new piston problem

When I pulled my block and pistons off to replace the block, I found new damage to the front of the right piston. This brand new piston has been in the bike for less than 15 miles

The rear wall of right cylinder looked like this when put back together 2 years ago:

 


The front wall of right cylinder today, 15 miles later.

 

The front skirt of right piston with only 15 miles on it, today.

There was no wear on the front of the old piston, so this is brand new wear on the front. What happened?  There is also rubbing on the rear side, but maybe that is to be expected considering the rear wall of the bore wasn't perfect.  I need to figure this out before installing the new block.

 

Sounds like time to measure everything.  You will need a micrometer that reads down to .0001", and a bore gauge.  Check for the proper cylinder to piston clearance, and make sure you have less than .001" of taper and out of round in each hole.

Some thoughts:

There appears to be galling as well as wear. Galling happens (IIRC) due to wear + localized heat. If you want to use that piston, I think that area will need to be smoothed/cleaned up.

There appears to be some discoloration on the oil scraper ring (the castellated one) directly above the worn area on the piston. This may be due to heat as well (or I'm seeing things). The oil control ring above that also shows some wear.

The piston wear is limited to the top portion (not lower on the skirt), which makes me think the piston might be slapping slightly in a front-to-back manner.

Everything looks really clean; did you wipe down the parts before taking the pictures? If not, it's possible the damage was caused by the parts being run dry. There doesn't seem to be any evidence of oil.

The mark on the piston appears to line up with the lack of a "carbon Band" at the top of the cylinder ( the clean part) suggesting combustion blowing past the piston. Where the ring end gaps correctly spaced ? Also did you put the rings in the bore and measure the end gap before fitting on the pistons ? The end gap on the top ring of the piston shown looks excessive

Nessism wrote:

Sounds like time to measure everything.  You will need a micrometer that reads down to .0001", and a bore gauge.  Check for the proper cylinder to piston clearance, and make sure you have less than .001" of taper and out of round in each hole.
 

I don't trust myself to do that kind of precision work. Maybe I'll take the pistons and new-to-me block to a local powers sports guy who is at least semi-competent, I hope. I've never used him. But I won't be using that pictured block again, so I don't need to measure it. Not sure I need to measure the new-to-me one that will be getting installed either, because I gave the Harley machinist the specs when he honed it. But I want to make sure this doesn't keep happening.

Nerdy wrote:

Some thoughts:

There appears to be galling as well as wear. Galling happens (IIRC) due to wear + localized heat. If you want to use that piston, I think that area will need to be smoothed/cleaned up.

There appears to be some discoloration on the oil scraper ring (the castellated one) directly above the worn area on the piston. This may be due to heat as well (or I'm seeing things). The oil control ring above that also shows some wear.

The piston wear is limited to the top portion (not lower on the skirt), which makes me think the piston might be slapping slightly in a front-to-back manner.

Everything looks really clean; did you wipe down the parts before taking the pictures? If not, it's possible the damage was caused by the parts being run dry. There doesn't seem to be any evidence of oil.

 

Its clean because it was a brand new piston and only has 14 miles on it. It did look rather dry, but it hasn't been started since 2021. I will take a closer look and see if I can see what you saw regarding the rings. I can't feel anything awry on the worn (or galled) part of the piston so I hope its superficial.

Wookie58 wrote:

The mark on the piston appears to line up with the lack of a "carbon Band" at the top of the cylinder ( the clean part) suggesting combustion blowing past the piston. Where the ring end gaps correctly spaced ? Also did you put the rings in the bore and measure the end gap before fitting on the pistons ? The end gap on the top ring of the piston shown looks excessive
 

That clean part of the carbon band is what I was trying to show in the picture. I had the Harley guy  install the rings because I am eminently unqualified for this kind of work and broke the first set of rings. But I do not recall giving him or having him ask about the ring gap specs so they might not have been set. And most definitely the piston to bore clearance was not checked when I put it back together 2 years ago.

Also I recall having some post-assembly anxiety about whether I had properly oiled the pistons before seating them in the bore 2 years ago...yeah, ouch...and I am just not sure. I may or may not have. Could not oiling them properly or improperly gapped rings have caused what we see on the piston? If so, I vote for those as the cause.
So I may be an idjit but it was my first time ever past valve covers, so I'll probably only make half or 2/3rds the mistakes this time.

It's all a learning curve, the piston should clean up using some 800 grit wet & dry with WD40 - 

Are there arrows on the pistons?
Here’s an interesting story on pistons which may or may not be relevant.
www.cycleworld.com/blogs/ask-kevin/motor...red-shape-explained/

Yup...arrows point forward

82KZ305:  With your experience or lack thereof in this matter, your REALLY need to find some competent help with this project.  I don't know where you live, but if you let us know someone may volunteer to give you a hand or at least recommend a person or establishment to give you some assistance.  Measuring engine dimensions is no easy task and requires some very specialized tools.  This is not an endeavor to be taken lightly and just anyone doing this isn't going to cut it.  You most assuredly don't want to install a new block or pistons and have the same thing happen.   Let us know where you are at.

Rick H. 

Rick H. wrote:

82KZ305:  With your experience or lack thereof in this matter, your REALLY need to find some competent help with this project.  I don't know where you live, but if you let us know someone may volunteer to give you a hand or at least recommend a person or establishment to give you some assistance.  Measuring engine dimensions is no easy task and requires some very specialized tools.  This is not an endeavor to be taken lightly and just anyone doing this isn't going to cut it.  You most assuredly don't want to install a new block or pistons and have the same thing happen.   Let us know where you are at.

Rick H. 

 

Oh I totally agree, been saying that from the start. I have a very good idea of my abilities and the finer points of engine internals is not among them. But I'm also poor so necessity dictated that either I did it or it didn't get done.

I took the piston to my local powersports guy and he didn't think the wear was much to be concerned about. The wear is on the points where the piston is widest, he explained, and he showed me other pistons he had laying around that had similar wear patterns, front and back only. Plus there's the possibility it wasn't lubed properly when I reassembled. So maybe this "new piston problem" isn't a problem? He showed me how to work with the rings and check their gaps and I ordered a new wrist pin, just in case there is wear that my existing tools can't measure. It was cheap insurance. He also gave me some assembly lube and said he would help if I needed more help. But I can't afford to have him do all the work.

Been thinking about the piston to cylinder clearance and wondering if I really need to worry about that. Its a new Kawasaki piston (with 14 miles on it) and a used block that has been measured and honed to specs. It seems to my (inexperienced) way of thinking that too little clearance is highly unlikely if not practically impossible with a stock OEM piston and a used, freshly honed cylinder. And there's nothing I can do if clearance is larger than specs, since going oversize is not an option. But feel free to correct any errors in my reasoning.

Cleaning the carbs now and was planning to start putting back together this weekend. I'm in NE MN, far from humanity.


PS-I also noticed when I took my carbs apart that the JBM vacuum diaphragms I installed 2 years ago don't really hug the slider cylinder very snugly. Caused me to wonder if that was part of the reason I couldn't get the bike above 45 mph, as that sounds similar to symptoms of leaky diaphragms...just a thought. I reached out to them but haven't heard back