Re-assembly: The Guts

The picture above shows the block with the crank back in, and the camshaft and balance gears also in place. This was one part of the assembly process that I was a bit nervous about because for the engine to run properly (and smoothly) the gears on the crank need to be properly timed (aligned) with the other gears. The last time I rebuilt an engine of this type (about 25 years ago) I recall some stress over this step, but it went fine today. The service manual for this engine is really superb. It was written back in the days when the people who actually designed the engine and built it also either wrote the manuals, or had a hand in them anyway. I had no problems.

At least, no problems with the procedure. I did run into a bit of a snag while torquing down the bearing plate. This is the plate seen to the right of the block in the picture above. It holds one of the main bearings that keeps the crankshaft in place. There is a specific torque spec on the four fasteners that hold this puppy in, and while torquing them down I got them to reach just about spec, then they softened and wouldn't come up. Damn! I first feared that the female threads in the block were giving way - that would have been really bad. I removed the plate and inspected the threads in the block - it was fine. Hum. Maybe a bit of OCD was going on, because I re-assembled the plate the same way and hoped for a better result the next time. Nope. Worse actually, one of the bolts broke. Well that explained what was going on, anyway. I can't quite figure out why the bolts weren't holding, that was a mystery. I noticed a second one was also about to fail. Anyway, I had some stock in the shop and made up four new ones and they torqued up just fine.

There is one gasket under the bearing plate, and a selection of paper spacers as well in the gasket kit - you select the right number of paper spacers to get the correct crank end-play. I happened to guess right with two spacers, giving a crank end-play of about 0.006". Nice and tight.

Painting pics

OK here we go! The picture above is of the block, of course, with the black areas masked off and after the third coat of Cub yellow. Yes, that is a balloon stuffed in there.

The picture below is of the cooling shroud and air cleaner, after three coats of Cub yellow.


In the next pictures the masking has been removed and the important surfaces have been cleaned up with emery cloth. The engine block is now ready for re-assembly. It looks sharp enough that you could almost take this thing out on a date!

Block update

I dropped the block off for machining on Monday, and picked it up today. Pretty good service I'd say! It looks, well, the same really. With the 0.020" overbore, the new piston sits in there nicely and the valves look to be perfectly ground in their new seats and guides. The ball is back in my court now, just some assembly is required and we'll be good to go. I plan on putting up some pictures after I've washed the block of sand blasting grit and oil, and finished the painting with some Cub yellow.

Some painting

I was going to paint the parts and engine block after the block came back from the machine shop, but because of the eBay delays I decided to go ahead and start the painting now. No pictures yet (I'll save that for when I'm done) but so far the high-heat black paint is on the top part of the block (cooling fins) and muffler. It looks pretty nice.

An eBay experience

I elected to purchase the rebuild kit (piston & rings, gaskets, rod etc.) from eBay -- I found the best price there and figured it's time to try eBay out. The Kohler-genuine rebuild kit was about $200 online (this would have been factory-original parts, my first choice) and the local small engine shop would sell an after market kit for the same. On eBay I found one for $105US; after shipping and currency conversion it ended up being about $140 Canadian.
The eBay seller uses a "blanket" item number for the K301 engine's kit -- you need to use eBay 'messaging' to contact the seller after the sale to let them know the exact overbore size and rod size that you need. Sounds simple. I made the purchase as a 'guest' (didn't want to sign up) but they don't tell you that you can't sign in to the message service with a guest account, you need a full account. Argh. They'd beaten me, so I signed up. Then you find out that the message service is disabled for 3 days after initial sign-up, so I couldn't reply to the seller with the details. What fun.

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 ... :)