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Flying Blind - Overhauling Kohler Engine
I have taken my 1983 Kohler K301 12HP cast iron engine off my Gravely and have taken it apart in the basement. I will have a neighbor come over to give me an assessment of the piston, rings and valves. I am anticipating having to purchase a master rebuild kit for about $100 which has all gaskets, pistons, connecting rod and rings.
What I am perplexed about is that the sellers ask you what size piston you want - standard or .01, .02, .03 oversize. I understand that these are options for worn cylinder walls, but how do I assess which would be best for me? How do I measure the cylinder size? Is there a special tool I need to purchase to do this? Once I know the diameter of the cylinder, how do I know how much clearance I need? Do I subtract the clearance required from the cylinder diameter and get the largest piston under that size that will fit?
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Flying Blind - Overhauling Kohler Engine
You use an inside micrometer to measure cylinder bore. The manual will have specs. On automotive engines if the cylinder is beyond a certain spec you usually have it bored out to the next standard oversize and install larger pistons. I imagine the Kohler is similar. If the bore is within specs you hone it out and install the same size pistons.
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Flying Blind - Overhauling Kohler Engine
Thanks Ken, I bought a service manual for this engine. Maybe I can find a range for the specs in there.
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Flying Blind - Overhauling Kohler Engine
There are probably special tools for measuring but the inside mic always worked for me. It's a good idea to measure it at several different positions to check for out of round and also at the top, middle, and bottom of the piston stroke to take the widest measurement. They will usually wear more at the top than bottom. It's been 30 years since I've done that kind of work and I sort of miss it.
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Flying Blind - Overhauling Kohler Engine
The best way at this point is take the block to a rebuilder/machine shop. Most auto supplys can steer you or check the yellow pages.
You will not have the machines (well maybe you do but then you'd know this anyway) to bore the cylinder or be able to check for minute cracking.
They'll tell you how much boring is needed/was done then you can order your piston set.
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Flying Blind - Overhauling Kohler Engine
You can get a pretty accurate inside reading with an extendable rod that will touch both sides of the cylinder wall. There are expensive manufactured units but you can make one easily out of a couple of pieces of metal. It would be like sliding a pencil into a drinking straw and then threading a screw in the pipe portion to clamp the inner part at the desired length. The pipe portion shouldn't be much larger diameter than a drinking straw. Of course you still need a micrometer to measure the apparatus length when you remove it but you can get a pretty inexpensive vernier or dial caliper for $15-$20 if you don't have one. They're good to have around. You can get an idea of the wear by measuring the upper & lower part of the bore vs. the middle. Your overbore would need to be more than this difference for equal diameters throughout.
The company might also have a maximum piston to cylinder wall gap. Just put the piston without rings into the bore and measure the excess gap with a feeler gauge. Again, you can get an idea of total wear by sliding the piston up and down and measuring to find the difference in bore diameter. There will be little or no wear above the point of ring travel. If it's out of spec, you'll still most likely have to have the boring done at a machine shop. If it's in spec, you can just hone the cylinder walls and install new rings.
These specs could be available on the net depending on how popular this motor is. Have fun, there's nothing more satisfying than hearing a freshly rebuilt engine fire for the first time.
Dave
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Flying Blind - Overhauling Kohler Engine
All you need to measure the bore is a telescoping bore gage and a micrometer or caliper to measure it with.
Link:
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Flying Blind - Overhauling Kohler Engine
Most machine shop around me will bore a cyl for about $40. If possible I would only go .010" or 10 over. That saves room for future rebuilds. Get the spec out of the manual of ask the parts guys and they would be able to get for you.
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Flying Blind - Overhauling Kohler Engine
Before you do any boring, or order any parts, it is CRITICAL that you measure the cylinder.
You have to take three measurements in two directions each. Top, middle and bottom of the cylinder. Take each of those measurements twice, 90° from each other. This will tell you how much the cylinder is worn and where. The cylinder does not wear uniformly top to bottom.
You want to bore the cylinder JUST enough to get it back to straight, parallel walls.
That will determine how much you need to oversize.
Any good automotive machine shop will do the whole job for you for not much money if you leave it with them as a "fill in" job for a few days. Just explain what you want to do, and the range of over-sized parts available and they will determine the amount of bell-mouth to the cylinders and machine the block accordingly.
Best of luck.
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Flying Blind - Overhauling Kohler Engine
Engine assembly is complete. It turned out that the top piston edge facing the exhaust valve was worn somewhat. It looked almost as it had been scuffed over a one inch range. The piston is aluminum but the block is cast iron and the cylinder was fine. I installed a new piston, rings, new valves, new fuel pump, new fuel lines, cleaned carburator, new gaskets etc.
I got myself a telescoping guage, dial caliper and measured about 30 times at various positions in the cylinder. I was not happy with the quality of the telescoping guage but according to my measurements it appeared the cylinder was in decent shape andd within specs. So I got a standard size piston and it seemed to fit well.
Tools cost me as much as parts - telescoping guage, dial caliper, ring expander, ring compressor, valve lapping tool and compound, 2 types of valve spring compressors - one to get them out - one to get them in, fuel line, emery cloth. Parts cost me about $200 total - tools $200 total. About $200 more than I was hoping to spend.
It was too late to try putting the engine on the tractor today - that requires some additional work, new fuel line from tank - and this engine is heavy. So next weekend will probably be the day. Wish me luck!
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