XR660R ate itself - why?

Hi all,

I just bought an XR600R ('89) from a guy who said it just lost compression going down the road at highway speed, but the bike was in great shape.

I bought it, tore the head off and found it had a Thumper Racing 660 10.5:1 kit in it and the edge of the piston was burned off pix here: XR engine

I called the guy I bought it from and he said he only got one tank of pump gas through it before the meltdown, do you think this was the only problem, or should I look elsewhere? The head bolts weren't very tight, could it be an air leak problem? Also, from the pictures is the rod supposed to be blued like that? There's no bad play in the rod or anything else.



San Diego

That looks like a 2 stroke seizure. It looks like what is known as a 4 corner seizure. There are a few different things that could cause it. First is too little clearance between the bore and piston. It heats up and jams. Second is excessive heat from being jetted too lean. Perhaps a combination of the two. Ask the previous owner if he put richer jets in there after the 660 kit was installed. I would imagine that the 660 needs richer jetting.

Maybe he ran regular 87 octane in it? I don't know if low octane gas would it to fry so fast, but it is a possibility. What does the plug and the head look like?

Head leaks on an air cooled engine don't cause the same problems that a water cooled engine has with a leak. A small leak is only a loss of compression. If the leak is big, then engine will not even run. I had a old XL250 that had a head gasket leak for months before I fixed it.

While you have it apart, take a quick look at the transmission gears. Look for pitted/broken teeth. It's pretty common on XR600s that have been ridden a lot.

I don't know if it's normal, but my crank/rod have the same blue coloring.

I uploaded pictures of the head too:

XR engine with head

It may be hard to tell from the pix, but it looks like there is part of the dissolved piston embedded in the head, is that cleanable?

I suspect there wasn't a problem with piston-cylinder clearance because the kit was done by Thumper Racing in 3/97 (date engraved on cylinder). Though it looks like it wasn't ridden that much, I suspect a 1 tank seizure type failure would've occurred since the install.

Also, the grooves in the cylinder wall are barely enough to catch your fingernail, is that honeable, or do I need a resleeve? What is the pain level with a resleeve?

I don't see any signs of gear wear, I'm guessing I would've seen metallic slivers in the oil, but the oil was in very good condition.



San Diego

The erroded ring land looks like detonation to me. 10.5:1 should be ok on premium pump gas--although it's the dynamic compression that really matters. Dynamic compression changes with cam specs and elevation. And then there's other factors like ignition timing, air/fuel ratio, ambient air temps, and more. They all contribute to an engines octane requirement.

If it was jetted lean, had 87 in it (even fresh 87, old 87 could really be 85 or even less), and was being run hard at speed--ping, ping, ping, ping, ping...silence.

The damage on the head--are you talking about the what looks like pitting around the very outside edge? That coincides with the damaged portion of the piston, right? It's hard to say without being able to see it first hand, but that looks like more deto damage.

Looks pretty lean too which helps create conditions that favor deto.

Just for comparison, here's a couple pics of deto damaged pistons:




Not that I've had a bit more time to think about it, I'm convinced you had a detonation failure caused by too lean jetting and too low octane.

and the best fix is to resleeve and use a new piston & rings.

To me that looks like it ran lean. If he did all the mods and ran it hard without rejetting, I could see that happening.

I would have to think u have 2 problems at hand. One undoubtably is a lack of fuel or not high enough octane causing detontation. The other judging by the blued crank and rod is a loss of oil to moving engine parts as well.

The guy said it kicked over first time and ran great, my guess is if it was running lean there would've been problems with it. I'll check the jets when I get the carb back from being ultrasonically cleaned (after it sat for 2 years).

Yes the piston edge was ate away like the pix posted above, so I guess whatever it was probably was related to detonation, though I'm still curious why it would happen so fast.

I had another 600 that had a blued rod and I heard it was okay, you sure the temper and bearing surfaces are bad? I don't feel any play, other than a couple thousandths side to side, though I can't get the wrist pin out without a press, which doesn't really seem normal...hoping not to get into the crank on this thing :ride:



San Diego

"XR660R ate itself - why?"

