KX 250, After a Fresh Rebuild... it now Knocks?

is :whoa: it the pistion rings

The knock may be a lean condition.

You could have an air leak (left side crank seal? or something around the reed cage?)
or
The pilot is too lean for the 'new' amount of compression?

Did you check the play in the connecting rod?

I'm just thinking out loud here.

dave

Why did you replace the cylinder head?

What end gap did set on the rings?

How much piston to cylinder clearance was there?

If its not detonation, maybe its the sound of your engine running better than ever before? Some pipes its like the sound and pressure wave converge/separate and it sounds odd? How is the silencer packing? And second the ring gap,piston skirt clearance and thrust washer clearance on the big end of the rod.

Alin99 said:

Hi,

I just performed a top end rebuild on my 2000 kx250 and while that solved the problem with lack of compression and the bike not starting, it now knocks very loudly. :yikes:

Click to expand...

Sounds like the rebuild was forced due to a seize. Did it lock up or did it just lose compression and stop running?

If it locked up while you were running there is a good chance that the rod bearing was damaged. Like TWRT said, did you check the rod play before you put it back together?

Another thought: Any chance you left out the wrist pin bearing?

When you took it apart, was the piston in one piece? Were all the pieces accounted for? If a chunk of piston is still down in the case the rod could be hitting it as it goes by.

If it is knocking you need to tear it back apart. Don't run it anymore, that will just damage otherwise good parts.

Rod

Knock,you are just lucky it is still running! Was the cylinder cleaned up/honed, and the bridge relieved in the exhaust, you have to measure the thrust washer clearance with a feeler gauge, some rings are too tight and the gap must be filed to obtain proper clearance, and the bolt missing from the reed cage will cause a lean condition with a little dirt to boot. And the bent ring thing, lots a luck. A well worn motor can look great, actually measuring is not difficult, but does remove all doubt.

The big end of the rod has thrust washers for clearance in between the rod and crank halve, hence, thrust washer clearance. It will tell you one part of possible wear. The other is the up and down movement, none. The axial is how much the top rocks from side to side, a little harder to measure. If you have been missing a reed cage bolt, and running it, it would be a good idea to remove it, clean it, new gaskets and sealer, properly torque it back together. Where did the debris come from that destroyed your engine? The pics are pretty blurred, looks like it ingested a lot of sand/dirt. On the exhaust side of the piston, is there a wear mark down the middle? So the new piston was not checked for skirt clearance, and the new ring gap was not checked also? And the foreign material that destroyed the top end, also went through your crank also.

If there was scoring on the cylinder, it may have needed honing, or a re-sleeve. Adding an oversized piston and ring set is not going to increase the amount of power all that much. It's not a small block chevy. I second the thought that you should tear the engine back down (I know it sucks) as you do need to determine the reason for the knock. When you're putting it back together, go very slowly, following the manuals every instruction. If something doesn't make sense to you, hop on this forum and ask, someone here will be willing to assist. Don't use power or air tools to take it apart, or put it back together, as they are unforgiving, and crankcase halves are frightfully expensive. Check your ring end gap. If it is within the recommended tolerance, ensure that the cylinder is smooth as glass, no verticle ridges. Honing should be done with extreme care (if you have a nickacyl cylinder, take it in to be done). Also, ensure that the exhaust port (and intake) doesn't feel as though it is protruding into the cylinder, use a finger nail and guess. If it at all feels rough, it will destroy your new rings. As you have stated that you are new to the game, you will likely not be able to guess on whether the piston is teh right size for the cylinder, so test the ring gap in several locations from the top to the bottom, as it may no longer be true. If you have any doubts on the sizes, take the cylinder, head, piston and rings in to a reputable repair shop, and have them measure everything.

Alin99 said:

As for the ring gaps, I didn't pay too close attention to this, as I bought all the parts from a KW dealer and the parts are all KW for the 250.

Click to expand...

You absolutely need to tear that top end back down. The knock for one thing but you skipped a very important step and it needs to be done.

Every top end I have done I have needed to file the ends of the rings to get the proper end gap. I just rebuilt the top end on my 250 two stroke. The required end gap is 0.022" and as supplied the rings were 0.008". It only took a few strokes with a file to get the ring gap how it should be.

This is the way it should be done. The cylinder has a certain tolerance on its size and you want the end gap on the rings to be as small as possible. The rings are intentionally provided oversize so you can fit them to your cylinder.

If you don't file the ring ends to get the required gap you have a high probability of seizing the engine the first time it gets to full operating temperature.

Alin99 said:

They fit pretty snug around this little raised dot thing on the piston. Is that bad that I don't know the clearance?

Click to expand...

Yes, it is bad that you don't know the clearance. I am not sure what you mean about the raised dot on the piston.

The rings fit in a groove around the piston. There is a pin in the groove where the end gap of the ring is to be. This locks the ring in place away from the intake/exhaust ports. Speaking of which, the piston also has a front and back side, and needs to be install the correct way. They typically have an arrow on the top that needs to point towards the exhaust port.

Part of the trick of assembly is keeping the rings in place long enough to get the cylinder on. If the rings don't line up the cylinder probably won't go on and if you force it bad things can happen.

Alin99 said:

One of the rings might have an issue, I hope not as we do have lots of compression, but when we put the top half of the engine back over the piston and rings, one of them got caught. It was bent it back into shape and placed in there.

Click to expand...

That ring is toast and needs to be replaced. As I mentioned above, if the ring isn't positioned so that the end gap is over the pin bad things will happen if you force the cylinder on. I am surprised it bent, I would have expected it to break.

Having been bent twice (out of shape and then back) it will be weakened. You do not want that ring breaking a short time down the road.

Bottom line is that the top end needs to come apart again so you can figure out what the knock is. Then replace the rings, gap them to the cylinder, install them with the gaps over the pins, install the cylinder carefully.

Rod

Rebuild

Alin99 said:

Hi,

I just performed a top end rebuild on my 2000 kx250 and while that solved the problem with lack of compression and the bike not starting, it now knocks very loudly. :yikes:

Any ideas? It did not knock before the rebuild when the bike ran. The piston, rings, cylinder head, and spark plug were all replaced during the rebuild.

I'm really afraid to keep it running with the less than a second repeating knock, in case it's wrecking something.

Click to expand...

hey I did the same thing on my 2003 KX 250 and the knocking was annoying and loud. I've rebuild my motor countless times. I found out that its the crank berring that is gone. the knocking went away once the crank berrings were replaced.