A bit of TLC

This is the cooling shroud that covers the cylinder head - it has a few cracks that I thought I'd repair.

OK the brazing is a little rough. It's not because I'm out of practice, I'm just a ham-fisted welder.

What was gold is now silver

No more Cub yellow here - a few hours with the sand blaster and the old paint and rust are history. A few bottles of spray paint and these parts will be looking like new.

Trying to save the coupling

(Sorry for the blurry picture.) This is the main coupling that connects the engine to the transmission. Two of the four mounting bolts were missing when I removed the engine from the tractor. They were twisted off, probably broken during use I'd guess. I tried to recover this part by using a bolt extractor to dig the broken pieces out, but they weren't budging and I broke the extractor bit inside one of the bolts. Ho hum. For the moment I'm going to let this piece just sit. I'll price out a replacement - it'll have to be pretty expensive to warrant any more work on this thing.

Home measurements #3

The last measurement I was able to make today was the clearance on the connecting rod big-end. In the photo:
(1) Is the connecting rod -- connects the crank to the piston
(2) is the oil dipper -- it helps to splash oil around and lubricate stuff (this engine has no oil pump)
(3) the crank (if the piston is the heart, this is the legs)
(4) the rod little-end (where the piston goes)

I like doing this measurement because it's like opening up a present -- you never know what you're going to get until you take it apart. In the photo above I've installed a small strip of plastic called a "plastigage" inside the joint between the connecting rod big-end and the crank. I've re-installed the end-cap (where the screws are, near the dipper (2) ) with the plastigage in place. I torqued the screws to their spec -- this squishes the plastic strip. Take it apart again and ...

... a paper gauge that comes with the "plastigage" kit tells you the clearance. The more it's mushed, the smaller the clearance. Cool! Result? You can't see from the photo but the width of the smushed plastic shows about 0.003" of clearance -- right on the wear limit called for in the Kohler manual. Right on the edge. This is one measurement that is OK and we don't need to do anything about.

Home measurements #2

OK we need a re-bore. Damn. That means a new piston and new rings. Just for the heck of it I measured the wear on the piston rings. The rings sit in grooves in the piston (see an earlier post for a pic) and seal it to the bore so as it goes up and down gases don't escape around the edges. Very important. In the photo above I've removed a ring from the piston and have installed it in the cylinder by itself. The rings aren't solid like a ring on your finger, they are cut and spring open a bit when you hold them in your hand. When installed in the cylinder they compress and fit very snuggly. The end-gap, shown clearly in the photo above, is an indication of how much the ring has worn. This is an important measurement because you can imagine gases leaking by the little slot that this end-gap leaves open. The gap should be about the thickness of two sheets of paper put together (0.006") but it's more like 0.060" -- ten times too big. The photo below shows how this is measured using feeler gauges.

What does this all mean? A large end-gap like this means lots of gases can escape past the rings, and the engine doesn't perform as it should. Basically an internal combustion engine is a gas pump -- as the piston moves up and down it pumps gases around. Leaky rings means it isn't efficient and has less power than it could.

Home measurements #1

With a micrometer I measured the diameter of the piston. This is not my micrometer but a loner from work. Subtracting the two diameter measurements gives the bore clearance. I won't put meaningless numbers here, but the clearance is out-of-spec which means we need to do something about it.

My friend Surinder

The machinist/engine mechanic had the tools to measure the bore -- this is the diameter of the big shiny hole where the piston goes. I hadn't counted on him doing those measurements for me, but he was willing (call it marketing I guess, since he is the one who'd machine the bore if needed). But I was headed to work anyway as my friend Surinder, a metrologist, had kindly volunteered to measure the bore for me using a hugely expensive piece of equipment called a co-ordinate measuring machine (his baby). I kick myself for not taking the camera into work because it was very cool watching him do this using this very cool machine. Basically the machine is a robot with a little glass ball on the end of a tiny rod called a probe. Think Demon Seed, except there were no unwilling human participants and certainly no DNA involved. Anyway he moves the probe and every time the glass ball touches something it stops automatically and records the position where it touched. Do this a few times, and the machine has enough information to figure out many things -- in this case bore diameter. It is 3.3769". Ooooh! What does that mean? Nothing without more measurements ...

(Incidentally, this measurement agrees with what Fred the machinist measured using a $200 tool, but I feel so much better getting the number from a machine that costs something on par with a nice new BMW sedan ... :)

Machine shop visit

I took the block to the local machine shop for a professional assessment about what it needed. I was there mainly to have the machinist look at the exhaust valve seat, but he gave the whole thing a once-over. He thinks the seats are OK. But, after seeing the intake valve his first impression was "new valves, since it's apart." This scared me a bit since the machining cost will be "about $80 per valve" (and there are two). That blows my thumb-in-the-air machining budget of $100 out the window. Also, he recommended a re-bore as well at a cost of $50 (more on that later).

After the machine shop I took the block into work for some more measurements ...

Bath time

Stinky varsol bath.

Rinsed and dried, ready for bed.

Next steps

1. Measure the cylinder bore. Need to do this to figure out the piston/bore wear and ensure it's within spec.
2. Measure valve guide diameters -- to ensure everything is within spec.
3. Clean
4. Get quote for replacement parts
5. Order parts
6. Back together again!
   

Shiny happy face seats

After seeing that valve the other day I was really worried what the seats would look like -- the other half of the seal formed with the valve face. Luckily, the seat (shown in this photo) looks to be in great shape. It must be made of a tougher material. Since the exhaust valve must be replaced, this seat will need to be professionally ground. I may have the intake valve done as well while it's all apart, if the extra cost isn't too much.

To remove the bent valve I cut the end off with a dremel tool and it popped out pretty easily.

What's going on

This photo shows the edge of the piston that has been worn/burned away. Apparently very common and nothing to lose any sleep over. This piston is reusable.



There are signs that this poor tractor has had a rough life - like in this photo of the PTO end of the crankshaft (valve tappets alongside). At some point in the past the clutch either came loose or the key gave way -- in any case the engine was operated while the clutch ground away on the shaft, nearly ruining it. The red arrows show a clever repair job where the shaft has been pounded with a centre punch to restore its diameter (the key has also been welded permanently in place). This is an expensive part so worth the repair attempt. Likely the person who repaired the shaft was not the same one who bashed the shit out of the end of it.

Mostly empty crankcase

On the bright side everything else looks really good. The cylinder bore is smooth, with almost no detectable wear. All the bits (connecting rod bearings, piston wrist pin) look to be in great shape, this is looking like it will be an easy rebuild after all.

The problem

(Click to zoom)




This exhaust valve is telling a story. The red arrows indicate some serious wear on the face (this area is supposed to be flat). The yellow area shows an edge of the valve that is entirely burned away. This valve is f$%&#@ - it's warped badly enough that it won't come out of the block. I'm not sure what has happened exactly, possibly the engine overheated or was run with the wrong oil (or no oil).

Anyway this explains the oil consumption, oil smell, and backfiring.

Removing valves

Very handy valve spring compression tool saves knuckles.

Flywheel off

Finally got the flywheel off. The process was too messy to describe ...