rate of valve wear over time on 87 K75

[48882]

When my bike was newer, I was riding it regularly and having it serviced regularly. During this time, it almost never needed a valve adjustment, going more thank 40k before needing an adjustment.

During the last few years, the bike has spent most of it's time in the 'centerstand' position, and I've ridden probably less than 5k miles since the last valve adjustment, probably 5 or 6 years ago.

Currently the bike has ~90K miles. It was running poorly with cold bloodedness that has gone on for a year or two and more recently, very poor running which had me bring the bike to the shop.

The mechanic did a full service and now the bike runs great. He pointed to the valves as the main culprit as they were tight, especially the exhaust valves.

Obviously, I need to sevice the bike properly. My question/concern is the interval that the valves needed adjustment. After being so trouble free all those miles, it doesnt' make sense (to me) that they needed an adjustment after so few miles. Is this normal behavior or is there something else that I should be looking for as the root cause of the valves going out of adjustment after such a short time?

Thanks!

 

[roncooper]

There have been several reports lately of K75 exhaust valves tightening up. Mine did. I checked them at 40,000 miles and 2 intakes were slightly tight and adjusted.

3 years and 34,000 miles later, one intake and all the exhausts were tight(#3 at.003 in.) I don't know what would cause such a large change in 34,000 miles.

 

[98lee]

There have been several reports lately of K75 exhaust valves tightening up.

More like several reports of K75s actually having to have normal maintenance performed on them.

Since the demise of leaded gas and the propensity of early K owners to totally disregard any type of standard valvetrain maintenance on their bikes, there have been reports of valves way out of adjustment.

What it's called (on the exhaust side) is valve recession. That is the exhaust valve sinking farther and farther into the head. This causes the exhaust valve clearance to tighten up. This is also caused BY the exhaust valve clearance being too tight.

It's a spiraling type of thing. The valve clearance is too tight. That causes the valve to sink further into the head thereby tightening up the clearances that cause the valve to sink further (and faster) which tightens up clearances even more.

The reason too tight a clearance on the exhaust is so bad and accelerates the wear is because the exhaust valve is in VERY HOT exhaust gas flow during the exhaust cycle. The ONLY thing that helps get rid of that heat is seat time (time when the valve is fully closed on the valve seat). The tighter the clearance the less the seat time. With cold clearances of .004", the valve is actually NEVER fully closed once the engine heats up and takes away that .004" with expansion.

Bottom line , the tighter the exhaust valves, the quicker they will recede into the head.

Plus once the process of recession starts the quicker the rate.

ALL early Ks should be checked every 10-15K because they are all starting the recession process by now and the closer you keep the exhaust valves to .012" the longer they will last. If you wait until it doesn't run well before you check the adjustment, you are severely damaging the engine and shortening it potential life.

 

[ted]

Oddball question. Might it also be helped along by aging rubber allowing air leaks that while somewhat masked by the Jetronic compensation still cause enough of a lean running bike to result in increased heat at the head -- and the accompanying valve seat wear that comes from not increasing the valve adjust intervals as a result?

 

[98lee]

Oddball question. Might it also be helped along by aging rubber allowing air leaks that while somewhat masked by the Jetronic compensation still cause enough of a lean running bike to result in increased heat at the head -- and the accompanying valve seat wear that comes from not increasing the valve adjust intervals as a result?

A separate issue but with possibly the same result. Proper valve adjustment and air leaks causing a lean condition are both all too common ignored items until the bike really runs poorly.

The problem is the Jetronic does NO compensation for air leaks, thereby causing a lean condition, more heat and a much more hostile environment for the valves.

Since the early Ks have a well deserved reputation of being almost indestructible (how many non BMW early Ks do you routinely see with WELL over 150k on them and running like new??), too many people translate that into meaning that NO maintenance is necessary to get the same high mileage results. Far LESS maintenance is required than a lot of different models, but only adding gas and changing oil will eventually lead to problems that could have been avoided.

 

[LOSTBOY]

I've seen several high-mileage K75s with disappearing exhaust valve clearances (all with well over 100,000 miles). No matter how frequently the clearances were adjusted, the cylinder heads eventually needed service/replacement.

Why is this happening when K100s have yet to exhibit similar problems? K75s run less ignition advance, which may cause an increase in temperature. They also rev higher than K100s.

 

[roncooper]

More like several reports of K75s actually having to have normal maintenance performed on them.

Since the demise of leaded gas and the propensity of early K owners to totally disregard any type of standard valvetrain maintenance on their bikes, there have been reports of valves way out of adjustment.

What it's called (on the exhaust side) is valve recession. That is the exhaust valve sinking farther and farther into the head. This causes the exhaust valve clearance to tighten up. This is also caused BY the exhaust valve clearance being too tight.

It's a spiraling type of thing. The valve clearance is too tight. That causes the valve to sink further into the head thereby tightening up the clearances that cause the valve to sink further (and faster) which tightens up clearances even more.

The reason too tight a clearance on the exhaust is so bad and accelerates the wear is because the exhaust valve is in VERY HOT exhaust gas flow during the exhaust cycle. The ONLY thing that helps get rid of that heat is seat time (time when the valve is fully closed on the valve seat). The tighter the clearance the less the seat time. With cold clearances of .004", the valve is actually NEVER fully closed once the engine heats up and takes away that .004" with expansion.

Bottom line , the tighter the exhaust valves, the quicker they will recede into the head.

Plus once the process of recession starts the quicker the rate.

ALL early Ks should be checked every 10-15K because they are all starting the recession process by now and the closer you keep the exhaust valves to .012" the longer they will last. If you wait until it doesn't run well before you check the adjustment, you are severely damaging the engine and shortening it potential life.

