Let's talk pistons and rings...

[Zman]

From the shop manual. Piston to Cylinder clearance (measured at smallest area of cylinder, and largest of piston)
300cc
STD: 0.06 mm
Limit: 0.10 mm

Your variances range between .457 and .406 mm so a total of say 0.05mm

How does the skirt of the piston look? Hows the engine been sounding? Any piston slap? Remember at 0.05mm with feeler gauges it could even be human error (or at least a factor of).

Jakobi is correct! ring gap is only part of the concern. Piston skirt and bore wear below the top can lead to piston slap and eveltual piston skirt breakage.

I pulled apart my big bore XC300 327cc after too many years of use, even though it ran well and had good compression. The piston skirt had worn which allows the piston to rock within the bore and that can cause ring leakage even with a good ring gap. I noticed some loss of power and it was starting to rattle a little. I was amazed that the ring gap was what Wiseco reccomeded for new installation even though the piston skirts were worn and it was time to be replaced. I noticed a nice increase in power and the slap noise is gone.

Broken piston skirts create a big mess.

 

[sweeper]

Okay, I ran a quick calculation (14 hours later, he said non-ironically) to determine bore diameter deviation based on ring end gaps.

Biggest deviation is 0.016mm diameter, smallest is 0.008mm. This assumes a round bore. I'll see if I can get the cylinder and piston to the machine shop tonight and have them give it the old once-over.

I purchased a top end kit as well as a set of rings, just because. If I can get away with using the bore as it is, then the rings will give me another 20-40 hours. If the piston has to be changed, then I'm set for that. And if the plating has to be redone, well, I'm prepared for that.

"Loaded for bear," as my old man would say.

Most machine shop guys are realistic,Ill bet they say run it.
The tapers you are talking about are reasonable.Youd know if the piston was hammering,and you were having those kind of problems.

Braaaaaaaaaaap

 

[pscook]

Initial report from machine shop is 0.006" difference between piston and bore. Bore looks good, but piston is a bit undersized, apparently. I'll be picking up the bits this afternoon and see what the true numbers are.

 

[Moto7man]

Initial report from machine shop is 0.006" difference between piston and bore. Bore looks good, but piston is a bit undersized, apparently. I'll be picking up the bits this afternoon and see what the true numbers are.

So, .006 converts to .15MM and the cylinder to piston clearance limit is .10MM, correct?

 

[Moto7man]

Update........?

 

[pscook]

Update........?

New piston and rings, no replate. Engine is together but I am going to pull apart the waterpump to see which impeller I have. If it's plastic with rubbing then I need to contact Mark and see if the warranty stuff is still going.

Oh, and I ran some solder pieces for squish but I haven't measured them yet. Busy with other stuff (family, work, other bikes, etc).

 

[Moto7man]

New piston and rings, no replate. Engine is together but I am going to pull apart the waterpump to see which impeller I have. If it's plastic with rubbing then I need to contact Mark and see if the warranty stuff is still going.

Oh, and I ran some solder pieces for squish but I haven't measured them yet. Busy with other stuff (family, work, other bikes, etc).

Sounds good, make sure you check that impeller, mine was rubbing before I installed the warranty kit. I also installed an alloy impeller and spacer from motocross center.com. No problems since install.