/2 BMW motorcycle pistons by Nural, especially the R60/2 and ring orientation.
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. Some info on the 1974 R90/6 and the R90S that leaked oil from the rear main seal.
The early (1955-61) pistons usually had 5 rings, 3 compression, and two oil control rings. One oil ring above the wrist pin and one below. In about 1960, depending on the model, they changed to 3 rings. In 1960 only, the R60 had 4 rings, 3 compression rings, and one oil ring. In 1961 they were only 3 rings, 2 compression rings, and one oil ring. As far as I remember, all of the other models went from 5 to 3 rings.
In general, the pistons seemed to hold up well. By 1967 I was in the BMW repair business in San Francisco, California. I began seeing a lot more pistons seizing on new bikes with low mileage than on older bikes with 75-100 k miles. It was usually on R60/2 models, but most of the bikes sold were the R60/2.
At least two of these four common factors always accompanied the seizure.
1. Nonstock mufflers, often Dixie mufflers or Hoske.
2. The advance mechanism’s flat advance limit spring (question mark shaped) was broken.
3. The wrong heat range of spark plug, usually not a Bosch.
4. Running out of fuel and having to go to reserve.
Sometimes, but rarely, only one of these factors would be found. It was discouraging and had me stymied. I kept all of the bad pistons.
The Marusho was a Japanese copy of the R50. It had been around 65 or 66 but quickly went out of business. While it was a copy, its history goes back to BMW. An enthusiast bought up all of the USA distributor’s stock and continued to supply parts but didn’t have any pistons. From time to time, he would get pistons from me. I was importing OEM Nural pistons directly from Alcan Aluminum Werke in Nurnberg, Germany, and kept a large (50-100) stock.
Eventually, he began asking for old used pistons to reuse on the Marushos of his customers. To sell him all of my remaining junk pistons from the R50s, I had to separate them. The 500 and 600 were in different piles and then pulled the “S” pistons out. To see the separated piles of pistons was alarming. About 75% were R60 pistons, about 8-10% R69S, about 8-10% R50, and the rest an assortment of /3, singles, and R50S. I estimated the distribution of models among my customers to be about 40% R60, 20-25% R50, and maybe 20% R69S and the rest assorted /3, singles, etc. While I was aware of the R60/2’s failing at a large rate, I wasn’t aware of just how bad it was until I saw the pile of pistons. Some of the large pile of R60/2 were “holed” and reminded me of a real disaster. Most were only galled from a seizure.
After boring, I picked out a set that seized in the first 250 miles and looked new. I never knew why that had happened. I had the set examined by my machine shop. These guys only did work on exotic and expensive engines for repair shops in San Francisco. I was the only bike repair shop that was a customer. The owner looked at the pistons for only a couple of seconds and began removing the rings. He put a micrometer on it and immediately showed me the problem. The diameter just above the oil ring was only a few thousandths of an inch smaller than the piston skirt. It was supposed to be about .009″ smaller. He pointed out that the seizure showed between the two rings. It had started there, spread to below the oil ring, and widened to the whole skirt at the bottom. His evaluation was that the Nural pistons were made incorrectly and couldn’t work. It was another example of asking the expert.
I was shocked. He suggested that we turn off about .004″-.005″ and use them as before. Since I didn’t have a lathe, he suggested we remove the rings and use a file to remove a bit of metal. There would be no penalty for taking off twice as much as needed. The ring land only keeps the rings apart, and the diameter was only important if it was too large and caused a seizure.
After the modification, we bored many more sets and never had a piston failure. If I owned an R60/2 with good original Nural pistons, I would modify them to be safe.
Technical drawing by Chris, thanks
This shows the correct orientation of the original rings on a Nural piston.
From then on, we filed each R60/2 piston upon rebuild or even a valve job. I never saw one seize after that. Now I wonder if that was the reason that the R50S seized so often. The few that I saw, or owned, had all seized pistons at one time or another.
Nural is only the trade name for the piston. The factory name was Alcan Aluminum Werke, and located in Nurnberg, Germany. On my next visit to the Nural piston factory, I asked about this. I got a sort of hesitant response, and the salesperson disappeared and brought back an engineer. I explained this to him in detail and only got the answer that they had to make pistons to the customer’s specifications (BMW). Without using any names, he was telling me that they were aware of it, and it wasn’t their fault.
