Hey, do you like riding your XT600? Don't know, it's broken at the moment. This has been a sort of running joke between me and several people and quite frankly it has gotten a bit old. Given the existence of this post, you can probably guess: it's not really getting better.
But let me explain, the the last time I overhauled the engine, which was only roughly 3 months ago, I used a very inexpensive, cast piston. Long story short that cheap piston had developed a nasty habit of expanding rather erratically and semi-seized in the bore a few times, which stopped the rings from moving freely in their grooves, which meant no power and massive amounts of blowby through the engine breather. To top things off, I went with a rather close tolerance, because I wanted to get some longevity out of this case piston. The result was a mechanically quiet and completely gutless engine, which started spewing oil out of the breather after less than 200kms.
After quite a bit of thinking, I decided to rebuild the engine and I put on my thinking cap trying to remember what worked and what didn't. To be fair, the old Wössner +1.00mm (501D100) oversize worked an absolute treat after the Wiseco had failed previously .
So I went ahead and bought it's bigger brother 96.50mm +1.50mm (501D150). I am
not absolutely sure just how much the coated piston skirt actually helps, but
it is a piston finished to a very high standard and unlike the JE-piston with
its 11.x:1 compression ratio, this one sports a more modest 10.8:1, which
should leave some breathing room for poor quality fuel or a bit of carbon
buildup on the piston crown.
Initially I played around with the thought of swapping the liners between my reinforced 97mm cylinder and the older 96mm one, but as this would have meant skimming the cylinder, I ultimately decided against it. (All the threads felt good enough, so let's hope this isn't going to bite me at some point.) With a gas-powered BBQ it's a ten minute job really. All you need is a couple of fire-proof spacers.
Flipped it over and once it came up to temp, you could hear when the liner
Then drop the cold liner into the hot cylinder.
A left over liner.
It has to be said that I gambled a bit, as up to this point, I hadn't taken
the engine apart. Bore didn't look too bad.
Conrod small-end showed no signs of damage and the conrod itself felt normal
But the piston had weird seize-marks on all four corners, indicating both a
rather tight bore (I knew that beforehand) and a somewhat erratic expansion.
Both compression rings were stuck in their grooves.
You know things are good, when some of the most important calculations of the whole engine build are scribbled onto the packaging of the piston.
Now all I had to do was to file/sand the rings to size. I used 400 grit paper around a file as the top ring with its hard-chrome layer was way too hard to file. (The second ring appears to be made from cast steel and is an absolute beauty to get into dimension.)
By the way, a stone on a dremel works really lovely to re-size the ring gap... not. The rings will overheat and once hot literally vanish. Oh well, now I have set of oil-scraper rings, should I need fresh ones down the line.
Assembly was fairly straight forward, but have I mentioned that the gudgeon pin (piston wrist pin for you 'Muricans) is machined conically on the inside? Thing of art.
The following pictures are mostly for me so I can look at them every time I feel like the engine is lacking power and to prevent me from having to take the engine apart "just to be sure that the timing is correct" or the like.
Cam neither advanced nor retarded.
Correctly timed on "T".
At this point the question was still open: why did the JE-piston fail on the Autobahn. Please pay special attention to the head pipe in the following two pictures. (A fresh piston and rings will always create excessive heat...)
If you follow the header pipes to the actual muffler at one point exactly this bluing will stop. To be fair I bought the muffler as "rebuilt" and whilst not entirely sure how the did it, I have the nagging suspicion that it's not supposed to look like that.
Judging by the wool, it was definitely repacked.
Oh well, I still have a stock muffler. (Might be better for the "Pickerl"/Austrian MOT anyway.) By now I have bought some perforated tube and will rebuild the internals. So what else to do than to take the old girl for a quick spin and as the odd "up and down the road" didn't reveal any real nastyness, why not turn it up one notch and take it home?
I do have a talent to break down at some scenic locations and the lighting wasn't bad either.
Turns out it leaked some air past the left carb and thus got a bit tight and whilst I caught it just in time, it was still a bit too tight to kick over or in other words, I broke the kickstart lever. (Thanks to my dad at this point, who took off the one from his XT and came along with some tools.)
After realizing that the lower joint on the kickstart must be made from pure gold or the like (120 Euros), I decided to give it a shot and TIG-weld it together with some ER70S wire. Also no fancy cool-down procedures as for one I am sure that the thermal mass of this bit is just too small to make a difference (unless you chill it in water or oil) and also I am pretty sure this is just plain old cast steel. Closer inspection revealed that it had been welded before, which I thought was rather confidence inspiring.
This worked rather nicely and meant that all I had to do was tackle another elephant in the room: Whilst not an immediate MOT-failure, having no Neutral light was a tad annoying. Whilst I had visions of unwrapping the whole loom it turned out that the old neutral switch had failed and that was the only source for the catastrophic short.
To finish this post with a slightly sarcastic note: How do I make an XT600 engine last an extra two weeks? I don't ride the bike. 😐 Not entirely sure how things will progress in the long run, short term, I have a new downpipe and I will rebuild that muffler, because I have all the bits I need. I suppose, if someone threatened me with a handful of cash, I probably wouldn't say no. (Even though frankly I haven't got the slightest clue what to get instead.) On the other hand and that's the big dilemma, aside from the "reliability issues", I have pretty much molded it into the kind of 2-wheeled-walking-boot that I think is what truly the nature of this XT600 should be...