XR600 Teardown


Posted by JAW JAW nipped out by himself one Saturday to test some changes he'd made to improve the "ease" of starting the big bore. While testing a ugly new problem reared its head - the bike was stuck in 2nd gear :( On arrival home closer inspection was undertaken. The foot selector moved up and down okay, it just didn't do anything. The clutch cover was whipped off for a closer inspection.	Under the clutch cover.


Close up of the end of the gear selector - the stopper plate and shift pawl. The guts of the selector (the shift drum and spindle assembly) are inside* the engine...*	A socket on the back of the gear selector drum reveals all cogs are working, selectable and accounted for. however, the foot selector shaft did not be connected to the drum anymore. It sounds pretty simple but... XR's use an internal gear selector mechanism. You have to split the crankcase to get to it. In order to split the crank case you have to remove the barrel. In order to remove the barrel you need to remove the head. In order to remove... I think you've got the gist now. For the sake of a circlip or a nut that has unwound or some other petty nastiness you have to do the equivalent of a rebuild.


"I'll come over Saturday, we'll pull it down" Wolfie says. Top man Wolfie. Saturday arrives and Wolfie rocks up with JAW still pulling the fuel tank off and draining the sump. "I thought you'd have it on the bench by now..." Wolfie jests. There are no suprises, all the bits are removed that join the engine to the frame; clutch cables, wiring, carby, exhaust, levers, etc. The exhaust proves a bugger, the muffler doesn't really want to separate from the headers. Dropping the monoshock off and removing the airbox allows the full exhaust system to be removed. Removing the top end while still in the frame seemed like a good idea. However no amount of wiggling and repositioning will give you enough room to pop the cylinder head cover off. However with all the engine mounts removed the engine dropped a centimeter and she slipped out.	It was close, but no. You have to drop the engine mounts to get the cap off.


Have a quick study of the timing marks before dropping the cam out.	Wolfie takes the time to note the timing marks. There are two obvious ones on the cam gear, the crank timing mark is cunningly hidden on the harmonic balancer, on the other side of the engine. Being a 4-stroke timing is a part of life. If you don't know what timing is about, nip out to some other site and read up about the operation of 4 strokes... ;) The top end engine configuration consists of an over head cam spinning with two end roller bearings and an integral cast centre bearing. There are four cam lobes, one for each valve. A rocker sits above the cam and an assisting sub-rocker arm pushes down on the top of the valve.


Looking at the top of the head with the cap removed. The four valves sit radially as the combustion chamber is hemispherical.	The underside of the cylinder head cover. Note the cam, bearings, rocker arms and sub rocker arms.

With that out of the way it's time to nip the head off. Watch out for the two little nuts on the outside of the engine. Lucky Wolfie is paying attention. "I'm glad you're here Wolfie :)"

This engine alledgedly had been bored out and this troubled JAW. "How easy is it going to be to get parts Wolfie?". Once the head was cracked and removed JAW was greatly relieved to see a nice copper head gasket. "You'll use that again", Wolfie exclaims, "but anneal it first." Huh?

"Heat it up red hot, and then quench it in water. That will make it go soft again; they get hard in operation." You the boss Wolfie, I'm the T/A :)


JAW grabs the head, wobbles it off the locators and plonks it on the bench for inspection.	The copper head gasket. Anneal it and reuse. And what's the real capacity JAW? bore=96.89mm, stroke=80.17mm giving a total displacement of 591cc. It's not a 630 afterall, she's just a stocker.


The engine is now fairly simple to remove from the frame and pop into the vice on the bench. There is something very, very cool about having a single pot big bored air cooler 4 stroke engine sitting in your vice on your bench in your shed. "UG!". Wolfie inspects some of the parts. Wolfie has a broken hand and is taking it pretty easy, he's gotta get back to work come Monday. There are no parts that have made him wince so far. "Start writing a list on your white board. Nip valves out and re-lap them. separate the exhaust. Reweld that broken engine mount. Lightly sand the clutch operating shaft, it's a bit stiff. Get some new bushes in that shock." Taskmaster! ;)	There is is, but still only halfway to the problem...


Wolfie examines the clutch plates.	The clutch is removed. The plates are okay but the clutch housing and pressure plate are a bit worn. Since the clutch hasn't felt shuddery and the cost of new parts is likely to be expensive, it will do. ...besides, the clutch is a reasonably easy old job any time. The clutch bolt starts proving difficult. Jigs and braces and some Wile E Coyote hari brained schemes cannot break the nut on the cutch housing. Always a pain these things as the engine spins while you try to undo the nut.


