How Far Will a KLR Go If Properly Maintained?

So the head was basically junk at 136k kms?

The one that was hit by the Ford Exploder had just over 50k on the clock. The '99 I bought to replace it reached 130K before I decided it was so down on power that I needed to rebuild the engine. Now it wasn't real down on power. It took a decent uphill and riding into a stiff headwind before you realized it was getting well worn. When I tore the engine down the ring end gap was still within service limits and my thinnest shim was only a 2.15.

For my experience the secret to longevity is three fold. First, knowing you have a old technology engine I broke it in the way. First few time I road it I brought the engine up to temp and then shut her down until the cases were cold to the touch. We each repeative cycle I rode it a bit longer before shutting it down to cool. Once I got to where I was riding 30 minutes at temp, I started going the distance on long 3-4 hour trips. I found mountain roads that kept me going on and off the gas to help seat rings and valves. I kept the engine rpms under 4000 for the first 1500, then under 5000 for the next 1500, then under 6000 for the next 1500 and pushed it occasional to red line in the next 1500. I pulled on oil change at 500, 1000, 3000 and 6000 miles. Second, after the initial 6000 miles, with the 4th oil change I switched to a full synthetic. And third, I treated 5500 as a orange line on the tach. In the old days tachs used to come with orange and red lines. The orange line was the limit for normal operation. You only ran between the orange and red lines for brief periods. It also seems if you keep the KLR below 5500 you avoid the problem with oil consumption, which is more a problem of pressure of descending piston 6000 times a minute the blowing oil out the blow-by tube into the air box.

Other than that I changed the oil and oil filter every 6000 miles (MCN oil tests have shown that a good synthetic with 3000 miles of use can outperform the best full dino oils fresh out of the bottle so why not go the full Factory recommended 6000 mile interval) as well as checking valve clearance. I always re-shimmed the valve whenever the intake was tighter than 0.15 mm and whenever the exhaust was tighter than 0.20 mm - I believe in loose valve clearance.

Oh, and one thing I learned from the 1st bike was never run a K&N filter if you ride off road. I did on the 1st KLR. After the accident we tore down the engine and found signs that dust had been getting by the K&N, accelerating the wear and tear.

Not really junk but all the shims were down to the 2nd or 3rd smallest size and I had the head off anyway to replace the cylinder so I thought might as well swap the head over since I already had it. I'd put it back on tomorrow if I had to.
The KLR is my only motorised transport so I can't afford for it to be off the road for any lenght of time ie waiting for head work or a rebore....

There have been many reliable sources that have reported that K&N filters will cut the rings out of any motor. I wouldn't use one if you gave it to me. YMMV