Does this make your bike a cannibal? :ride:

I had another 600 that had a blued rod and I heard it was okay, you sure the temper and bearing surfaces are bad? I don't feel any play, other than a couple thousandths side to side, though I can't get the wrist pin out without a press, which doesn't really seem normal...hoping not to get into the crank on this thing :ride:



San Diego

I think that the blue coloring has to do with heat treatment that is part of the manufacturing. My XR600 has nearly the same coloring pattern, and it's been fine for a long time. The few thousandths from side to side is OK. To measure the up and down play in mine, I used carb cleaner to get all the oil out of the big end rod to crank area. With the oil gone it's much easier to feel the play. I think that the spec is pretty large, like .003 or something. I think that your crank will turn out to be OK.

Agreed with the above. The color of the rod looks normal to me.

I haven't closely read all the responses, but, if the cylinder liner has alum piston galled onto it, muratic acid will melt it away and not eat the liner. It is very VERY dangerous stuff, and makes nasty fumes but does the job. If the bore is in spec you'll still need to hone it for the new rings to seat. If not in spec., send it off to be sleeved and bored out to the max, I have over 10k running a resleeved 10.25 to 1, about time to freshen it up after my next trip. :ride:

Have to agree that it is damage from a detonation/high heat problem due to a very lean mixture. Every XR engine I have ever had apart (quite a few) has the bluing shown in the pix, it is normal.

I have had 4 XR600r's over the years, all have had the blueing on the rod and crank, and all run perfect, the one I have now is the same blueing and a 632 kit, I run 91 pump gas and have it jetted spot on, I ride a ton of desert and fast, she runs perfect and never gives me fits, still is solid after three years in the desert. I agree with the other guys, deto problem.

While we're on the subject of detonation: The big air-cooled XR engines are some of the least tollerant of low octane. Large bore engines are more susceptable to deto than smaller ones. The greater distances in the combustion chamber allow more time for the air/fuel mixture to decompose into the unstable "end-gases" that actually cause detonation. You might be able to get away with 11:1 on an XR250 or even an XR400 with 91. But on a 600+ you may not.

note:That's just an example--I'm not saying those numbers are actual guidelines for compression and octane.

The blue color of the rod is normal. It's caused be the heat treating from the factory. Besides the piston damage, how is the head? If there is any uneven wear on the cam/rockers you may have an oil starvation problem.

The blue colour on the bottom of the rod is completely normal. I've had plenty of XR engines apart (at least 30) and every one has had this. It comes from when Honda heat the crank to install the crankpin during crankshaft assembly. As for the seizure I agree with the other guys here. Judging by the white carbon around the combustion chamber, the engine has been running dangerously lean, to the point where the piston would seize in time.

Great pictures! 1st off the coloring/blueing looks "normal" compared to all the motors I've seen apart without being cooked, teardown due to other issues,as said before, factory heat treatment. I haven't seen very many burn-ups but you could try & give Rob a shout at barnumspro.com. He 's got a lot of 600 experience. I hesitate to say it is 100% leaness caused. ( my only knowledge is from my own experience with a motor I burned up about 5 x. It was a v-4 evinrude outboard motor/watercooled & 2 stroke) It always fowled up plug pretty bad with alum deposits(BBs) if not caked up completely,(losing spark) & the middle of piston was eaten away.(in all 5 cases). Your piston looks scuffed really bad as if the friction melted edges . Did it seize/stuck to a stop or sever slow down & then you shut it down? I also wander if bore was too tight . Some one told me that if you dont warm up a cold engine before romping it, (especially a newly cut cylinder) the piston will heat up quickly being alum & the cylinder alum mass with the steel sleeve will still have not expanded yet at same rate(make sense to me )causing the severe scuffing & also possibly slight leaness (from blowby)after rings are stuck with alum deposits.

Did you make sure the oil was getting flow? my bud got his xr600 upted to a 660 at one of those 3 places out there. long story short he had trouble priming oil pump. Does your cam show lack of oil? Just a thought. I'm not an expert but just trying to give you a different point of view or angle to help the proccess of solving problem. My .02,take it with a grain of salt. Good Luck!

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