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Thread: Boring a Kohler cylinder
09-17-2005, 12:42 PM #1
My son has a 10HP Kohler engine on his 1982 John Deere lawn mower that has started to give up. It burns a good bit of oil and has lost power. I would like to rebuild the engine for him this winter and I need some advice. I can buy .010, .020, and .030 pistons for this engine pretty darn cheap. It has a cast iron sleeve so there is something there to work with. Can you use a cylinder hone to bore it, can I bore it on my milling machine, or do I have to take it to an automotive shop like NAPA to get it bored. Pardon the possibly stupid question here but I do not have any experience with this procedure. I rebuilt my 8N tractor but the cylinders did not mike out with any taper or appreciable wear. The only reason I mention this is so that you know that I at least have some idea of how an engine goes back together. Thanks for any help you can offer.
09-17-2005, 01:41 PM #2
I would suggest that you don't decide to bore the cylinder until you see such is needed. Much of the time in our engines the major problem is valve guide wear.
Otherwise, yes you can bore it on a mill if the clearnaces and travels will allow the tool access to the cylinder. Just indicate the cylinder for parallel and required appropriate centerline.
I also suggest you price a short block to see if such compares favorably to the work of a one-off major rebuild.
Sometimes we get a short block and modify it for longer life before putting it into service. Usually around the valves, bronze guides bronze seats, stellite valves etc.
We also "plumb up" the crankcase drains to make the P/M of changing oil most convenient, insteaad of a sloppy mess.
Synthetic crankcase oils also help the valve train on poorly cooled small engines last longer.
09-17-2005, 02:58 PM #3
Thanks for the information agrip. What led me to believe this engine needs a rebuild is not so much the oil burning but the fact that when it gets hot after running a while, it starts to miss and lose power and it seems to smoke more. I replaced the carburetor this year ($200.00), set the points and changed the plug for him. It seems like when it gets loaded down and hot, it immediately starts running poorly but will run OK again after giving it a rest of a day or so. I think some clearances must be out of whack but I can't say for sure. Thanks again.
09-17-2005, 03:14 PM #4
I think you have possibly two problems -- one might be oil burning, but I also think you have either a temperature sensitive coil or condensor which is what is causing the miss you are encountering when hot.
09-17-2005, 03:41 PM #5
On my 14hp Kohler (K321?), it was starting to burn oil. Turns out the piston was warn considereably, but the bore was in spec. The top ring land was warn away so much, you could see the ring with just the head off. Skirt was also warn. Most of the wear on the piston was near the valves (more carbon to be abrasive?). Re-ground the valves on a Soix grinder, re-cut the seats with a Neway cutter, honed the cylinder, and bought a new standard size piston and file fit rings. Guides were still tight, rod big end looked excellent. Runs like new. --Doozer
09-17-2005, 03:43 PM #6
I agree, you need to check out the ignition rigging.
If you replaced the carb for that much, you probably need a small engine manual.
Here is a couple sources parts and manuals
Jacks Small Engines
1747B WEST JARRETTSVILLE ROAD
JARRETTSVILLE, MARYLAND 21084
Somewhere around Cockeysville, N of Baltim'r
09-17-2005, 04:37 PM #7
Consider a compression test for this
motor. Do it dry, and then add in a bit
of oil. If the numbers come up then, it's
likely worn rings/bore.
09-17-2005, 07:30 PM #8
Before doing anything major, you might want to check valve clearances. The symptoms you describe can be caused by lack of clearance. As the engine heats up the valves begin to not seat properly and compression drops.
09-17-2005, 10:50 PM #9
Thanks very much for all the replies. I will take each into account.
09-18-2005, 01:15 AM #10
So far all information given you has been very good, however no one really answered your question about the ability to bore the cylinder yourself should it truly need boring. My answer is yes, it can be done. I did a 8 HP Briggs and Stratton on a 10" Logan lathe after making a boring bar to fit in the chuck and attaching the engine block to the cross slide and making repeated cuts. I finished it by using a cylinder hone in a drill to achieve the proper cross hatch pattern and final size. If you have a milling machine the task should be even easier. Go for it!
09-18-2005, 01:44 AM #11
If this is the k series kohler-the 10 hp will be a k241--24 cu in.I have worked with quite a few of these and the larger brothers.I have a cylinder boring bar but I prefer to use a sunnen hone of the positive expanding type, electric drill powered--coarse stones to within .0015-.002 of finished size,medium stones to finish, and then polish with fine stones or wrap the mediums with 400 grit wet or dry paper.If the stones are in good shape you can have it opened up .010 in 5 min and you should use the hone to finish anyway if you bored it.You can't get a finish for chrome rings too smooth with polishing stones and remember the ring life is increased with a smooth finish to start with.With this hone you can't detect the finished bore out of round by .0005 and you can easily keep the taper top to bottom within .001.Last clean up with soap and water several times.Measure the crank throw carefully-- they can look perfectly smooth and be out of specs on taper and roundness.Good luck.
