Clearances, valvetrain wear, lifespan?

It's been about eight hours since the last thread about valve clearances... so time for another one!

Between 5000 mile checks, how much can you expect the clearances to narrow?

Assuming a constant rate of gap narrowing, let's say 0.01mm every 5k, at what point is the valve/seat relationship considered worn enough to pull the head and replace valves/have seats recut or replaced?

Or could one continue to keep riding indefinitely (not infinitely, of course), assuming the clearances are kept within spec?

Since I no longer own an XJ, this may seem moot, but the theory interests me and probably has applications elsewhere.

 

Additional questions -
* Are the more aggressive riders experiencing more wear??
* Do people with different fuel blends (different regions of the country) have more wear??

Chacal would know - as he ships the needed shims and knows all our secrets.

 

Don't assume a "constant rate of gap narrowing."

The more miles one of these motors gets on it, the more the valves tend to "stabilize." Provided you've stayed on top of the adjustments, and haven't run any out of spec for too long.

My '81 550 has 27K on it; the last time I checked its clearances none needed to be adjusted, although 5 of the 8 had become a bit tighter since the previous adjustment. Two are on my "watch list" and may be out of spec by the next check.

The most important check is the first one; unfortunately a LOT of these bikes make it to 15K or 20K without being touched, before they start running ragged or having "hot start" issues because of it.

 

That makes sense; I could see a long plateau of very minimal wear before a failure. At what point is a failure likely? Yes, if I started to notice rapid gap-narrowing, then it's time to crack it open, and that would be the only realistic way of actually pinpointing the beginning of the end.

But, at what point during the plateau could one determine it's time to take it apart and install new valves?

I don't doubt that for a second. :lol:

Being a dual-sport guy myself, with a kickstart-only, street legal Honda XR400, valve adjustments happen on fairly regular intervals. Partially, because anything less than in-spec valves makes kick starting a big ass dirtbike that much more enjoyable. Also, it doesn't have shim-on-bucket lifters and can be accomplished in 15 minutes with a 10mm wrench, a screwdriver, feeler gauges, an 8mm hex key to remove the plug to see the TDC mark, and a 30?mm socket to remove the caps to reveal the adjusters. Not having a permanent odometer (I have a VAPOR speedo/tach/odo/timer that works great until the battery dies and it loses any mileage info) makes scheduling this on regular events impossible, but you can hear subtle changes in the valvetrain and know it's time. Usually off by only 0.001-2 inches, sometimes none at all. The intakes often loosen up.

 

I don't think just keeping an eye on clearances would be an indicator. Quite honestly, I think that there'd be other reasons to pull the head sooner, like a valve stem seal letting go, that would prompt a "no sense not doing it while apart" valve job.

Properly maintained, these things appear to be very long-lived. I know my '81 550, with 27K on it, is just a puppy yet. And the valves have pretty well "bedded in" not changing much at the 5000 mile intervals.

Shim-over-bucket is NO COMPARISON to rocker-arm systems; the cams and shims just DON'T wear; at least not appreciably so. My pushrod rocker-arm Norton has a 2500-mile adjustment interval; and they are ALWAYS loose.

The valves themselves don't wear much once bedded in.

 

Yes, they are pretty much bulletproof. Only problem, really, is the owner. "You have to measure stuff, write it down, take it apart, write THAT stuff down, and then order stuff? Naw it runs fine."

Throw some lifters on a cam, with pushrods, with rockers, then onto the valves... Yes. A heavy valve train, stiffer valvesprings needed to keep it all under control, multiple wear points, RPM limited without going to lightweight alloys in extreme cases. A (relative) ton of adjustment needed frequently.

Honda single cylinder 4-strokes (in the dirtbikes, anyway) have an overhead cam with rockers, with the rocker pivots in the upper casting. Nearly maintenance free (change the oil, and don't run it low, and occasionally clean out the frame tube screen), the cam and rockers simply don't fail until you run it out of oil and burn it up (which happens. Pull the dipstick. It's not hard. Still, owners run them ragged until they seize). I'll estimate that I adjust them about every 1500 miles, but that's because I have no idea how many miles I put on it, it takes 15 minutes, and when I get it out on some trails, I'm not very kind.

They will only rev to about 8 grand, and are not happy about it. They're tractor engines. Low end torque. I've got the biggest front sprocket and the smallest rear sprocket they make (without custom machinework) to lower the cruising RPM.

Yes, this is an apples to oranges comparison, and this is an XJ forum, not a honda XR dirtbike forum, but I like the theoretical nature of how different systems behave over time.

 

I have 650 XJ Maxim with 900 motor on it. 900 motor had 21K on it when I got it. Got running. Put another 15K running mostly 84 miles round trip to work. Checked valves. All ok. Only one close to tight but in spec. Runs flawless and still need to sync carbs & colortune... Great Bike. Will run till the wheels fall off.