BMW /2 motorcycle cylinder heads, butterheads, loose spark plug inserts, warped heads, gasket leakage
The BMW motorcycle /2 cylinder heads, or “butterheads”
This page is about the BMW motorcycle models R26, R27, R50, R60, R69, R50/2, R60/2, R50S, R69S, R50/US, R60/US, R69US. This page is to help with information concerning the evolution and problems with the various heads for BMW’s made from late 1955 through 1969. My perspective is that of a mechanic of that time. My shop had four full time, year round, mechanics that only worked on BMW’s, nothing else, ever. We saw the problems.
The early heads from the /3 up to about 1961
In the early days, it was common to find cracks in the head. At first, I was very worried about this.
The first real problems came up in about 1963 and reached gigantic proportions by 1965. Nearly all twins would have sticky valves within the first 5,000-10,000 miles. They would stick in the open position and in the worst cases, not close in time for full combustion. Instead of following the cam ramp down and closing gently, they would snap closed and make a giant bang. The owner might limp home, but many feared catastrophe and trailered them to my shop.
We would remove the heads and valves for cleaning. The carbon would build on the valve stem till it wouldn’t slide closed, even with the great spring pressure trying to close it. The build-up of carbon would hollow out the inside of the valve guides. To clean off the valve stem it would take a wire wheel on a grinder. We had a special 7 and 8 mm reamers to clean out the guides.
The first attempt by BMW to solve this problem was to redesign the valve guide. The new guide has a recessed end that basically made it shorter. A shorter guide would take longer to build up enough to stick again. It seemed to “sort of” fix the problem. Owners were also told to use fuel additives, as the fuel was the problem. Now the valves would go farther but still stick again around 10,000-20,000 miles. It was only a patch, not a solution.
The next attempt was in 1966. All BMW motorcycles came out with rotating valves. The idea was that if the valves could rotate, then the carbon would get burned off equally all the way around. The idea was attractive, as a sticking valve would have more carbon on one side than the other. BMW changed the valve, keepers and retainer washer/cup at the top. This solution worked well.
The question came up “Why didn’t they stick on the earlier bikes?” and that is a very good question. I don’t have the answer. Now to talk about the other problems. These problems were much greater on the R50/2 & R60/2 than the R69S. The R50S had its own set of problems.
The term “butterheads”
Butterheads is not a German technical term, but one invented by me. Back in the 60s when I saw the problem, I would tell people that the heads are made out of an alloy that is similar to butter. More recently I coined the term “butterheads” while on the /2 list.
We started to have trouble keeping valve clearance. Bikes would often come in with very low compression. Usually, we could adjust the valves and fix it. Sometimes they would already be burned and would require a valve job. The valve adjustment would “run out of threads” and we had to put washers between the end blocks and the rocker stands. That would get it back into the adjustment range.
2. The heads would warp easily and bubbles would escape from just above the spark plug. We would lap it flat again.
3. The exhaust threads would easily peel off when removing the exhaust nut, especially on the R50/R60. The old exhaust threaded portion could be machined out and a new piece could be welded into place.
The is a welded in piece. This solved the problem of the soft exhaust threads.
4. The spark plug inserts would become loose, start to leak a bit and finally come shooting out.
Here is a typical one that is just starting to come out. It will work itself out further and further and will start leaking gases. Eventually, it will blow out of the head.
One of the solutions to a loose steel spark plug insert is to remove and weld it over. The localized heat can distort the valve seat and it may not seal or it may fall out. The thickness of metal added in place varies. Some only try to restore the original 1/2″ thickness. Some try to approach a long reach plug thickness. If you head is not the LK type but seems to need a long reach plug, this may be due to the repair.
If you still have short reach plug heads and the spark plug inserts are stable, consider yourself lucky. In that case, you may be able to reduce the risk of loosening up the insert by only removing/installing them when cold. Always use some anti-seize product on the threads, especially on the long reach heads. Keep a close eye out for evidence of leaks around the plug. Do not over tighten them, as 10 lbs of torque are enough. I have never seen a spark plug fall out of a BMW. Over tightening is far more dangerous than under tightening.
This shows the spark plug hole welded up. It will be drilled and threaded.
The repaired hole will look like this. The depth is slightly over 1/2″, but well under the later 3/4″ hole.
