Fresh Kohler rebuild knocking

I know threads are in abundance but I want to share my findings and look for a resolution. This is a K301 and I had the block bored .010, installed the "new" style Mahle piston with new style rod, everything else checks out in regards to rod clearance, crank end play, etc.

I read that changing the point gap from .020 to about .012 cured this, so I changed to .012 and the knock went away with only an occasional knock. So I pulled the head to find the piston is rocking back and forth and with it pushed all the way to one side or rocked to one side, I have .021 gap at the top of the piston.

So now I need to discuss options....what should the bore be cut to? Should it have a tapered bore? The skirt checks out but apparently the piston is larger at the skirt and looks lie it was bored to the skirt dimensions.

With the points at .020 it knocks terribly, sounds like the rod is about to let go but the rod checks out. Can I even use a .020 piston now? Will another .010 clean it up and does this need a tapered bore?

 

Are you seriously asking this question? How would a tapered piston slide in a tapered bore?

While I know nothing about this particular engine the ONLY time I have known of using tapered pistons is when the operating temperature of the engine is so high as to cause the upper part of the piston to expand significantly more than the skirt area. Maybe when your engine is operating at rated load it will be okay but my gut feeling is that you have defective pistons.

As for changing the points gap...I assume that you are referring to the ignition points, that large a change has definitely changed the ignition timing. Incorrect timing will cause all sorts of problems.

 

Yeah.

These Kohlers tend to wear the piston closest to the valves and render the cylinder to a slight egg-shape due to the shape of the combustion chamber. The New Mahle pistons have longer skirts to help against the piston rocking and and wearing that side of the crown.
I wasn't sure if there was a process specifically for these Kohlers where the bore may taper very slightly towards the bottom....which is why I asked.

One would think the rings would be stressed this way but I was thinking the area below the oil ring could possibly be tapered maybe a thousandth nearest the bottom of the bore.

I was thinking the piston could possibly be out of spec, I hear that the top of the piston above the top ring can be smaller than the skirt.

But the bore seems to be right at 3.385 which is roughly .010 from stock. The piston is stamped .010 but I have quite a gap at the top and is the same all the way down. I can easily rock the piston back and forth, ring gap is .013.

Seems that changing the point gap to .012 retarded the timing enough to reduce pressure on the piston which is why it was quieter.

Top pic is at TDC, top ring is clearly visible, 2nd pic is BDC.

 

As far as the tapered bore, my take on it would be no, you wouldn't machine a tapered bore. That in itself would be complicated.

As far as the bore being tapered now it could very well be due to wear - a typical wear characteristic of a piston bore. The piston could be mismarked or the machining on the piston bore could have missed the mark.

You can mic everything to check the fit or put the rings in it and check the ring gap for fit to specs. Check for taper by checking the gap at the top of the cylinder, then at the bottom.

I suspect what happened is, if you said to overbore .010 it wasn't enough to take out the taper worn into the lower piston bore.

That brings you to the problem with the knock. I would say a poor fit of the piston to bore and the bore wasn't made large enough. The knock being piston slap.