"Leave it JAW, I'll take it to work and rattle gun it off. I'll do the harmonic balancer and crank nut while I'm at it, undoubtably they will be hard to remove as well." JAW pops the engine in a box in the boot of Wolfies car. "I'll drop it off Monday, you pull the rest down and I'll come past for another look after that." Thanks Wolfie, did I mention that you are a top man?	Boxed up and taken away for a good rattlegunning.


ah HA! An important piece is clean broken off the gearshift spindle.	With the clutch and crankshaft nuts rattle gunned off things were looking brighter. Until it was time to remove the alternator rotor. Sure the bolt comes out easy but you must have a puller to remove the rotor. Not one of your claw type put a wind in bolt arrangement. I don't have one. yet. Didn't stop the crankcase being separated. Make sure you find every last one of those pesky 5mm allen key bolts, and drop off the shift pawl and the stopper plate. I was convinced there were a few other parts trying to keep it together but no, it was just the dowel/locators being stubborn. A bit of unheeded hammer and screwdriver violence at the engine mount points (where the 3 dowels are) soon saw an end to the crankcase being one piece.

So what was in there JAW? Why were you stuck in second gear? Well as the photo plainly shows a piece of the gearshift spindle assembly has clean snapped off. Bad luck that. May have been my continous stand-on-the-selector to change down a gear, but it has always been more difficult to go down gears than up. Probably a long time pre-existing weakspot, half bent already, I just happened to be the one to finally finish it off.

Fortunately the broken piece didn't go anywhere nasty and do further damage.

Now comes the hard part - deciding what else is going to be replaced or tweaked up while the engine is completely dismantled...

What does the gearshift spindle do? It transfers foot selects from the selector shaft to the shift drum. If you are familiar with the operation of car gearboxes then it will be apparent what the shift drum does. As it rotates, the worm holes in the drum itself force the selector rods backwards and forwards, working either up through the gears or down through the gears. Cogswapping is otherwise the same. Nifty huh? :)


New gear, Old gear. Oil surface tension from "bad" oil can cause the hardening to come off as discovered on the 2nd gear, making a nasty grinding noise and failure on it's way...	With all the pieces neatly piled and organised on the bench it was time for a Wolfie inspection. "Those bearings are okay. Those ones aren't. That second gear pair has to be replaced - that's where you are getting that grinding noise from. Replace that seal, and those while you are there." So the list added up to $130 top gasket set, $90 gear selector spindle, $70 in assorted bearings $150 for new 2nd gear set, $30 in some oil seals. The inside was suprisingly good - rings were as new, main bearings as new also. It would seem the engine had been down not that long ago. Since this is just a "cheapy" bike (?!) I kept the cost to what was needed only; the nice to haves will stay nice to have.


Replacing bearings scared me at first, but in hindsight it was a snap. I used my little propane blow torch to warm the area up a bit, hot to touch but not so that you are going to get 3rd degree burns. All bearings can be tapped out from the back, and when warm they come out with a few sharp knocks. I did both the balance shaft bearings and the counter shaft non drive end. Freeze your new bearings in the freezer overnight. Grab yourself a socket the same size as the outer race of the bearing. Line it up in the crackcase sitting square and with the socket and a hammer knock it home. With any luck being much colder it will practically drop in like mine did.	Fitting the balance shaft back into the freshly popped in new bearing.

"What's the balance shaft JAW?" heh, you'll find balance shafts on quite a lot of larger capacity engines. It is there to counteract some of the harmonic vibrations that would otherwise shake your engine to bits. Basically the big counterweight on the back of the crank takes the main brunt of the nasty out-of-balance action that is a piston engine, and the balance shaft takes out most of the other harmonics vibrations that the counterweight does do. Make sense?

Too easy - the marks on the split balance shaft under gear (right) and the marks on the top gear line up to keep the balance shaft timed correctly. The balance shaft must be timed correctly to the crank - otherwise it it would cause worse vibrations than it fixes! The balance shaft has a direct drive to the crank drive. It uses a very cute split & sprung gear that puts force on the leading *and* trailing edge of the crank cog. To stop any backlash I guess. Line up the timing marks, use a big screwdriver to force the cogs apart and slot it back in!

