Hi Frank. I'm glad you enjoyed the article. Rob and I had a great time "gathering data" for it......;~} I'm copying Rob this reply for comment. I believe the reason Rob chooses not to use the BMW filter is actually not based on the filter itself, rather it's location. BMW places the filter in the tank, making filter changes difficult. The purpose of changing to a different filter has to do with mounting the filter outside the tank for accessibility. The mounting location he had selected for the outside filter was on the right side, just behind the cylinder and filter size was a consideration for the location. The first filter he tried was - he thinks - too small and flowed to little fuel, which - again, he thinks - caused the bike to run in a fuel starved condition and hot. Now he's using a very large filter, located on top of the air cleaner housing, under the seat. One thing that doesn't show up very well in the photo's is the left head. Since taking the engine down, I went over to Tucson to look at the old heads. Looking closely, I found that the other head [left side] was also cracked in the same location. Very small, very faint line, but a crack nonetheless. The crack is so small that Rob overlooked it, even when inspecting the heads after replacing them. Look closely at http://www.nmpcs.com/50CC/head%20crack.jpg between the intake valves one the head on the left at the same location as that circled on the right. He was going to take the heads down and have them magnafluxed but I don't think he has had time yet. When that's done, we'll post the results to the site and try to get the word out that it's there. I'm not convinced that the failure's cause is completely accurate at this time and I don't know if we'll ever really have a clear picture of it. I don't know that I can completely agree with Rob's assessment of the fuel starved condition causing the heads to overheat to the point of those failures [heat related cracking]. My opinion is there is something missing here related to the cracks, but I don't know what it is. I can speculate though. Here's what I know that is factual: BMW had [and may still have some] problems with castings on the early Oilheads. I personally owned two new motorcycles with casting problems, both - one a May 1993 and one a December 1994 [my current Oilhead] - had porous heads. Rob's bike is a 1993/4 model [not sure of the exact mfg. date]. I know for a fact that at least some bikes had porous heads cast during 1993 and 1994. I would speculate that Rob may have had a casting problem and the failure could be related to the casting of the heads. In the event you aren't familiar with the result of porous heads, if the heads are porous, when cold, the engine runs normally. As the engine warms to operating temperature, the heads heat up and expand slightly, opening minute airways allowing outside air to enter the combustion chamber behind the throttle bodies after the correct mixture at the throttle body has already been determined by the motronic system. The motronic system is incredibly fast at recognizing the mixture is incorrect [O2 sensor] and makes a fuel/air mix ratio adjustment based on what the injector sends to the combustion chamber and incorrectly adjusts to balance the exhaust gases. The riding result is something similar to constant throttle adjustments or the bucking bronco effect that makes the "surging problem" we all know about look like smooth sailing. On my first Oilhead, the problem was so dramatic that the bike was simply not ridable at any speed and at 100 mph on the freeway, it constantly "surged". The bike I'm riding now also had the heads casting problem but not as bad. It just couldn't be smoothed out. Testing for a casting problem showed clearly that they were porous. They have been replaced and the bike is very smooth. It's important to understand that not all heads with casting problems are porous. It's possible for a casting to be faulty and the owner never have a problem or notice anything abnormal, it depends on where the faults in the casting are. My speculation is the heads on Rob's bike may have had a casting problem that though he didn't experience air entering the combustion chambers abnormally, he may have areas in the heads that have voids, creating a honeycomb of passageways that can expand and contract with heating and cooling. If that condition exists, the heads would then have areas that don't dissipate the heats properly and could cause cracking. Hope this answers your question. Kinda long, but then again, this is a complex problem. I would expect Rob to comment. -- I don't suffer from insanity......I'm enjoying every minute of it. http://www.doomed.com/sanity/sanity_1.htm Steve Aikens, Clovis, NM steve@nmpcs.com > Hi Steve, > I just read and enjoyed your article in the ON about your > 50CC run with Rob Lentini. I also went to your site to read > about Rob's engine failure. Is there any specific reason > that Rob doesn't want to use the stock BMW fuel filter?