AUTO & MOTO ARTISANS - AUSTRALIA
Handy XR600R rebuild tips
XR600Rs are pretty bullet-proof but when something does wear out or break there’s nothing much easier to work on (short of a two-stroke) than Honda’s big air-cooled single.
There’s no better guide to rebuilding an XR600R than the genuine factory service manual, but there are a few tricks that aren’t in it...and some anomalies that are.
This is by no means anything like a definitive guide but, in an effort to share the love and make some XR600R owner’s life easier, here are a few handy hints and tips (all care, no responsibility accepted etc., etc.).
Let’s start with your basic top-end refresh
Time for a refresh?
If something’s gone bang and you’ve got a rod poking out of the crankcase you know you’re up for a rebuild, but the most common reason for unbuttoning an XR600R engine is a top-end refresh.
A smokey exhaust and increased oil consumption are the first signs you’ll need to do some work – worn valve seals and piston rings are the usual suspects.
Oil consumption varies. Old (aren’t they all) XR600Rs can use as much as a litre of oil every 1000km and still run pretty good...but it’s a sign they’re well on their way out. By comparison a freshly rebuilt engine will probably use around 200ml per 1000km.
Valve seals seem to get blamed for smokey exhausts more than they really deserve, but they do wear and overheating can cause them to fail...but worn rings are more likely the cause of smoking issues. That said, valve stem/guide wear can be a factor, but only as mileages really add up
Common bits of advice to diagnose which is which are (a) if it smokes on acceleration its rings, if it smokes on deceleration it’s valves, and (b) if it smokes on start-up it’s valves, if it smokes all the time it’s rings.
A compression test (tricky on the auto-decompressor XR600R, even with the right equipment) isn’t necessarily the definitive answer – low compression could be down to poor valve seating as much as worn rings.
It’s really neither here nor there because you’re going to have to pull the top end off to fix either issue. Bottom line is that you can live longer with worn valves than you can with worn rings.
The simple answer to rings or valves question is to change your oil and filter (you’re doing that every 1600km or so anyway, right?) and see how long the oil takes to colour up – the sooner it turns black the more of a worn-rings issue you’ve got and the sooner you need to get the spanners out.
Take your top off!
If things have got to the point where you think it’s an issue, it probably is an issue – pulling the head and barrel will confirm what you need to do.
The official Honda factory service manual makes the process all pretty clear. And, yes, it can all be done with the engine in the bike. The only vaguely tricky ‘in situ’ bit is where you have to loosen and raise (you can’t remove them just yet) the bolts in the centre of the rocker cover (Honda calls it a cylinder head cover) while you juggle it past the frame.
Honda would have you believe you need a special ‘cam chain tensioner holder’ tool to get the cam out, but if you undo the cam chain tensioner screw, and withdraw the tensioner shaft, the tensioner assembly comes straight out (with a bit of a 'spang')
Handy hints the manual misses: loosen the vale clearance inspection covers before you undo the rocker cover bolts, back off valve clearances (so the valves are fully closed), and use kick-start compression (gently) to ‘lift’ the head once you’ve loosened all the bolts.
The cylinder should come out of the cases pretty easy but, if someone’s been a bit too overzealous with the gasket goo or there’s a bit of corrosion, don’t start whacking the fins with a hammer (not even a rubber one) – use the head as a template to drill some holes in a piece of 4x2, bolt it in place, and apply force (evenly) to the wood until you crack the ‘seal’.
And here’s another handy hint: grab yourself a cardboard box, trace out a rough profile of the head and barrel, mark which way is front, and punch out some holes to match where the bolts come from. That way you can be sure of getting everything back together correctly (some of the head bolts are different lengths) after waiting a week for your barrel to be rebored.
Likewise, when you pull the head apart note where each valve came from and keep the valves, springs and collets together – that way they’ll go back into the right guides and all the wear patterns will match up.
It's all about tolerance(s)
Speaking of valves, it’s a good bet the springs will have sacked out and need replacing. Honda’s service manual specifies valve spring free length at 36mm for the outers and 35.1mm for the inners (with a service limit one mill less), but don’t be surprised if your outer springs are longer and the inners shorter.
A brand new set of ‘parts correct’ Honda replacement springs measured up around 38.6mm and 34.3mm straight out of the bag, so maybe there’s been a running change that didn’t make it into the manual. Others report similar measurements, so presumably those figures are pretty right and the one-mill service limit should still apply.
Unless you’ve got a decent set of micrometers the only other things you can realistically check are valve seat/face width and ring gap and clearances.
A visual inspection will show if you can get away with cleaning up the valves and seats and lapping things in, but if the seats are pitted they’ll need refacing. Honda says the valves can’t be reground, but you can get away with a light reface without things going pear shaped.
Best bet is to take copies of all the relevant specs from the service manual and have your friendly local engine reconditioner measure everything up. If they’ve had plenty of bike experience they’ll tell you where you can fudge things a bit...or if you really do need new valves, guides and seats.
It all boils down to time/cost/benefit. Just slipping in a new set of rings when your piston and barrel are worn is false economy though – they’ll wear fast and, when you add up the time involved in another teardown plus the cost of two sets of gaskets, you’ll regret not forking out for a rebore and a new piston and ring set to begin with.
For a basic ‘refresh’ consider yourself up for a rebore, new piston, pin and rings, gaskets and valve seals (definitely), valve springs (probably), and a valve grind/new valves (maybe).
While you’ve got the top off take a look at the cam chain guides. The front one (that lifts straight out when you remove the barrel) shouldn’t show too much wear. The rear one provides cam chain tension so you might expect signs of wear.
