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, and R69US. This page is to help with information concerning the evolution and problems with the various heads for BMW 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 BMWs, nothing else. 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 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 large 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. It would take a wire wheel on a grinder to clean off the valve stem. We had 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 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.
“Why didn’t they stick on the earlier bikes?” that is an excellent question. I don’t have the answer. However, there were other problems. These problems occurred more often on the R50/2 & R60/2 than on the R69S. The R50S had its own set of challenges.
The term “butterheads.”
Butterheads is not a German technical term but one coined 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.
We noticed that the bikes started to have trouble keeping valve clearance. Bikes would often come in with very low compression. Usually, we could adjust the valves and fix them. 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.
The heads would warp easily, and bubbles could be seen escaping from the head gasket just above the spark plug. We would lap it flat again.
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 photo shows a welded piece. This solved the problem of the soft exhaust threads.
The spark plug inserts would become loose and start to leak.
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 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 your 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 can reduce the risk of the insert loosening by only removing or installing the plugs 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 overtighten them, as 10 lbs of torque is enough. I have never seen an under-torqued spark plug fall out of a BMW. Over-tightening is far more dangerous than under-tightening.
This photo shows the spark plug hole after it is welded.
After drilling and threading, the repaired hole will look like this. The depth is slightly over 1/2″ but well under the later 3/4″ hole.
The heads wouldn’t hold torque. The rocker stands would sink into the head, and the head bolt torque to the cylinder would be less than the spec. It was often challenging to get the bolts out of the heads after they had been removed. They should slide out; instead, we could easily measure 15-20 lbs of torque from the carbon build-up in the passageway. This friction 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. A combination of aluminum and carbon pinched the head bolts. The carbon was the result of the warped head. We had a special drill bit to drill it out again so that the bolt would easily slide in and out. Now the head could be torqued repeatedly until the aluminum moved again, which it always would.
A valve seat would fall out and hold the valve in the open position. The piston would hit against the open valve, lock up, and prevent the engine from turning. The typical scenario was that the rider would stop and leave the bike for 15-20 minutes. It wouldn’t turn over upon returning to start the fully warmed-up bike. The kickstart lever would freeze, and a prudent rider knew that it was the time to stop. Some riders would try to bump start it. You should see the inside of those heads. Don’t bump your bike if it locks up solid.
The center valve cover studs were pulling out often and easily. They had always been a problem caused by over-tightening, but it all at once became much worse.
Another related problem exacerbated these symptoms. The (mostly the R60/2) bikes started seizing up and scouring pistons. This is a big problem for anyone owning an R60/2, and it must be modified.
Historical background
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 first that didn’t improve much. That also explains why the old heads didn’t have those problems. 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.” These spark plugs had a longer (3/4″) reach sparkplug. The letters LK cast into the metal are below the spark plug hole. These heads worked much better than the older ones with the short-reach plugs.
A 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.
Eventually, 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 dozen) off of salvaged BMWs, so we would use those older cracked heads, and our problems seemed to disappear.
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 some of them failed, although at a lesser rate.
The R69S butter heads still held up better than the R50/R60 series by a lot. Because of better design and better cooling, the typical faults didn’t show up in a few thousand miles. However, the inserts came out and still tended to lose exhaust threads. The rocker arm 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. 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. The years to avoid are 64 thru 67, 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 heads 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 and not a mechanic. 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
This useful plug color chart is from Champion.
Updated 14 July 2022