Spent most of the afternoon replacing the head gasket in my Craftsman lawn tractor which I use for blowing snow in the winter. You might remember this - it was a freebie, and when I got it home and pulled the cover off the top of the engine I was greeted with this mess:
It was packed in solid and was not a fresh nest - it has some oil soaked into it as well as dirt through out which told me that the engine had been run a fair amount with the nest in place.
I cleaned it all out and repaired the chewed-off kill switch wire and amazingly enough, the tractor ran great. I mowed grass with it that summer and cleared my driveway all winter with no problems.
Over this summer, however, I started noticing a chuffing sound under the hood when it was cranked over or as it slowed down when the engine was shut off - a head gasket leak. I was not surprised in the least, given the abuse it saw. That it ran this long with no issues is a testament to the durability of the Command line of Kohler engines.
With winter approaching, I knew I'd have to repair the leaking gasket. I found one on eBay (genuine Kohler factory part) for $10 and free shipping. Today was the day to make the swap. I've never worked on an overhead valve small engine before, but I had a factory repair manual and I'm pretty handy, so I wasn't too concerned.
With the hood off and the front wheels up on a set of car ramps for easier access, here's where we start:
Blower shroud and valve cover off. I don't think I have EVER seen an engine that was so clean inside. Wow! I think you could eat off of that.
I cleaned the nest out, yes, but look at what probably years of exposure to mouse urine before I got it has done to the cylinder fins. Yuck.
Head is off. Do you see the problem? Luckily, the marks are just that and not channels cut into the head like I thought they were at first. THAT wouldn't have been good.
And, the cylinder with the gasket still attached:
A closeup of the cylinder wall. Looks great, with the cross hatch still visible. Again, a testament to the Command line of engines for it to be in this condition.
After lots and lots of cleaning with a razor blade to get the gasket remnants off of the head and block as well as a general decarbonizing of the piston and cleanup of the tinwork, assembly was pretty much the reverse of disassembly.
One thing I ran into, though - this engine has hydraulic lifters just like a typical pushrod automobile engine. Here's a picture to show how they work:
The rocker arms on this particular engine are non-adjustable; according to the manual some versions were. When I removed the head, the lifters "pumped up". In other words, the internal spring in the lifters forced the plungers (which transmit the motion of the cam lobes to the pushrods) to the maximum extension. As a result of this, when I reassembled the engine and went to start it, the engine spun over very easily, would not start, and backfired badly though the exhaust and carburetor because the lifters were not allowing the valves to close fully. The manual didn't say anything about how to deal with this. I know that with an automotive engine, you generally loosen the rocker arm adjustments and then once the engine is running you crank each one down until the tapping stops, then slowly turn one more full turn on the rocker arm adjustment to push the plungers down in the lifer bore somewhat - this gives the lifter some range of adjustment for it to do its job. You have to do it slowly to allow the oil in the lifter to bleed off past the plunger, too fast and it will leave the valve hanging open. In an engine with very little clearance between the piston and valve, you can get into trouble if you are too hasty.
My solution was to loosen up the screws that fastened the rocker arms to the head until I felt some free play in the rocker arm, than start the engine and slowly tighten each bolt down until it ran out of threads and tightened up. While this worked, it did take time and was kind of messy from all the oil being pumped up through the pushrods. I ended up stuffing some paper towels around the rockers to keep as much oil as I could from dripping onto the muffler while I slowly tightened up the bolts a fraction of a turn at a time.
With a single cylinder engine, you don't have any other cylinders to help motor the one you're working on along while the valve is hanging open until the lifter bleeds down, so I had to go slow. Too much, and the engine would die. I finally got it, though, and it runs like a top now.
Here's a video I shot that shows how the compression release works. There is an auxiliary exhaust lobe on the cam that only comes into play when the engine is stopped or turning over very slowly, like at the speed the starter would turn it over at. This cam lobe partially opens the exhaust valve at the beginning of the compression stroke, effectively reducing the compression ratio which makes it easier to turn over while still maintining enough compression to start up. As soon as the engine fires up, centrifugal force pulls that auxiliary lobe out of the way and the valves work as any normal engine would. I think it is a pretty neat idea.
It's not far removed from the function of a Jacobs engine brake, except that a Jake brake pops the exhaust valve open at the top of the compression stroke, not the bottom.
Wrapped it up just as it was getting dark - now all I need to do is get some oil and a filter, change the oil, and attach the snowblower and I'll be ready for winter.