09-18-2005, 02:08 AM #12What led me to believe this engine needs a rebuild is not so much the oil burning but the fact that when it gets hot after running a while, it starts to miss and lose power and it seems to smoke more. I replaced the carburetor this year ($200.00), set the points and changed the plug for him. It seems like when it gets loaded down and hot, it immediately starts running poorly but will run OK again after giving it a rest of a day or so.
If you had a valve that stuck some when hot it might do that. They can be very intermittent in my experience.
Ditto maybe for rings sticking along with some wear.
You changed the plug...they do that too. Get hot and mess up.
Worn bores AFAIK don't tend to change much, they seem to pretty consistently do what they do.
09-18-2005, 03:59 AM #13
I have a 35-year old Cub Cadet lawn tractor with a Kohler engine that burns oil so bad, it looks like I am fogging for mosquitoes. Except for lacking a little power, the engine starts easily and runs good. If it wasn't such a good running little engine, except for having to clean the plug every so often, I'd overhaul it.
As for the miss when hot, another thing to look at is the spark plug. Even brand spanking new Champion plugs have given me problems in small engines -- I have heard it has something to do with low compression engines and the "new and improved" gasoline formulations. I have had real good luck with Autolite plugs.
09-18-2005, 09:17 AM #14Rustystud Guest
I worked in a small engine shop while in highschool. In fact that is where I developed my interest isn machining. If the engine has as much wear as you say I would concider another option and that would be to short block the motor. You have 5 areas of concern with a 10 hp and up engine. Ususally the valves give up first. At they give up the back preasure does the carburator. If the rings have gone you should also be concerned about the crank journal, connecting rod and main bearings. I have rebuilt hundreds of small motors. Do you have a mill and a boring head large enough and the personal time do it your self? Before you order any parts measure the concentricity of the cylinder at both the top end and the bottom of the stroke. If your mill is not long enought to bore the cylinder in one stroke take it to a machine shop with a real boring machine. You might find that after you replace everything and do the machining the short block with a warranty is a good investment.
09-18-2005, 10:17 AM #15
Gentlemen, I am a bit confused in one respect. I think enginenut is saying you can repair the cylinder with an expanding type hone (three spring loaded vertical stones) which I have. Others are saying you can bore it or have it bored. I would think that if the cylinder was tapered then the hone would just follow this taper and make a bigger taper. I think it will fit under my mill and I know it would fit on the carriage of the lathe. I know a short block would solve my problem but I have more time then brains and I would like to accept the challenge of fixing this motor. I have replaced the coil, condenser and adjusted the points. What is kind of wacky about this problem is that it will be running along pretty good until you hit a chunk of really high grass or some other obstruction and then it runs weak and smokes. It is not really a miss. It is more like it will not take gas all of a sudden. It always idles nicely but when it acts up, advancing the throttle sort of chokes out the engine. That's why I replaced the carburetor. I also rebuilt the fuel pump and checked the delivery pressure. I do not at this point think it is starved for gas, it just won't take fuel. I think something is sticking or changes quickly. J Tiers is I'm sure correct when he says that something worn out is not going to change conditions quickly. The engine smokes some all the time but seems to smoke more when it starts acting up. A neighbor of mine said he had an engine that had a partly sheared key in the flywheel and it would do strange things some times when the flywheel would shift a little. I might be going out on an ignorance limb here but is it possible for the valves or rings to stick or something like that? Again, I can't thank each of you enough for your input. I take all of your comments into consideration and I will try to take an educated guess as to what is wrong. I am relying on you guys for the education.
09-18-2005, 11:04 AM #16
You can rebore it with a hone, but not a spring loaded hone. It's slow, but can be done. A spring loaded hone will follow the taper and out of round. What those 3 stone flex hones should properly be called is a glaze breaker for re ring jobs where the cylinder isn't worn enough for a rebore, and really they don't work all that great for that. For a glaze breaker, one of those abrasive ball tipped brushes, or what I call a berry hone, works a lot better. What you'd need to use is a rigid hone such as a Sunnen or Lisle. It'll take a good heavy duty 1/2" or 5/8" drill to power it. This is not really a good job for a mill, a drill is a lot easier, faster, and more convenient. Start with it tight at the bottom of the cylinder, and keep it tight, working upwards as the bottom of the cylinder becomes larger using coarse stones, checking your size often, and switching to finer stones, 400 grit or even 600 are fine to finish. When finishing, it's best and most accurate to extend your strokes so that the hone extends about halfway out of the cylinder at either end, although that's not always possible at the bottom end, due to possible interference with the crankcase casting. Flood the bore with either kersosene , diesel fuel or light engine oil to keep the stones from loading. Done carefully and with good stones, it'll come out round and straight within a couple of tenths max. Even if you bore it first, you'll need to stay at least .0035 undersize with the boring in order to clean up the boring toolmarks with the hone afterwards. Don't worry about coarse crosshatch honing marks. A good straight cylinder does not need them to seat the rings. What people refer to as ring seating in actuality is cylinder seating, as the rings are a lot harder than the cylinder. If the cylinder's straight and round, the rings will seat immediately. Wash the cylinder with warm water, a brush,( small scrub brush or even a fingernail brush works well) and dishwashing detergent afterwards. Wiping it out with oily rags will never get all the grit out of it like soap and water will.