I took the word of internet "experts" who said "40,000 miles is a good interval for checking valves, since they seldom need shimming". I now wonder about the air filter, "they" also say it is "50% over sized and is good for 50,000 miles"

 

[kentuvman]

Where to buy Shims?

I plan to check valves on my '87 K75S with 43K (did it last year and they were okay).

If I need to adjust, where, besides BMW dealer, are other sources to purchase shims?

 

[98lee]

I've seen several high-mileage K75s with disappearing exhaust valve clearances (all with well over 100,000 miles). No matter how frequently the clearances were adjusted, the cylinder heads eventually needed service/replacement.

Why is this happening when K100s have yet to exhibit similar problems? K75s run less ignition advance, which may cause an increase in temperature. They also rev higher than K100s.

I don't know if or why there is any reason for a difference on the 100s. I personally (on this forum) have only heard of one K75 that ran out of adjustment. And it was more than likely a case of neglect during some point in it's life.

I don't believe any of the famous high mileage K75s had ever had the head off (Paul?). I believe they were examples of proper maintenance.

To point out that heat is an issue: You'll notice that on a K75 it is almost ALWAYS the #3 exhaust valve that is the worst or first to show problems. I don't know about K100s. Their cooling jackets might be designed much differently.


Several points a professional engine builder friend of mine pointed out that MIGHT be relevant:

Carbon: When the clearances tighten up and are left that way for a while, carbon builds up on the seat (remember, as clearances tighten at the cam, it's actually not allowing the valve to fully close at the seat). This acts as an abrasive on the seat and the valve causing the valve to recede faster into the head. Proper clearances diminish this effect.

Exhaust seat metallurgy: These seats were designed when unleaded fuel had recently become mandatory. Fuel formulations have changed many times since then and the possible effects on exhaust seats is possibly a factor. If any of you have old muscle cars that have had the engines rebuilt, you know they had to put hardened exhaust seats in the heads to cope with the unleaded fuel, but prior to the rebuild there was enough "residual" lead that had been hammered into the seats to get by. Once that layer wears or is ground away the seats wear VERY rapidly.

Oils: No, I'm NOT going to start an oil thread here! These bikes were designed for oils high in the anti wear elements Zinc and Phosphorus. Motorcycle oil for most bikes has to deal with wet clutches, so most bikes use motorcycle specific oils. Motorcycle specific oils still have high Zinc and Phosphorus in them. Since Ks have a dry clutch, people believe that they can use any off the shelf oil. MOST oils have had most of the Zinc and Phosphorus removed or greatly reduced (by LAW because it plays havoc with catalytic converters in cars). Reduced anti-wear properties in the oil could lead to valve guide wear and accelerated seat wear. Lower quality oils (that don't have the high heat capabilities) could lead to more carbon build up and coking around exhaust valves (especially pre pinned ring bikes that are allowed to suck in oil when left on side stand after getting hot).

Once more back to seat metallurgy: The exhaust seats are hardened. They get that way by being heat treated. The process involves getting very hot and cooled rapidly (to very exacting specifications). The opposite process is called annealing which is basically getting very hot and cooling much slower (kinda like what happens in your engine). It might be possible that running the bike for a while with such tight exhaust clearances that the valve has not enough seat time to cool would change the hardness of the seat or valve (think about how a valve "burns" yet not quite that extreme) causing them to wear faster from then on.

Taking valves that have run for some time at WAY too tight a clearance and then setting them to the proper clearance MIGHT not necessarily reset everything. Damage may already been done that will show up as more rapid changes (perceptible or not) in valve clearances for the rest of its life compared to a bike that has ALWAYS been kept within limits.



Bottom line, you can do what you want and you might get lucky, but you can definitely increase your odds by doing basic maintenance and being proactive not reactive.


PS: Any dealer or mechanic that lets your bike leave his shop with an exhaust valve at .010", after a valve adjustment, is doing you no favors. He should have set it to .012". By the book, he's right but since most of the cost of is just getting in there, he should be proactive.

 

[98lee]

I plan to check valves on my '87 K75S with 43K (did it last year and they were okay).

If I need to adjust, where, besides BMW dealer, are other sources to purchase shims?

$4.17 from BMW dealer.

Kawasaki also makes a 29mm diameter shim that will work.

 

[PHMARVIN]

Hi, Kentuvman,
As 98lee said, Kawasaki used 29mm shims. I believe some models of Honda (may be Yamaha) did, too. If you need a shim NOW and your dealer doesn't have the shim(s) you want, find an independent shop which does a lot of Japanese bikes. They will probably have a bunch of shims and will probably sell you what you need.

 

[kentuvman]

As always, thank you gentlemen!

 

[kentuvman]

Which is cylinder #3?

Is cylinder #3 forward or aft?

 

[roncooper]

Is cylinder #3 forward or aft?

Aft.

 

[48882]

thanks for the great discussion.

what I think I've heard is that the lack of maintenance becomes cumulative over time. Heat damage to the valves becomes a loop, where the heat causes damage, which increases the heat, etc.

Where I'm still confused is that I had been keeping up the maintenance on the bike until my riding became significantly reduced. The heat argument would not seem to apply if the bike is not running.

Perhaps its just 'one of those things'. I'll definitely keep a close eye on it, though. I originally ignored the cold blooded problem because it started almost immediately after the temperature sensor at the bottom of the oil/water pump was replaced and I though that the sensor might have been in a different temperature range (I now think all the sensors were the same range).

 

[PGlaves]

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