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Revised by the Author 09/20/2007
Disclaimer:
- This entire article is my personal opinion.
- Use of aviation fuels, in the U.S., and possibly elsewhere's, on road vehicles, is likely illegal.
A considerable percentage of BMW airhead motorcycles have had various valve and/or valve seat problems, up until the 1985 models. This article will try to explain the background, and why the various problems developed, and why some have problems and some do not. Note that air-cooled motorcycles are more subject to 'top-end' stresses, than water cooled motorcycles.
Tetraethyl lead, dangerous if absorbed through our skin when in pure form, is a form of lead previously used extensively, and highly diluted, in gasoline. This compound has TWO major effects. First, in sufficiently high percentage, it can raise the octane value a fair amount, very important in WWII aircraft engines, especially those that were supercharged; some were both supercharged AND turbocharged. The original purpose for using tetraethyl lead in gasoline was specifically for raising octane. Some of those aircraft engines required 145 octane gasoline. Usage for the purpose of raising octane was carried forward for decades for cars, because it was cheaper, even when the lead was sold to the refineries expensively by the then Ethyl Corporation, than using only refining methods to increase octane. Many decades ago, premium (higher octane) gasoline's were simply called "Ethyl". Usually those gasoline's had MORE TEL (Tetra Ethyl Lead). There is a lot more to this story, this is a simplified version of lead usage.
Lead is not good for human beings, whether breathed in from the air, or by skin or other contact. Lead has been removed from common paints for walls in your house, and lead has been removed from road-vehicle fuel. Lead is responsible for shorter spark plug life. Lead conducts electricity, and is slowly deposited in metallic form on the center ceramic insulator of the spark plug, and when enough is deposited, it will electrically short out the spark plug. Deposits of lead and other compounds, some of which are metallic, are also left on valves and pistons, and generally any place combustion products can get to. Various chemicals are added to gasoline to minimize this, a famous one is Chevron's "Techron".
This means that, via blowby and some other means, the lead would contaminate the engine lubricating oil. Some deposits even manage to get inside valve guides, promoting wear. They get under the valves, making for deposits that can have various bad effects. These various deposited compounds can raise compression ratio, cause some other types of wear, have 'hot carbon spots' causing detonation,degrade the oil....ETC. Some lead was also spewed out in the exhaust, contaminating the air you breathe.
The 'seat' and 'valve' protection effects were something noticed later on, after WWII [I think]. Tetraethyl lead leaves a microscopic layer of lead on the valve and seat; some folks call this cushioning. It was generally originally accepted that this effect, a 'lubricating', if you will, prevents microscopic welding of seat and valve, or at least reduced metal to metal microscopic contact, sort of how oil works. This is a GROSS simplification!
Little bits of carbon trapped in the valve edge and seat area might even help promote that welding. It was thought that this occurred microscopically here and there, then the valve lifts again and some microscopic metal is removed leading to pits and valleys. That this really happens in our Airheads is debatable...read on....
Lead is, at very high temperatures, a LUBRICANT, AND A TERRIFIC CONDUCTOR OF HEAT. The conjecture was that a small amount of lead makes the seat and valve much less likely to weld at microscopic scattered points, and that the valve head heat is more readily transferred to the seat. Lead was thought to generally totally prevent the welding.
However, new theory has it that in our pre-1985 airheads, it is the normal rotation of the valves (the rocker arm tip strikes the valve stem OFF CENTER, causing the valve to rotate), that is causing the wear, wiping away, so to speak, microscopic bits of metal, when the lead is not there.
Whichever it is, the results are described as the exhaust valve eating away its associated valve seat and the valves slowly dropping downward into the valve seat area.
HOWEVER... BMW really had TWO problems. The problems began showing up first on the 1980 and later models. It did NOT show up initially at that early period of time where folks worrying about upcoming lead-free gasoline's thought it would first show up; the 1979 and earlier, which had cast iron seats.
BMW changed the 1980 and later seat material, IN ADVANCE PREPARATION for the soon-to-be ONLY type of car gas, unleaded gas. In the U.S., lead-free gasoline's were introduced in 1989...or, better said, road use leaded gas was outlawed, and full compliance, started earlier, was completed in 1989.
The new seat material BMW introduced in 1980 was a type of stainless steel, a high nickel-chrome alloy. Problems with the 1980 and later models soon began, and became more and more evident, and the dealers and BMW blamed things on poor maintenance and rider abuse...for the problems showing up on U.S. motorcycles. Many of these engines developed deformed and overheated exhaust valves...some were swallowed up in the seat bore. The R100 models were the worst. They develop more heat than the smaller engines. There were other contributing factors, including lean running engines, the pulse-air injection system, high power output on some models, etc. Thus, the valves were 'deforming' ...sort of like a piece of plastic that is overheated, and started sinking into the valve seat bore.
The 'improved' valve SEATS that BMW introduced in 1980 were not damaged but somewhat at the edges, certainly not like the pre-1980 models valve problems (which showed up later on).