Honda’s service manual seems to gloss over acceptable chain and guide wear limits, but while many cite cam chain ‘noise’ as indicative of a problem you shouldn’t necessarily overplay it. Replacing the cam chain and tensioner guide is a covers-off job, so unless things are looking dire it’s really more of a full-rebuild thing than a simple refresh.
Couple of notes about the photo on the right. Clean inlets and dirty exhausts suggest worn rings. Scars on the combustion chamber are down to choke plate failure (see XR600R carb issues for more) – if you smooth down the high spots (possible pre-ignition hot spots) you're good to go.
Bolting it back up
Reassembly is all pretty straightforward If you follow the manual.
To make sliding the barrel over your shiny new piston and rings really easy cut a piece of wood board (about an inch thick) to support the base of the piston while you lower the barrel over it – it’ll save a bit of juggling and prevent any damage or scratches.
Reinstalling the cam chain tensioner without Honda’s ‘special tool’ is down to using a zip tie to pull the tang on the spring up to the tensioner arm before you drop it in place. It takes a bit of experimentation but it’s no real drama...unless you drop it down the cam chain tunnel when you snip it off.
Make sure you get the correct timing mark lined up when you install the sprocket, and give it a ‘reassurance’ check after the tensioner is back in.
Following factory torque specs goes without saying, but unless you’ve got some really trick tools you’ll have to use some ‘mechanical sensitivity’ on the small external bolts...tighten them up again after a few heat cycles.
You might just get away with re-using the rocker gasket (with a clean-up and a smear of silicone) but forget about trying to re-use the base and head gaskets or those totally-crushed copper exhaust pipe things. You should also check the condition of the oil-feed line washers and replace or anneal and clean-up as necessary.
Which brings us to...
Some notes about oil
You pick your own poison with oil, but when it comes to break-in don’t use synthetic and don’t run a ‘high zinc’ diesel oil. Synthetic oil is ‘too good’ and doesn’t allow the rings to bed in properly. Diesel oils have added zinc (good) but have added detergents which keep the bore ‘too clean’ during break-in. Do use a good assembly lube if you’re installing a new camshaft.
In addition to slathering lube on your new parts, it’s good to pre-fill the crankcase before you pop the barrel back on so that nothing is running dry from scratch – takes around 800cc before it starts to come out of the crankcase level inspection plug (near the brake lever).
This trick cuts down the number of kicks you need before oil makes its way up to the feed pipe junction at the head. That’s something you should check whenever you do an oil/filter change anyway – pull the sparkplug, loosen the oil-feed bolt at the head and kick away until you see oil weeping out.
Honda specifies the XR600R’s oil capacity at 1.9 litres, 1.95 litres with a filter change, and 2.3 litres for a full engine rebuild. Pre-filling the crankcase means you can dump Honda’s recommended litre into the frame, fire the engine up and add the ‘missing’ oil without shutting it down too soon (see break-in below).
Assuming you’ve turned it over, felt no unexplained resistance, not heard any strange noises, seen oil coming out of the feed pipe at the head, checked drain plugs and fasteners and popped the plug back in, it’s time for the moment of truth.
There are two main schools of thought on running-in an engine – the “treat it gently” camp and the “hammer it” guys. Both will warn against lugging a fresh engine, and the smart ones will tell you not to let it sit idling for any length of time or keep stopping and starting it.
Modern metallurgy and manufacturing tolerances make the ‘go hard from the get-go’ case fairly compelling when it comes to getting good ring seating, but it does gloss over practicalities like checking oil levels, finding any leaks (oil and air), basic carb adjustment and, if you’ve dropped in a new cam, recommendations like running at light loads above 3000rpm for 30 minutes
If you’re not part of a team of race mechanics clustered around a factory dyno after a blue-printed rebuild, there’s a happy ‘home garage’ solution (worked here on a variety of bikes, but hey, no responsibility etc., etc.)...
Tank off and set up a remote fuel feed so you can see everything. Grab a fan and set it up near the front wheel to blow across the cylinder and head. Set the bike upright, start it (minimal choke, you don’t want fuel washing oil off the cylinder), and set the throttle (tape or a mate) somewhere comfortable between quarter and half throttle.
Stand back and congratulate yourself that it runs (expect a bit of smoke as the lube you liberally applied clears) and check the time. Add some of the ‘missing’ oil as the engine warms and oil starts to circulate.
Check for air leaks around the inlet manifold/carb boots (ether, WD40 etc., for any variation in rpm). Check the time...goes really quick when you’re a bit tense doesn't it! When the engine is warm check for oil leaks (drain plugs, oil lines, head and rocker cover gaskets).
If you’re hearing nasty noises, or the exhaust is smokey, you’ve done something wrong. If it’s all good at this stage (you’re probably around five minutes in) you can step up the rpm. Go to half throttle and hold it sitting there for a minute or two.
Okay. Rap it out a few times (just a quick twist of the throttle, don’t hold constant high rpm), settle back down around half throttle for a bit, rap it out again, ease back to quarter throttle, then back it down and adjust your idle speed. When you’ve got a decent idle speed, run it back up to half throttle, then quarter throttle (you’re maybe getting around the 10-minute mark), then let it idle down and shut off.
The theory is that everything is working under a light load but with sufficient heat and pressure to start seating the rings properly...plus it’s a controlled environment where you can shut down if things go bad.
Ideally you’d run through this on a dyno, gradually applying load once you’re sure everything that should be inside the engine is staying there, but in the ‘average bloke’ world this is a pretty good start.
Once the engine has cooled you can check fasteners, look for leaks, whack the tank back on, go for a ride and start applying some load. Second and third gear mid-range roll-ons are the ticket to begin with...keep varying rpm, don’t lug the engine, and don’t hold high rpm until you’ve got some time up.
Running-in new parts generates heat and heat kills oil, so don’t leave an oil and filter change too long...first tank of gas after a rebuild if you’re OCD.
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