5. The heads wouldn’t hold torque. The rocker stands would sink into the head and the head bolt torque to the cylinder would be reduced. This is the reason that when the head was removed it was often very difficult to get the bolts out of the heads. They should just slide out and instead we could easily measure 15-20 lbs of torque from the carbon build up in the passageway. This was caused by the aluminum that gave way from under the rocker stands and migrated to the only space available, the partially filled head bolt hole. The head bolts were pinched by a combination of aluminum and carbon. The carbon was the result of the warped head. We had a special drill bit just to drill it out again so that the bolt would easily slide in and out. Now the head could again be torqued accurately until the aluminum moved again and it always would.
6. A valve seat would fall out and hold the valve in the open position. The piston would jamb up against the open valve and lock up the engine. The typical scenario was that the rider would stop and leave the bike for 15-20 minutes. Upon returning to start the fully warmed up bike, it wouldn’t turn over. The kickstart lever would just freeze. That was the time to stop. A few anxious riders would try bumping it. You should see the inside of those heads. Don’t bump your bike if it locks up solid.
7. The center valve cover studs were pulling out often and easily. They had always been a problem caused by over tightening, but all at once it became much worse.
These symptoms were exacerbated by another related problem. The (mostly the R60/2) bikes started seizing up and scouring pistons. For anyone owning an R60/2, this is a big problem and must be modified.
In 1960 and 61, Germany started to become concerned with air pollution. One of the areas to get new regulations were the smelters and foundries. The metallurgy for the heads was changed and BMW didn’t know about it. I was told that the sulfur content was reduced. It took some years for them to find out. The result was that the aluminum turned to butter when hot, hence the name “butterheads”. The metal just moved away from any stress such as torque or pulling. That is why they tried simple patches and didn’t really improve much. That also explains why the early heads didn’t have the problems in those days. Now they are really old and any of them can have problems.
See the upside down cast in “LK” letters?
BMW’s solution was to redesign the heads. Sometime in 1968, all twins came out with the new heads. We call these the “LK heads”. Removing the spark plugs revealed a longer (3/4″) reach sparkplug. Just below the spark plug hole are the letters LK cast into the metal. These heads worked much better than the older ones with short reach plugs.
I have no knowledge of problems that have arisen since 1980 on the /2. An apparent problem that BMW heads had, especially the R50/R60, was that they developed cracks that emanated out from the spark plug hole and headed towards the valves. We just ignored the cracks and went on with life. They didn’t seem to cause any problems and I have seen many go another 50,000 miles with cracks and no problems. These cracks were in many heads from the first ones in 56 on. I can’t remember if they happened to the /3 (51-55) series.
We realized that the “good” heads were those that cracked. I know that this seems funny now, but we didn’t have the newly designed LK heads. At the time I had a large number (several dozens) off of salvaged BMWs and so we would use cracked heads and our problems seemed to go away or at least be greatly reduced.
If I had heads from a 63-67 R50/R60 I would try to get the newer LK heads or older ones. Those heads can’t be fixed because the metal is butter. The R69S heads are sometimes workable, but many of them failed, though at a lesser rate.
The R69S with butter heads still held up better than the R50/R60 series by a long way. Because of a better design and therefore better cooling, the typical faults just didn’t show up in a few thousand miles. The inserts came out and they still tended to lose threads off of the exhaust. The rocker stands didn’t sink nearly as badly causing the valves to close up so much or as fast. The LK heads were just better in about every way. We noticed it in the R60/2 first because it was the one that failed so quickly. I never knew exactly when they changed the metallurgy. I have been told that it was reformulated by knowledgeable people, that is all. One service bulletin alludes to an alloy change.
Don’t personalize this by feeling that your machine has been attacked. The best model is the R69S with the harmonic balancer. Years to be avoided, 64 thru 67 for sure, all models except the R69S. This is especially true of the R50/2 and R60/2. The R69S was a lot better, but the heads were still not as good as after the LK came out. Don’t get the idea that 1963 had great heads. I don’t know how to exactly determine the advent of the “butter” metal in them. I was there and couldn’t figure it out at that time because I was just a rider. It was years later that I started working on them professionally. It may have started as early as 62.
Spark plugs for the /2 BMW motorcycle
I just deleted an old conversion chart for plugs. Now it is 2017 and I don’t know anything about what plugs to use. Ask Vech.
This useful plug color chart is from Champion.