Some of these 1980+ machines got valve jobs, and the bad problems started up again. It seems that BMW made an 'improvement' for the 1980 models that made things worse. There is a German word for that sort of thing....a 'fix' that makes things worse. It is Schlimmerbergesserung; or, more properly, Verschlimmerberbesserung. BMW never used that word. BMW hardly really admitted to having a problem.
The first indication of either of the two types of valve problems is almost always rapidly closing-up of valve clearance settings, PRIMARILY the exhaust valve. This is true for both pre and post 1980 models.
Note that a BMW airhead with mechanically sound valve gear: good seats, valves, guides, of the later proper materials, or, earlier parts (pre-1980) with leaded gasoline's; will normally not have its valves close up more than about .001" to .002", per 5000 mile adjustment interval. Those riders that like to use the area near or at red-line rpm, may find the valves closing up a bit faster. In fact, with a good top end condition, closing up of as little as .0005" per 5000 miles is NOT unusual. The situation with the bad 'new' seats might find valves closing up entirely in a thousand miles OR LESS.
Meanwhile, the 1979 and earlier bikes, were still mostly OK. Mechanics KNOW that cast iron seats won't hold up to unleaded gasoline. What was going on? Well, there was plenty of lead still around. It takes only a minuscule amount of lead to protect the CAST IRON valve seats.
***About 3 ounces of aviation leaded gasoline, in the common grade called 100LL, is needed, per gallon of unleaded car gas, for good protection on the 1979 and prior airheads. On a practical basis, less than this amount seems OK, and many have had good results by simply adding a cupful (a cup is 8 ounces officially, but in practice is 5-8 ounces) every few tankfuls. NOTE that this usage for road vehicles is illegal in the U.S., possibly elsewhere's too. At the time I did research that went into this sort of thing, it was UNCLEAR if off-road use was illegal.
***For the technical types out there wishing more exacting information, aviation fuels containing either 4 or 8 grams of tetraethyl lead per gallon are available at airports. It is commonly accepted that approximately 0.1 gram (some say up to 0.3 gram for high protection levels) of tetraethyl lead per gallon is enough to provide reasonable valve seat protection. This is FAR LESS than that required to boost octane any appreciable amount. Using the 4 gm/gal specification type of av-gas, only .025 gallon of av-gas is needed per gallon of unleaded car gas. A gallon is 128 ounces (U.S. measure). Thus, only 3-1/4th ounces of av-gas per gallon of car gas is required for likely acceptable protection. NOTE that this is helpful ONLY on the PRE-1980 models with the original valve seat material.
In early 1989, most major gasoline producers suddenly entirely eliminated leaded fuel. Residual lead in tanker trucks, tanks, etc., were suddenly thinned out of their lead content. Some suppliers had already started selling UNleaded fuels some time before this date.
Suddenly 1979 and earlier bikes were having problems with valve settings closing up. Usually seen was major wear patterns, sharp and well defined steps in the valve SEAT. The exhaust VALVES in the 1979 and earlier models were generally OK. The 1980 and later models continued to show damage as previously described, mostly to the VALVE.
Fresh valve jobs were not lasting as a repair, problems returned shortly thereafter.
SOME pre-1980 bikes had no problems. Apparently BMW had more than its share of problems in heat treating the original cast iron valve seats.
So, there were TWO distinctly different problems, depending on whether the engine was a 1979 and earlier, or a 1980 and later. There were sub-problems, in that SOME 1979 and earlier cast iron seats were 'harder' than others, and that some engines ran much hotter. Thus SOME folks had NO problems with the UNleaded fuel on PRE-1980 bikes.
The BMW PROPERLY DONE 'production fix' occurred about 1985. This happened right about the same time that BMW upgraded the valve rocker gear for the shim method for end float, not that THAT had anything to do with the problems being discussed in this article.
The fix for BOTH problems is to put in the latest VALVE SEAT that BMW sells now. This material has a vastly improved thermal conductivity, and is immune to not having lead in the fuel.
It is possible that using a tad of tetraethyl lead in today's lead-free gasoline MIGHT make our updated valve seats and valves last even longer, but I have no information, and this is pure off-the- wall speculation, without a shred of proof on my part. In fact, it could be the opposite, with more wear in the guides, and possibly elsewhere's in the engine. What IS true is that the early (PRE-1980) engines don't have these specific valve problems if their is some lead in the gasoline.
The new seat material is not what folks generally think of as a Stainless Steel (SS); rather, it is a 'tool steel'. In installing the new seats and doing a valve job, sometimes the early valves, if in good condition, are reused. Installing valve seats in Airhead heads is a job for a specialist who is VERY familiar with our Airhead heads....I do NOT care if your favorite machinist or automotive overhauler 'can do it' or not.....My recommendation here is very clear and very strong....Let someone who REALLY knows Airhead heads do your valve jobs!!! I recommend such as OAK Okleshen; Ted Porter (Beemershop)....
