I am soon going to be parting ways with my KLX so decided to try and return it to as stock a bike as possible recover what I can from my various upgrades. A mate of mine wanted the 351 kit for his KLX so I gave him my bike to remove it. Yesterday he showed me the piston and cylinder and pointed out some scuffing on the side of the piston. When we looked closer there is corresponding marking on the bore sleeve too.

You can see the scuffing on the piston under the rings.


In the right light you can see where the piston has been rubbing on the sleeve and it has worn the cross hatching away altogether.


To me this looks like there was a problem with my piston, but I'm no expert. Whatever the case, owners of 351 kits might want to keep an eye on this.

 

Wow!...that's on the side(s) that usually indicates rocking of the piston in the bore from too much clearance and/or serious wear. I know you have no reason to pursue it, but it would be good to know what the clearance in the bore to piston is at the points 90-degrees to the wear marks. If those are within proper spec, then it indicates the bore to piston clearance was set up properly at the time of boring. I'm not suggesting Bill didn't have the right clearance at the time of assembly, but it would be interesting to know those measurements. It would take inner and outer mic's for exact readings.

Arctra, how many miles on it? I'd think you might have heard some piston slap with that wear, but maybe not. The KLX piston...and the 351 unit...is not really a short skirt item...like say a Honda XR250R in the 80's...and many other modern 4-stroke performance engines. That is the normal first place to see wear between the piston and cylinder over time, but it usually takes some use. Did you have any notable oil use?

 

It has about 1000km, which is about 600 miles. I use hours more for my servicing, and it's got about 30 hours on it. I can't say I noticed any noise from piston slap. I didn't notice any oil usage either, but to be fair I have done 2 oil changes since it was installed so it's not like I really let the engine go long enough to use enough oil to tell.

It's just something for other owners to be aware of I guess.

 

Question: What is a ‘scuffed’ piston?

Answer:A scuffed piston is one that has been damaged by rubbing against thecylinder wall. The metal-to-metal contact smears the metal on the skirton the piston and damages the piston.

Scuffed pistons can becaused by too much heat in the combustion chamber, engine overheatingor inadequate lubrication. The piston-to-cylinder clearances in mostlate model engines is much less than it used to be to reduce pistonrock and noise. Consequently, if the piston or cylinder gets too hot,the clearance goes away and you get metal-to-metal contact.

Ininstances where piston scuffing occurred due to a loss of lubrication,the underlying cause may be a low oil level in the crankcase (due to alack of maintenance or an oil leak), low oil pressure (a worn oilpump), poor oil quality, or oil breakdown (not changing the oil oftenenough).

When diagnosing a scuffed piston, note where the pistonis scuffed. If the cause is overheating, the scuffing will mostly be onthe upper ring lands and on the sides near the wrist pins.

Theremay also be oil carbon and lacquer burned onto the underside of thepiston indicating it got too hot. Scuff marks on the lower skirt areaoften indicates a lack of lubrication (check the oil pump and pickupscreen). Scuff marks on the edges or corners of the thrust sides of thepiston may be the result of bore distortion. Scuffing on both thrustsides would indicate binding in the wrist pin.

Some originalequipment pistons and many aftermarket performance pistons now havespecial anti-scuff skirt coatings to reduce the risk of scuffing.

Cut and pasted from Uncovering the Causes of Burned or Scuffed Pistons - Engine Builder Magazine

Piston Scuffing
This is a sure sign of overheating and/or insufficient clearance. When an engine runs hot, the pistons swell up reducing the clearance between the piston and cylinder. The cylinder bore can also distort from localized hot spots, further adding to the problem. If a piston gets hot enough, it will scuff the wall wiping material off the sides of the piston. Where this occurs will give you a clue as to the cause.80125agif_00000003707

If overheating is involved, the scuffing will be primarily on the upper ring lands and on the sides near the wrist pins. There may also be oil carbon and lacquer burned onto the underside of the piston indicating it was unusually hot.

The presence of scuff marks on the lower skirt area would tend to indicate a lubrication problem rather than an overheating problem. Scuff marks on the edges or corners of the thrust sides of the piston would indicate bore distortion as the culprit. Scuffing on both thrust sides would indicate binding in the wrist pin.

Any time you find evidence of scuffing, therefore, it’s important to diagnose and correct the underlying cause to prevent a repeat failure.

If the engine was recently rebuilt, the scuffing problem may be due to insufficient assembly clearances. The only way to know is to measure the pistons and cylinders. Measure the pistons at the top, center and bottom of the skirt area in two directions (parallel to the wrist pin and perpendicular to the wrist pin) to see if the pistons are within the manufacturer’s specifications.

If the pistons are the correct size, then measure the cylinder bore at the top, middle and bottom, also in two directions (parallel to the crankshaft and perpendicular to the crankshaft). This will tell you how much taper is in the bores, and if bore distortion is a problem. By subtracting the piston dimensions from the bore dimensions, you can figure the piston-to-cylinder clearance. If the clearance is within specifications, the underlying problem is overheating.

Many aftermarket replacement pistons today are available with moly coated skirts to protect new pistons against scuffing. The coating also allows you to run slightly tighter clearances to reduce piston noise when the engine is cold, and blow-by. Upgrading to coated pistons is probably a good idea for any engine that has a history of scuffing or may be prone to detonation or overheating. Think of it as insurance against piston problems.

Cut and paste from Back to Basics: Preventing Piston Problems - Engine Builder Magazine

 

That scuff mark that's on the cylinder, that would be where the piston is at or near TDC? If it is, that is where the angle of force from the conrod changes which will try and tilt your piston a different way.

 

Well why is there crosshatching still in that cylinder? It appears to me that the rings never seated.. Don't tell me you ran synthetic or synth blend from the moment you installed the kit....On second thot, if you did, you can tell me, but don't tell anyone else..

 

I have seen similar wear when I had to tear my 351 down. I think that it may be due to the short skirt/rod angle. I asked Bill about adding a dry film coating to the skirt to minimize the wear-I think that it would help the problem.

 

With only 600 miles I would expect to see some cross hatch. Many engines show some of the original cross hatch after many thousands of miles, not prominent, but they can show it.

Is it possible the piston or bore was oval? Otherwise clearance would be of issue, probably too tight. Rocking, as I would see it, would be too much clearance and shouldn't show a scuff the length of the skirt like that, but I could be wrong.

 

I've seen cross hatch on engines that have many hours of use on them. Just a thought, could it be possible the piston was put in with the forward facing mark going to the rear?

 

I think we need a picture of the internal side of the piston from the bottom.