I have been thinking about Jeremiah’s engine with the valve issue and what likely happened, so get out your darts.
https://www.ihcubcadet.com/forum/messages/4/289670.html?1409061193
It appears the valve hit the head many times and the valve stem is bent (well most likely).
Some believe that that the valve did NOT hit the head and that is almost impossible. However the head had witness marks right above the exhaust valve, more on this later. A loose impediment floating around the combustion chamber would have left marks everywhere and not just over the exhaust valve. Normal engine operation does not allow the valve to hit the head, something else had to happen. The mostly likely candidates are broken valve stem, retainers, spring, etc.
It is now later. The retainers failed and the valve is free to go upward, but limited by the head. An engine running at 3600 rpm has an exhaust valve rising every other cycle. This means the valve rises 1800 time per minute or 30 times per second. The floating valve would cause loss of compression and the engine would quit. Let’s say for argument sake the engine took 5 seconds to come to a stop. An engine with compression loss will continue to rotate longer than one with good compression. Assuming the engines RPMs fall in a linear manner, the valve will strike the head 75 times in 5 seconds before the engine stops.
The witness marks on the head are not in the same location as you might expect, there is this possibility. Imagine trying to drive a nail with a hammer into a piece of steel plate. No matter how square or true you hit the nail on the head, it will eventually bend. This is the same for the valve (nail) and the engine head (hammer). In this case the nail is moving and the hammer is still, but the principle is still the same. The valve started hitting the head directly above the valve. Then the valve stem began to bend just a little and the valve started hitting the head in a slightly different spot. The lifter and valve rotate due to the cam’s contact with the lifter is not centered. Thus the bent valve makes contact in an offset rotating manner as indicated by the witness marks on the head. The valve bends more and more and the offset hit become more off center ( directly above the valve guide). The engine quits and the valve is resting off the seat
Does the 42# valve spring rate force relate/counter the force the cam imparts on the lifter and valve? The valve springs main purpose in an engine is to keep the lifter riding on the cam lobe at all operational RPMs. When RPMs exceed a certain level the lifter will no longer ride on the cam lobe, the valve will “float”. The spring force has to be great enough to keep valve/lifter riding on the cam lobe, especially when the valve changes direction and is closing. When the cam lobe surface is traveling away from the lifter, the springs force keeps the lifter moving at the same velocity and direction, downward in an upright Kohler. As engine RPM increases so does the velocity of the cam lobe, and thus acceleration is present (F=MA). The spring needs to accelerate the valve and lifter fast enough to keep up with the cam lobe surface heading in the opposite direction. The upward force of the cam imparts on the valve and lifter is generated by the engine’s combustion. Through the gear reduction (crank and cam) and small height travel of the lobe when compared to the crank shaft rod journal height travel, the force applied through the cam to the valve and lifter is greater than the valve spring rate. The spring rate force on the valve train is not as great as the cams force. The cam has enough force to slam an unrestrained valve into the head, likely enough to launch it out of the valve guide. Assuming the head is missing and RPM = 3600.
https://www.ihcubcadet.com/forum/messages/4/289670.html?1409061193
It appears the valve hit the head many times and the valve stem is bent (well most likely).
Some believe that that the valve did NOT hit the head and that is almost impossible. However the head had witness marks right above the exhaust valve, more on this later. A loose impediment floating around the combustion chamber would have left marks everywhere and not just over the exhaust valve. Normal engine operation does not allow the valve to hit the head, something else had to happen. The mostly likely candidates are broken valve stem, retainers, spring, etc.
It is now later. The retainers failed and the valve is free to go upward, but limited by the head. An engine running at 3600 rpm has an exhaust valve rising every other cycle. This means the valve rises 1800 time per minute or 30 times per second. The floating valve would cause loss of compression and the engine would quit. Let’s say for argument sake the engine took 5 seconds to come to a stop. An engine with compression loss will continue to rotate longer than one with good compression. Assuming the engines RPMs fall in a linear manner, the valve will strike the head 75 times in 5 seconds before the engine stops.
The witness marks on the head are not in the same location as you might expect, there is this possibility. Imagine trying to drive a nail with a hammer into a piece of steel plate. No matter how square or true you hit the nail on the head, it will eventually bend. This is the same for the valve (nail) and the engine head (hammer). In this case the nail is moving and the hammer is still, but the principle is still the same. The valve started hitting the head directly above the valve. Then the valve stem began to bend just a little and the valve started hitting the head in a slightly different spot. The lifter and valve rotate due to the cam’s contact with the lifter is not centered. Thus the bent valve makes contact in an offset rotating manner as indicated by the witness marks on the head. The valve bends more and more and the offset hit become more off center ( directly above the valve guide). The engine quits and the valve is resting off the seat
Does the 42# valve spring rate force relate/counter the force the cam imparts on the lifter and valve? The valve springs main purpose in an engine is to keep the lifter riding on the cam lobe at all operational RPMs. When RPMs exceed a certain level the lifter will no longer ride on the cam lobe, the valve will “float”. The spring force has to be great enough to keep valve/lifter riding on the cam lobe, especially when the valve changes direction and is closing. When the cam lobe surface is traveling away from the lifter, the springs force keeps the lifter moving at the same velocity and direction, downward in an upright Kohler. As engine RPM increases so does the velocity of the cam lobe, and thus acceleration is present (F=MA). The spring needs to accelerate the valve and lifter fast enough to keep up with the cam lobe surface heading in the opposite direction. The upward force of the cam imparts on the valve and lifter is generated by the engine’s combustion. Through the gear reduction (crank and cam) and small height travel of the lobe when compared to the crank shaft rod journal height travel, the force applied through the cam to the valve and lifter is greater than the valve spring rate. The spring rate force on the valve train is not as great as the cams force. The cam has enough force to slam an unrestrained valve into the head, likely enough to launch it out of the valve guide. Assuming the head is missing and RPM = 3600.