One more point, do NOT measure the worn cylinder and buy an oversize piston based on that measurement before boring/honing it oversize. In my experience on automotive engines, if theres enough wear so that theres a detectable ridge, it'll take at least an .030 overbore to clean up the cylinder.
All this is not as hard or time consuming as it sounds. I've done it in a pinch on quite large V8's and if I was set up, ready to go with tools at hand I could have had one bank about done in the time it took me to type all this lol.
09-18-2005, 12:19 PM #17
I think you need to determine for sure what
is wrong with this machine before you fix it.
First off you need to do a compression test
and a leakdown test, both cold and after it
has been acting up.
The leakdown test will tell you if there is
a compression loss, and if it is from rings,
intake, or exhaust valves.
If you are not blowing air out the crankcase
vent, then there is nothing wrong with the
Is this motor hard to start when it is hot?
If so then suspect the mag coil. I've seen
more carb and ring problems go away when
a weak ignition is repaired.
I will also relate a very recent series of
events that happened to me. This bike:
had the ignition advance unit blow up while I was
out riding one day. The timing was all messed
up but stupidly I tried to limp it home. There
was major damage to the pistons so I wound up
having to do a top end job on it. Bore, hone,
new pistons and rings.
AFter re-building it it had great power but one
jug used a lot of oil, and smoked.
Compression was fine, but the plug was wet with
oil on that side.
In frustration I finally tore that side down
again to inspect the scene of the crime. The
only thing I could find that was possibly amiss
was with the third, oil control ring.
This was of the newer, three piece design, with
two very thin scrapers and a wavy expander ring
that fits between the side rings. The expander
ring had the corrugations so that one end was
going "up" whereas the other end was cut so that
it was going "down" with the result that the
ends of the expander could slip by each other
Because I always save teardown parts, I inspected
the expander ring that I took out - it had both
ends directly opposed (both pointing "up") and
there was no chance the ring ends could catch on
In desperation I simply re-installed the older
The bike has not burned any oil since that time.
A very small thing, and I am not positive that
there was not some equally small change during
the teardown that mattered - but I think from
now on I will look at the expander rings very
carefully when installing them....
09-18-2005, 03:54 PM #18
OK we thrashed the cylinder resizing sufficiently I think.Now lets talk about your symptoms and an inherent thing we see often with this engine.For some reason the k series kohler likes to build stem deposits on both valves but usually it's the ex that suffers by leakage leading to burning.The ex valve expands a lot as heat, which is increased by added load, increases so we see a good running cold engine but when working hard the valve/guide clearances decrease,the valve seats poorly if it doesn't stick open, and the engine slows or dies completely.After cooling it goes back to work.If the exhaust weren't muffeled so well you might hear the leakage or it may fire out the pipe.The cure will to be remove and reseat the valves, clean or replace guides and reassemble with correct lash.The lash adjusting screw will be indented from contact with valve stem and will prevent proper lash adjustment with feeler but just resurface this screw in the stem grinding fixture of the valve facing machine.Correct lash is important to proper operation of the automatic compression release on these engines and affects the life and performance of the starter motor.While on the subjectur newer valve in head air cooled engines need more frequent lash adjustment, if not hyd done as some, to protect starter life and performance,valve life, and good engine performance.The old "flathead" had less wear points in the valve train so many times could go untouched between rebuilds but not so the overheads.Check your manual for the recommended adjustment intervals and believe me it is important.One of the first overheads we saw was the Tecumseh and many burned up the expensive starter due to lash increase which made the compression release ineffective.IMHO the 10 hp Kohler is even more durable than the 12/14/16 which are the same size and design block because it was less highly stressed if not overloaded.Good luck and don't scrap that great little Kohler.
09-18-2005, 04:03 PM #19
I should have mentioned if new guides are installed, count on passing a guide reamer through the newly installed guide as it will be slightly undersize.Any auto machine shop will have the necessary reamer.Also if you install new guide, I would make a shoulder punch and drive it with a pneumatic hammer-use antiseize lubricant and watch for signs of galling.I have seen blocks split when the guide being driven seized and a bigger hammer was used.I have pulled them in place with threaded rod or bolt.
09-18-2005, 04:07 PM #20
I'm glad to read the expert detail on this engine here. I have one of these 10 HP "K" series in an old JD and am printing this out to put in its file for future reference.
A friend who works on these occasionally, says to be careful with the governor and the crank balancer, as they are hard to figure out and get right even with the exact manual page in front of you, and a real bad idea to work on without, esp. if you don't already have a lot of small-engine experience. Your mileage may vary...