Use of aftermarket one-piece SS valves is GENERALLY a BAD idea. BMW uses a high quality 2 piece SS valve....with the two materials selected for the best performance in the area being used. SS ONE-piece valves may swell up in the stems area, seize in that area, stop rotation (you DO know that your rocker contact point is offset to make the valves rotate??). Stems will gall, tulips and seats will deform. Sometimes the swelling is so bad with SS one-piece valves that they are not easily removable, and might drive your machinist/mechanic nuts. One piece SS valves are a BAD deal...do NOT use them...and they are NOT compatible with the stock valve guides. Some aftermarket valves are probably just fine, and SOME might require a different valve guide material. Consult with folks mentioned in the previous paragraph. Do an Archives Search on the Airheads Mailing List.
Some people, illegally in the US, do use aircraft gasoline (100 or 100LL) as an fuel additive. It only takes a very small amount of lead to protect the PRE-1980 valves and seats, and it need NOT be in every tank, it was residual amounts in tanks and delivery systems that kept SOME 1979 and earlier bikes in good condition for YEARS after the lead phase-out (in many instances). I mentioned the amounts, etc., earlier...here is even more information:
'Race' additive WITH lead is still available, expensively. However, you need so little lead, that aviation fuel cost becomes vastly cheaper than race additives. Race fuel that contains lead is very expensive, and for off-road racing use, so their cans state. Aircraft gas (aviation fuel), typically costing 'only' perhaps twice as much as car gas, can be stored in a metal can for many years, it does NOT gum up like car fuel does...at least not for a VERY long time (many years)... or a cup or so per tank, or every couple of tanks or so, apparently does the job, as mentioned earlier herein but only for the PRE-1980 bikes.
Remember: ILLEGAL! and you are polluting the air with lead. That consideration aside, if you have a 1979 or earlier, and are beginning to see exhaust valve closure, you might be considering the addition of a cup of aircraft gas. Or maybe a ounce or 4, per gallon. Aircraft gasoline is found mostly in a grade called 100LL, meaning 100 octane (aircraft type measurement). The LL means Low Lead. There is a high amount of lead in this gasoline actually. 100 grade (not with the LL designation) may also be found. It has considerably more lead, and less is needed. Either of these two grades will work fine as an additive for the 1979 and earlier airheads...with the 100 [NON-LL] grade being the better, as less is needed. Again, I must caution, that this usage is likely illegal, and a small amount of deposits, and less spark plug life, is likely. HOWEVER, the deposits and reduction of spark plug life are quite MINIMAL, due to the very small amount of the lead actually used....FAR less than the old days when the lead was also used to increase octane.....after all, you are diluting it WAY out, using only a cup in a tank of unleaded gasoline. The chances are pretty good that you will not see oil contamination or spark plug fouling problems at all.
If you have a 1980 and later, this lead will probably NOT stop valve recession, and it is not clear to me if it will help in the slightest, as the problem is excessive exhaust valve temperatures for the seat material to absorb fast enough.
Unless a autoparts store sold additive states specifically that it contains lead, it is unlikely to do ANYthing to help the 1979 and earlier (or later models). There are some exceptions, some tout ValveTect, available, I think, at some Marinas. I have no personal testing information on ValveTect.
NOTE: It is my belief that the use of oxygenated fuels, sold to reduce smog, tend to make the engine run leaner....and hotter....especially with the leaner burning engines BMW started producing in the late 1970's....in other words, alcohol containing fuels (or MTBE, etc.) will make things even worse. Not to mention even poorer fuel mileage.
Why did not BMW do something sooner?? My belief is that BMW did NOT BELIEVE the problem REALLY existed, with no reported problems in Germany, where unleaded fuels were used by those with catalytic converters in CARS.
Note: It is a tricky job to PROPERLY install the updated valve seats in your airhead. Do NOT trust just any old auto repair shop, or BMW repair shop, nor even, necessarily, a 'recommended' shop that has done many airheads....unless you, after investigation, are SURE they are competent to NOT RUIN YOUR $$$ HEADS. There ARE competent airhead mechanics, and you can ASK on the Airheads LIST, for the latest recommendations. Once the new seats are properly installed, they will likely last several lifetimes.
Note: A more in-depth article on valve and seat problems will be found on the Author's website,
http://bmwmotorcycletech.info
BEAD BLASTING:::
Since you might be removing your heads at some time, here is my take on BEAD BLASTING ...
While many folks have gotten away with this, my personal opinion is that you should NOT bead blast heads using the normal typical blasting materials. No matter how thorough the masking and subsequent cleaning, it seems that small abrasive particles manage to get lodged in crevices and even in the metal, and come loose later...and abrade cylinder walls...valve guides, pistons, etc....and may get into the oil...and then the bearings. They may well IMBED themselves, doing considerable damage over time! If you MUST blast, use walnut shell medium or soda blasting..something less permanently abrasive. For just decarboning ....heads, pistons, etc., are best cleaned chemically...it takes a week or so, but is SAFE!!
Snowbum #1843 01/05/2005; revised 09/20/2007
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