(As with any balance shaft you could leave it out if you want to take a gamble - it has mass and therefore the engine can't rev as fast with it in place. However with it out there is a chance the engine may vibrate itself into oblivion - but I bet it would have a really cool lumpy idle though ;))

With much fiddling around the gearbox goes back together in one half of crankcase. Yes, you will fiddle about. Mind your thrust washers, keep 'em in the right place. Which of course you kept together and took photos of anyhow didn't you?
A nice shot here of the completed internals. You've got the balance shaft far right connected to the crank, and far left is the counter shaft with it's gears next to the main shaft with its gears. Selector forks just below that with the shift drum underneath and that pesky gear selector spindle in the bottom left. Grr! You little bugger!

The internals, just before putting the crankcase back together.


The nicely refaced valves and 3-cut seats.	In the meantime the head was stripped down. Standard stuff, use your valve g-clamp to get the valves out. Clean up the coke in the head and lap the valves back in. When you find, like I did, that after you have been lapping back and forth, back and forth with your little suction cup lapstick it hasn't quite touched all of the valve seat then get it down to the head shop. For $40 you'll get some fresh 3 cut valve seats and some nicely refaced valves. Pop on the new valve stem oils seals, pour oil over everything and put the valves back in!


How do you get valves in an out? What have you been doing all your life! You'll pick up a tool like this one for around $20. It is like a g-clamp except it has a quick release handle - you wind the valve face adjustment up until when pushed together it has pressed the valve spring down far enough that the collets can come off. Putting the valves back in is simply the reverse procedure...	A valve spring compressor


The underside of the head. Note the extra fins welded on for "more cooling". Does this make it a desert racer? ;)	...sadly when the coke was removed there was a tiny little crack between an exhaust and intake valve. This caused me great concern. While speak to Heads'r'Us the guru got out his magnifying glass and basically said "pfft" and explained that it was nothing to be concerned about. It was superficial and there was no way the valve seat was going to let go. If it was water cooled then water might have found a way in, but being aircooled I got the "pfft" call.


Skipping ahead, the crankcase is back together and the clutch, starter and main drive is back on. The barrel gets bolted up - don't forget to loop the timing chain through! The clutch and starter are such interesting clever little things that I will save them for their own stories. You'll probably forget the order in which things go back on, but don't worry, when you've put the main gear on, torqued that nut up, put the clutch on, installed all the plates and springs and cover then realised you needed to have the timing chain on first - you'll get good practise at these things ;)	Clutch and stuff back on, barrel torqued up.


Looking down the bore...	Examination and measuring up the ring gap showed the rings to be close on brand spankers. huh? Well, whos arguing. A light hone of the barrel was performed to make sure she's not shiny - just enough honage (did I just make up a word?) to scuff all surfaces. Sure, there will be some deeper scratches but don't try to take them out - leave that for the next rebore. Whats a hone? Man, I'm not going to explain everything to you this time...

We're getting close now. Just a matter of bolting up the head, and popping the rotor clutch and valve covers back on. In fact I was so excited at this stage I forgot to take any photos. Woops ;)

The complete engine is not all that heavy and slotted back into the frame easily. A bolt tighten here, and oil line there, here a connector there a connector everywhere a connector and boom! "kick her in the guts Barry."

Not being new to the whole engine teardown scene a great concern of mine was seeing oil pressure at the head before it starts. kickedy kickedy kick. and some more kicks. With the spark plug out of course...

And did you think I could see oil pressure coming out of the end of the external head line? (which I had left unconnected) Nup. So I ring up Wolfie the guru "Hey Wolfie, I'm kickin' but I'm not seeing any oil pressure?!" "Yeah JAW, you probably won't. Start it, it will be okay for 30 seconds with no oil pressure."

She fires up easy. I've got no exhaust and barebones bolts in - just in case I need to pull it out again. She's sounding good. I'm happy. I'm watching for oil to start pissing out the end of the head pipe. I'm counting. I'm waiting. I get to 30. doh! I shut her off.

"Wolfie, me again. Doesn't look as if it is priming? Any thoughts before I pull it back down again to re-check the oil pump?" "Um, pull out the oil filter and pack as much oil in there as you can. That's about it, they don't need to be primed as such."

Boomphf, she's alive again with oil packed in. I'm waiting. I'm counting. Again. Then suddenly I see pressure. Yah!

I bolt all the rest of the stuff up, and jump on her for a spin around the block. What can I say, she's back to her old self again and I'm ready for the next event...


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