Valve adjustment R90/6

I know the valves need to be adjusted regularly because they tend to close up, but I have never seen an explanation of why they do this. Could someone explain? Thank you.

Heat. The internal combustion engine runs a optimum heat range. The valves and seats are of a special metal composition, metalurgy, to process this in the engine. Stellite is the prominant material on the face of the valve tulip to seal the combustion chamber. Great development for longevity of seats and valves. But regardless over time heat cycles wear and deteriorate this material and the valve has be be adjusted for this wear. A lot of factors come into play with a aircooled engine to extend the life of the valvetrain. It varies from bike to bike, but 100,000 miles is a good figure to use and it can be more than 100,000. Valves adjusted on a regular timed sequence extend the life. But when valves need to be adjusted much more often than normal, this usually denotes that the head needs to be removed, inspected. Usually the valves and seats can be machined to a proper fit and reassembled. Good for another 100,000 plus. If the seat or valves have deteriorated beyound their life cycle, they are replaced. So, how many miles on your /6?

Reasons I can think of for valve clearances closing up:

- relaxing of the head bolts/rods...when the fuel explodes in the combustion chamber, the effect is like trying to blow the head off the engine and pull the heat bolts/rods with it
- further compression of the head gasket
- valve seat recession...slow wear of the valve seat allowing the valve to sink deeper into the head

As mentioned above, the latter problem will eventually happen to all pre 1981 heads depending on a range of factors.

This was covered extensively in the MOA magazine by Oak sometime in the early '80's. The valve seats in the pre-1980 bikes were cast iron metallurgy (shrink fitted into the aluminum heads) and this material was of insufficient hardness to resist wear when leaded premium went away. As a result of valve seat wear, the valve gradually embeds itself into the seat. This results in the valve stem becoming closer to the rocker arm tip with a resultant decrease in clearance. The only viable corrective action is to replace the valve seats. Oak's preferred replacement seat was made from Pleuco, a proprietary alloy.

swall said:

Oak's preferred replacement seat was made from Pleuco, a proprietary alloy.

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I would also think that the metallurgy used by BMW beginning with the 1985 models is a suitable replacement. Once BMW figured out what was wrong with the '81-84 heads, the new replacement parts have stood up well since then.

They don't all do that. But, as mentioned above, the two main reasons are valve seat recession and valve head deformation. Back in the days of leaded fuels the tetraethyl lead cushioned and lubricated the valve heads and seats. With the removal of lead from the fuels the older softer seat materials will wear - physically wear. As the head of the valve "recedes" deeper and deeper into the worn/wearing seat (thus the term "recession" the gap closes up.

In the time period 1980-1984 BMW made a big mistake. Knowing that the existing valve and seat metalurgy was not going to work with the widespread and continuous use of lead free fuel, BMW began using hardened valve seats. We'll call these Generation 1 hardened seats. It was soon discovered that exhaust valve gaps were closing even more rapidly than before. Upon removal of the head and examination of the valves it was obvious that the heads of the exhaust valves were physically deforming in a manner called "tuliping" because the outer edges of the valves were cupping. When first discovered many a dealership replaced the "defective" valves leaving the seats in place, and in many cases the problem simply happened again.

After a while it was discovered that the culprit was actually the hardened valve seat. While they looked pristine when the damaged valves were discovered, the problem was that the seat material dispersed heat poorly. The brief moments of contact with the seats is how the exhaust valves heads are cooled. So with poor cooling the valve heads would deform. This was most often on the 1000cc models but also happened but less often on the 800cc models. By the 1985 model year BMW had figured the problem out and put into production the Generation 2 new hardened valve seats.

Other wear factors tend to cause the gaps to open: camshaft wear, cam follower wear, rocker arm wear, etc all cause a wider gap.

You've (swall) got the dates wrong, as it was the '81-'84 Nikasil bikes that suffered most from valve recession. The problem is better described as "accelerated" valve recession.

Nevertheless, nothing lasts forever, and a valve opening and closing a zillion times is going to create wear on both the valve itself and the surface it contacts (seat). The gap will close due to this wear and wear will be in fact accelerated if the gap is not maintained, since the gap provides some heat relief. In addition, it could occur that without gap the valve may never close at all and then obviously performance suffers. Finally a valve worn so much it no longer contacts the seat and instead retracts even farther than normal could see the keepers fall off, leaving the valve loose to fall into the piston.

By the time accelerated valve recession became a noted problem, it was mostly about the 1000 engines. The '81-'84 engines were jetted more lean than ever and had the air ingestion system creating further combustion in the exhaust system. The BMW solution was new valve seat material and changing the valve/seat angle from 45 degrees to 30 degrees to provide a larger contact surface. The "big-valve" R100 heads were discontinued as well.

In Oak's article on this, he differentiated the two periods of what lead to valve clearance reduction. Prior to 1981, it was what has been mentioned - valve recession. From 1981-1984, it was valve face plastic deformation (VFPD) or as Paul said, tuliping.

