Borrowed time on valve train?

[ohiorider]

My 1991 R100GS recently turned 123,000 miles. I've done some preventative maintenance (rebuilt carbs, replaced cam timing chain and tensioner, replaced front and rear tranny bearings 20k ago (but didn't have shaft machined for circlip). The original driveshaft did make it to almost 122,000 miles, and finally crapped out in central Indiana on homeward trip from Montana. I shipped it off for repair this Sept. to a shop in Green Bank recommended by Oak a few years ago. Repair looks good and runs well.

Re the borrowed time issue .... let me set the stage. The heads have never been off this bike. Compression when measured warm 10k miles ago was approx 140psi on each cylinder. Valves tend to stay in adjustment, and I ck them every 5k miles. Burns no more oil now than ever. Still wants to pull hard from 65-70mph in 5th gear. I realize common wisdom is to pull the heads and have them rebuilt at or before 100,000 miles.

Question - is the valve train in jeopardy of a catastropic (sp?) failure, due to a valve seat breaking off the stem, due to age and wear, or do the valves on higher mileage airheads generally burn out at their seats (non-catastropic)?

Bob

[bpeckm]

Sounds like things are working perfectly.... but not being an expert on catastrophic failures, as you asked about, I hesitate to say it, but I will anyway... heads and valves usually give lots of warning of impending problems, of which you have none. If it ain't broke....ride it, enjoy it!

[Yarddog]

Sounds like things are working perfectly.... but not being an expert on catastrophic failures, as you asked about, I hesitate to say it, but I will anyway... heads and valves usually give lots of warning of impending problems, of which you have none. If it ain't broke....ride it, enjoy it!

Agreed...if it ain't broke, don't worry about it...it's a simple enough thing to do when the time comes...and it's been my experience that when ya do thing that don't need doin', more often than not yer opening up Pandora's Box!!!

[carockwell]

Read Joel Rappoport's blog about the same issue. He had over 300,000 miles on his heads. He said that he stopped listening to the "experts" after that. You just have a good set heads. No need to pull them off unless the valves start closing up or you get low compression.

[88bmwjeff]

Best thing to to is just monitor the situation. Keep a good log, if you don't already. Before you adjust the valves, measure the gaps and note it down as well. This way you will be able to see a pattern before it becomes a problem.

Now ride the hell out of that bike and enjoy it.

[AntonLargiader]

Bob, how does the motor sound? Is there a clank that you can't get out by careful valve and rocker adjustment? If so, you could benefit from a valve job. If not, you're probably fine.

Look on my website under "Valves" for a mouseover picture of a very worn GS valve. It was around 50k miles, but I've seen them that bad at 20k miles.

[ohiorider]

Guys, thanks for all the comments. Anton, that is an interesting (somewhat scary) Javascript working pix of a valve in a bad valve guide. Re any 'clanks', hard to tell with this engine .... it was always a rattly beast. And no, I've never made any adjustments to the rockers, just kept the valve clearances adjusted.

One other question ..... although I know it is only one more step to pull the cylinders and install new rings, do the pistons and rings tend to hold up better than the valves? In other words, if I was inclined to do the valves/guides this winter, ride the bike next season, and put off reringing until 2011, is there a downside, aside from replacing head gaskets 2x?

I have my '03 Triumph T100 available for most of my weekend backroad riding, and will be taking delivery of a Guzzi 1200 Sport (new 2008) in the spring, so I probably won't put more than 5-7K on the Beemer next season. I suspect I'll be busy sorting out the Guzzi!

Thanks,

Bob

[mneblett]

Bob, how does the motor sound? Is there a clank that you can't get out by careful valve and rocker adjustment? If so, you could benefit from a valve job. If not, you're probably fine.

Look on my website under "Valves" for a mouseover picture of a very worn GS valve. It was around 50k miles, but I've seen them that bad at 20k miles.

My heads have just been pulled for dual plugging. My left head exhaust valve has at least that much lateral motion at only 16.5K miles.

My point: mileage does not seem to be a determining factor. I was told that the lateral motion problem became particularly notable when BMW went to 30 degree valves in 1988. I have no reason to doubt this, but as this is the first post-87 head I've observed with its valve springs removed, I have no basis to confirm/deny it. Nonethless, at the moment my 30 degree valves are being replaced with 45 degree unit, with corresponding seat work and guide replacement.

Mark Neblett
Fairfax, VA

[James.A]

The 30 vs 45 thing seems plausible to me. The machinist in the shop I use, however, is of the opinion that the angular/off-set attack of the rocker striking the valve stem is more likely to promote wear in the guides. Bear in mind that his shop builds all kinds of motors, not just airheads.

[mneblett]

The 30 vs 45 thing seems plausible to me. The machinist in the shop I use, however, is of the opinion that the angular/off-set attack of the rocker striking the valve stem is more likely to promote wear in the guides. Bear in mind that his shop builds all kinds of motors, not just airheads.

Interesting theory, but it doesn't square with the real world experience -- tens of thousands of airheads since at least the /5's release in 1969 with the same basic rocker-induced side thrust configuration, without this later-airhead issue.

Don't get me wrong -- I'm not unyieldingly in the "it's definitely the 30 degree valves" camp. If there's a better explanation for the issue, I'd like to hear it, as I'd like to address it while the heads are off the engine being reworked. I just haven't come across anything more plausible as yet.

[Yarddog]

While I have the engine outta the frame for a refresh, I've removed the heads and cylinders...I'm gonna have the heads gone thru, valves lapped and the heads generally cleaned up...since the second plug was already in the head, I thought about putting the second plug into play, but on retrospect...naw...I'd swap heads with someone who had a single plug head, but I'm not gonna work it real heavy...When I put it back together, I think I'll just hone the cylinders and not replace the rings...the top end was supposed to have been done not that long ago, and it looks just so...

Then I'll paint the engine black and call it good! Just the minimum as it was running fine when it came apart...