Kohler k91 rebuild

[Jeffn 1]

Hi all,
I'm new to the forum. I have a kohler k91 from the late 50s that I'm going to put on a vintage go kart. The top ring shattered and has scraped the cylinder about 1/4 of the way down. I've taken measurements, and the cylinder appears to be 2.375 to 2.376 at the top which is spec for a new bore. At the bottom it appears to be about 2.372, which is less than new spec. The scrape is about 0.004 to 0.005 deep. I'm wondering about honing it myself with a spring loaded 3 stone hone as i'd only need to take about 0.004. It seems to be round and the taper is within spec so I'm not worried about truing it up. That would keep it within spec for a std piston and rings. Thoughts?

Also does it make any sense that the bottom of the cylinder could be less than spec, or does that mean I'm measuring wrong?

If I do it myself I can get it done... If not I have to wait until machine shops can operate again.

Thanks for the help!

[WHX?? 48,767]

jEFF

I would think you would be ok just to try and hone the scrape out and see what shakes for a ring end gap. You really don't have anything to lose in trying it and as long as your not flying that kart at 30,000 feet. Just be careful you don't take it out of round or put a taper in it. 

[squonk 41,020]

If you don't mind taking it right back apart again try honing. Hones don't really take off much material unless you're using a Sunnen machine with the proper stones. Your arms will get a workout. A cylinder that "seems round usually isn't. I would bore that block honestly.

[The Tuul Crib 7,336]

3 minutes ago, squonk said:

If you don't mind taking it right back apart again try honing. Hones don't really take off much material unless you're using a Sunnen machine with the proper stones. Your arms will get a workout. A cylinder that "seems round usually isn't. I would bore that block honestly.

[The Tuul Crib 7,336]

I agree . The cylinder will be somewhat of an oval shape. All the wear will be on the front and the back of the cylinder.  The sides won't have as much wear.

[squonk 41,020]

With Those vertical marks in the bore, The piston has to be scuffed up too.

[Rob R 966]

I agree with some of the other wheelers hone it yourself after cleaning and using copious amts of WD40 or some light machine oil. Replace the piston and the rings and you should be a happy camper. p.s. just hone going up and down slowly in a continuous motion..... with medium rotation about 12 times...… DONE! 

[953 nut 55,065]

3 hours ago, The Tool Crib said:

I agree . The cylinder will be somewhat of an oval shape. All the wear will be on the front and the back of the cylinder.  The sides won't have as much wear.

The greatest amount of wear will be along the middle of the cylinder. When the piston is at the top or bottom of the cylinder the crankshaft and connecting rod are straight in line with the cylinder so no side force is being exerted. As the piston travels up or down it exerts pressure on the cylinder wall because the crankshaft is now off-set from the cylinder. This will make the center of the cylinder wall become egg shaped and result in compression loss and ring blow-by.

[The Tuul Crib 7,336]

4 minutes ago, 953 nut said:

The greatest amount of wear will be along the middle of the cylinder. When the piston is at the top or bottom of the cylinder the crankshaft and connecting rod are straight in line with the cylinder so no side force is being exerted. As the piston travels up or down it exerts pressure on the cylinder wall because the crankshaft is now off-set from the cylinder. This will make the center of the cylinder wall become egg shaped and result in compression loss and ring blow-by.

That's exactly what I was trying to describe but I couldn't quite put it in words!

[squonk 41,020]

All I know is I have a worn out K341 that has a bore 10 times better looking than that one 

[Howie 886]

What everyone elsse has said, also the three stone srping loaded hone is

only to break the glaze on the cylinder wall not remove metal. They are a 

fine grit stone on them anyway. If you had what they call a rigid hone you

could use the coarse grit stone and take out some. 

I have opened up cylinders here .010 with one of these, start at the bottom

where it is the roundest and work upward. And yes I have a dial bore gage

to check it with. 

[oliver2-44 9,679]

Trying to hone out .004 ths with a spring hone is a hard task even if you put on coarse stones.  Also as others have said a spring hone will only make an oval shaped bore even more oval.  

Boring is the way to go.  Be advised, . 010 or .020 ths oversize pistons and rings for K91's are getting a little hard to find and or pricey on ebay.   

[Pullstart 62,785]

 @Jeffn!

[tom2p 2,394]

score = bore 

 

 

[richmondred01 2,237]

I hate to be the bearer of bad news but Honing not going to fix correctly fix that. 

It needs to be bored.

You will need a new piston and rings too. While you are into the engine spec the crank it most likely needs to be turned.

Before you spend the money on machine work you may want to get the parts in your hand and have the machine shop fit them.

like was mentioned before, the .010 and .020 over piston and rings are difficult to find. The undersized rod will be even more difficult. 

 

 

Edited April 11, 2020 by richmondred01