Yes, I confess to not having fine tuned the dates I reported, and I likewise did not bring up the seat alloy change that led to the tuliping in the 81-84 bikes. As I understand the situation, the problem manifested itself mostly on R100 models. Yes, the valve seat alloy that was the first iteration to replace the cast iron seats had lower thermal conductivity that the cast iron. This was a high nickel alloy, not particularly high hardness. It may have been a proper choice from a standpoint of preventing adhesive wear between the valve and seat but the lower thermal conductivity resulted in the valve running hotter. Oak told me he never used these seats when he did valve jobs--I think he used seats for the oil head machines and had them center-less ground to fit the Airhead seat pockets. I'm not sure BMW ever went away from the high nickel, low thermal conductivity seat alloy as Airhead production phased out. Their response was to downsize the valve sizes and lower the compression. I welcome any tech corrections on this. BTW, I am glad to see there is such an interest in these technical details--as a metallurgist, I have a bit of analysis data from looking at valve seats and other stuff. Now that I am retired, maybe it is time to better organize this material.

I repaired a lot of airhead heads from 1992- 2003, the '81-84 were definately unique. The thing I saw missmatched valves and seats both ways. Sometimes the valve was pretty good shape and the valve seat was deformed and then I would see the opposite. The valve would be deformed and the seat good. Then again I would see some with what I cosidered to be equal in normal wear. These were all heads from '81-'84; 650's, 800's, and 1000's. I'm just guessing that different batchs of valves and seats came thru from the suppliers to BMW, most were missmatched and then again a few got thru that were matched properly. This was by pure happen stance. I guess BMW at that time probably didn't know which were or were not matched properly. They may have not known of the problem at that time. It also seemed that most riders were getting between 50,000-100,000 before thay pulled the heads. Not bad really. Most of my customers were on a budget. When I saw that the valves were bad, they obviously were changed. When the seat was still good; I gave them a choice. Change the valve and keep the old seat. The cost to change the seat was kinda expensive. And bad deformed seats were changed as a matter of fact. I think it is a little bit of a myth that 1000cc engines were that much worse. The 1000cc engines from '77-'80 were not known to have this problem. Endless factors come into play on this stuff. But hey what do I know, I was just a flunky machinist.

lkchris said:

You've (swall) got the dates wrong, as it was the '81-'84 Nikasil bikes that suffered most from valve recession. The problem is better described as "accelerated" valve recession.

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Sure - model year 1981-1984 is the time period 1980 - 1984 exactly as I stated, and the valve head deformation due to poor heat transfer is technically not valve recession. Yes the valves close up but for a totally different reason than recession, even though a lot of people call it that.

And in fact some valves do last seemingly forever. Classic K bike valves can be counted on to go 300,000 or 400,000 miles with only a few shim changes. F800 valves seem almost as robust. As much as we might like old Airheads, the valve train was one of the notable weak spots.

I think that the OP is asking why wear in the valve train results in tighter clearances vs. looser clearances. This used to puzzle me too. Wear on the cams, followers, both push rod ends, both ends of the rockers and on the valve stem tip would (and will) loosen the overall clearance. As has been discussed above, wear and/or deformation at the valve seat/valve head interface will cause the end of the valve stem to move upwards and tighten up the overall clearance. Because of the high heat and low lubrication at the valve seat/valve head interface, the whole valve train tightens up faster than it loosens in normal operation.

MonoRT said:

Because of the high heat and low lubrication at the valve seat/valve head interface, the whole valve train tightens up faster than it loosens in normal operation.

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I agree with regard to Airheads. With K bike and F bike valve train the wear seems to be offsetting and the clearances are quite stable.

So along these lines - can I ask:

Is there anything I should be doing on my new (to me) , pristine 1977 R100S with now 13,000 miles on it? I won't ever put a lot of miles on it as it is and will remain a play bike. BUT, should I use a fuel additive? Or, and I think I know the answer don't worry about it because in my case it will never become an issue? Thanks all -

mikeyork5 said:

So along these lines - can I ask:

Is there anything I should be doing on my new (to me) , pristine 1977 R100S with now 13,000 miles on it? I won't ever put a lot of miles on it as it is and will remain a play bike. BUT, should I use a fuel additive? Or, and I think I know the answer don't worry about it because in my case it will never become an issue? Thanks all -

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I'd start a maintenance log. Do you have any records with the bike? If not at next oil change I'd adjust the valves and make a note of the clearance on each valve. I'd then just ride it checking every year or two (or 5k miles - whichever comes first). If you're running out of adjuster or you see the valves closing up then have new seats, etc put in.

Just my .02

dmftoy1 said:

I'd start a maintenance log. Do you have any records with the bike? If not at next oil change I'd adjust the valves and make a note of the clearance on each valve. I'd then just ride it checking every year or two (or 5k miles - whichever comes first). If you're running out of adjuster or you see the valves closing up then have new seats, etc put in.

Just my .02

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And I'll add - if you start having hard starts or rough idle check the valve clearances before you mess with either the carburetor or ignition.