BMW Airhead Motorcycles Valve Seat Recession - A Definitive Reply
Originally published in 01/05/2005; revised 09/20/2007; 05/21/2013; 05/20/2017; 06/20/2018; all by the author, Snowbum
- 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, TEL, is dangerous if absorbed through your skin when in pure form and much less so in diluted form, and it is a form of lead previously used extensively, highly diluted, in gasoline. Way back when it was still in use (still is in some parts of the World) it was sold by the Ethyl Corporation, and such gasoline's had a nickname: Ethyl.
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 in most any form 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, usually beginning of which is intermittent and VERY wastefull of fuel. 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". Techron has been reformulated a few times over the years, particularly since TEL was eliminated from road-going vehicles.