45/315 and repeating would show no delay and that it was the gauge. (rare)

Put another way, if everyone pulled their head and did what you did, we'd have everyone pulling

cylinders and checking rods because they'll all do it to some extent. If your rod shows no play

on the indicator and your piston/rod are within a reasonable service interval, I wouldn't sweat it too much.

As long as we're talking about it, what were your piston/rod times and what type of riding again?

Did you check the rod play on the last top?

Edited by carver
Got your PM George and couple of things. Your piston/head look fine.

The piston movement you're describing is normal and there really isn't

an agreed upon spec/method for checking rock/rotation. You have to actually

measure the piston/cyl. and rod play. That said, I like to check for rod play at approx.

the 45/315 positions (TDC = 0) with a dial gauge in the plug hole while doing a service

mid-point between a top (20hr.). Basically you're looking for any stationary needle while you're

slowly reversing/rocking the crank slightly at those positions. (pin force ~ CL of rod)

There is a slight possibility a dial indicator could hang up, to which checking again

slightly outside the 45/315 and repeating would show no delay and that it was the gauge. (rare)

Put another way, if everyone pulled their head and did what you did, we'd have everyone pulling

cylinders and checking rods because they'll all do it to some extent. If your rod shows no play

on the indicator and your piston/rod are within a reasonable service interval, I wouldn't sweat it too much.

As long as we're talking about it, what were your piston/rod times and what type of riding again?

Did you check the rod play on the last top?

Hey Carver, thanks for joining in.

The bike had a new Wiseco crank and rod installed about 1 hour before I bought it (long story). I pulled it apart when I bought it, and indeed, brand new everything.

I have put around 25 hours on it since I bought it. I re-ringed it about ten hours ago when I had to replace a bent shift fork.

The first thing I did when I noticed this problem was to bust out my dial indicator. I checked for any needle lag as per your timing thread.

The needle worked like magic, stayed exactly with the rotation of the crank everywhere I tried.

Then I realized, it was making a slight sound every time I reversed directions. Almost like the piston was rocking against the cylinder.

Well, you were right, everyone would go pulling their cylinder. I did!

Absolutely no up and down motion in the rod. I could not figure out the piston rocking sound when I was inspecting everything.

As for piston and rod times, so far with this bike there hasn't been any. I ride strictly mx, not very fast though :thumbsup:

Edited by GMO397

Well, I noticed this. My finger nail will catch in it, but it is very small. Cylinder still looks great, no gouges, just typical streaks.

2011-05-03150514.jpg

The first thing I did when I noticed this problem was to bust out my dial indicator. I checked for any needle lag as per your timing thread.

The needle worked like magic, stayed exactly with the rotation of the crank everywhere I tried.

Then I realized, it was making a slight sound every time I reversed directions. Almost like the piston was rocking against the cylinder.

Well, you were right, everyone would go pulling their cylinder. I did!

Absolutely no up and down motion in the rod. But it does rock on it's axis slightly. I could not figure out the piston rocking sound when I was inspecting everything.

As for piston and rod times, so far with this bike there hasn't been any. I ride strictly mx, not very fast though :thumbsup:

Makes sense. The noise you hear while reversing the crank while checking the gauge is normal,

you're reversing quite a few parts with clearances in the engine.

The rocking on its axis is also normal. The small end free play limit is 2mm with up to 1mm being normal.

Your piston looks good except for that line that catches your nail......I'd check exactly what it is aligned

with in the cylinder. Looks almost like the beginning of a cold seize, but that's just a shot in the dark without seeing the cylinder.

Usually they'll be symmetrical around the exhaust port so don't know if you see the beginning of it on the other side.

I'm sure you warm the bike up before going hard, but just something to remember.

Since you're riding MX, have 25hr. on the piston and 10hr. on the rings along with that line and everything is apart,

I'd just top it right now. Identify what was lined up with the line in the cyl. and inspect it closely.

Based on the top of the piston, don't think a lean mixture/timing caused it, more likely a quick warmup with a forged piston.

Edited by carver

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