I had a recent problem with a Honda Xr600R. To make a long story short the bike became harder and harder to start. Finally it was a no go. I took the bike to a mechanic who found the badly worn parts and then we discovered that the oil flow to the head was almost nonexistent. What was damaged was the rocker arm/cam-riding surface, they were worn down to almost nothing. We discovered that the oil pump had failed. Now when it came time for me to get a refund on the deposit on the bike the other party claimed I had overheated the engine and as such he wasnt going to give me back a big chunk of change. The bike was never overheated at any time but the other party claims I must prove this to him.

I've received some comments from other boards but I wont post here because I dont want to be accused of tainting the results.

The 2500 dollar question is
If the bike was overheated what other damage should have been observed?

Any help here is greatly appreciated.

spark plug discolouration
bottom end damage- main bearings and bigends
valve and valve seat damage
oil separation and discolouration

but the most prominent: piston collapse and sleeve wear

If an engine overheated, especially an air-cooled motor, the piston tends to collapse. So by measuring the piston diameter compared to original dimensions, you'll see if it has shrunk. Most pistons will naturally be slightly smaller at the top (expanding during operations where its hottest), but a collapsed piston tends to be more like an hourglass shape in measurement.

One thing to keep in mind, an overheated engine's oil will have broken down and loose its 'cohesiveness' meaning it will not lubricate or protect the moving parts like it should, resulting in excessive wear on all components. Excessive wear or heat seazing will result in bits of metal going through the bike, and oil pump, thus buggering the oil pump, leading to less oil pressure, higher temperatures etc. etc.

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Squily

When my XR600 became harder and harder to start it was down to head bolts stripping threads and pulling out of the cylinder. I've had to helicoil all of them. The lack of compression is what stopped the bike starting.

I've been told that overheating the engine changes the metallurgical properties of the alloy, softening it, and that's what allows the bolts to pull out. I haven't got the equipment to check for softening so it may or may not be true but I know it does happen in overheated car engines.

As the XR's air cooled it's very hard to know whether you've overheated it or not. If your days consist of roosting deep sand in 1st gear when it's around 40C then you might well have pushed it too far but I never had any issues with normal trail / road use in UK temperatures.

What was wrong with the oil pump? It's a pretty simple system and unless you've stripped the drive gear it should work ok. If it's debris that's got in there is there much on the gauze screen (just below and to the left of the pump).

The oil feed to the head comes through the outer cover before it goes up through the bolt on pipe. The weak spot (other than the pipe blocking) is the O ring seal from the pump to the cover. If that's missing or broken it'll reduce the flow and it's easy to overlook when you're putting the cover back on.

Up in the head the two bits that go first from lack of oil are where the rockers and cam rub together and the centre cam bearing. The end ones are ball bearings and survive pretty well but the centre one can look like a ploughed field if it's oil starved.

If the oil pump is junk I'd be worried about the state of the gear teeth as well as one of the pump outlets feeds the gearbox. The early XRs were marginal on gearbox life, with gear pitting a problem, so an oil starved one wouldn't be good news.

Great info on the old XR ... unfortunately may not help the OP retrieve his lost cash.

XR's always ran HOT ... and the above description about the flawed radial valve head is spot on. Rocker/cam bearing wear is what I remember to be a common problem on XR's. My first XR type bike was actually an '87 XL600R, similar to an XR engine. I later bought a used XR600 in round '93, sold it off for an XR400R.

One thing I was taught to do on BOTH XR's was to take out the Dip Stick at the steering head (Oil in Frame) when the bike was running, and watch the oil squirt across the opening from the oil return line positioned there. Good way to ensure oil pump working. Should be a FAST, healthy squirt of oil!

My guess (for the OP) is that oil pump was clogged (or partially clogged) before you bought the bike and the motor melt down is not your fault. But proving that and getting back your money may be impossible. Sure, it was overheated, but not by YOU but probably by previous owner. Not hard to overheat an XR, and if you let them get low on oil ??? Disaster.

Best thing to do when you buy 2nd hand dirt bike you are not familiar with is take DIRECTLY to a trusted mechanics, fully check everything on the bike. It's worth the money ... even if it doesn't find any fatal flaws .... just for the peace of mind alone.

Sorry for the bad luck. Lots of old XR's around ... maybe you can find a nice used Engine somewhere?