I had been a small but frequent customer over the years, and they always accommodated me. They even provided me with a list of known inventory of pre-war BMW pistons in various warehouses around Europe and other continents. It was interesting to see many pistons for the late ’20s and early ’30s BMWs in stock, but nobody knew about it. I was able to acquire some for the old bikes that I owned. I wish I could have bought them all.
BMW offered pistons in standard, 1st, and 2nd oversize. Nural also made them for the aftermarket in 3rd and 4th oversize. Kolben Schmidt, the competition, also made pistons, marked KS, for the /2 and in all 5 sizes.
My experience with the 3rd and 4th oversize pistons wasn’t good, and I quit boring them out that far. I also saw problems with work by competitors who put sleeves in the cylinders. They seized up, and I was told that hot spots occur, resulting in improper cooling. I think that with water cooling, the fit of a sleeve isn’t quite as important as with air cooling.
Use only the BMW type of circlip to hold the wrist pin. The VW type with the bent ends for easy grabbing with needle-nose pliers can break off and score the cylinder. I have seen it happen three times, once on my R69. I have also seen them work. I have never seen a BMW circlip break and cause damage. You decide.
A problem with the R90 new series in 1974 led to some very destructive advice. The bikes were leaking oil from the rear crank seal. Butler and Smith, not BMW, suggested washing the cylinders in soapy water, letting them air dry, and getting a coating of rust on them. Then assemble, with no oil on the piston, and run it up in rpm for a minute or two to really “seat the rings.” To start with, this wasn’t even the problem, and secondly, one never assembles an engine without proper lubrication. This urban legend has evolved, and this crazy process has been applied to the /2. One must use common sense in these things. When something seems far-fetched, ask around carefully. BMW is made of normal materials and responds to science and physics, not mirrors, magic, and snake oil.
BMW motorcycle piston failures
BMW motorcycle piston “collapse.”
This mostly concerns the series that we call the /2 from mid-55 thru 69. The problems could occur in any model but were mostly in the R60.
The typical situation was when the rider was going down the freeway. The bike would seem to run out of gas, and the rider would reach down and flip over to reserve. The bike would run fine again, and all seemed OK. Often the rider would be surprised that it needed fuel so soon. At the next stop, the rider would fill up and discover that the tank didn’t need fuel after all. At the next cold start-up, the engine would have some piston slap. More riding would show that the bike was also burning oil.
The engine didn’t make any strange noises. The rear wheel didn’t lock up. The only apparent symptom was what seemed to be out of gas.
As a piston would overheat, it would get larger. When it gets too large for the cylinder, it must expand somewhere. It can’t go out, so the inside of the piston goes in. This gives the piston wall a newer and smaller “center,” so to speak. When the piston cools down, the “new” center also shrinks, along with the entire wall. Now the piston skirt is smaller than before. When the engine starts up, the piston will flop around. That is called piston slap. If it expands from heat enough, it may stop the noise and run just great. No real damage has been done, and the noise may be “fixed” in one of two ways. An inspection of the piston will show nothing unusual. The piston can be replaced, or the old one may be knurled. That is a process where the piston is squeezed (stressed) across the wrist pins, and a tool knurls the now wider skirt. This allows the stressed piston skirt to permanently relax into the new larger skirt size.
BMW motorcycle piston “seizure.”
A seized piston is when the piston gets so large in the cylinder that some of the sides scuff off onto the cylinder walls. Poor design is one of the most common reasons for /2 BMW motorcycle piston seizure.
Typical seized piston. The white line running under the wrist pin is a crack and on both sides. This photo by Ben, thanks.
Another typical piston seizure. See it seized just above the top ring? I suspect that the design is poor. Photo by Jim Shaw, thanks.
A rare find. The exhaust valve head broke off and beat a hole into the piston crown. This would result in a ruined lower end too.
A typical “holed” BMW motorcycle piston. Photo provided by Ed Timm. Thanks
Sometimes a hot spot will develop on the piston top or crown. It can melt it to the point of failing. The metal never really gets to a molten stage. When it gets to a crystalline stage, the pressure of combustion is so strong that it blows out and down. It fails before it can melt. The particles look like sand and blast the entire inside of the lower end. Some of them go through the oil pump and into the slingers, filling them up even faster. Some particles always get into the rod bearings, and within 10,000 miles or less, they fail too. This means a full lower-end rebuild. A holed piston often shows no sign of seizing. The heat was localized at the crown. I saw one piston from an engine that was working well. It had almost holed. The metal was concave at the crown. It cooled off just in time to not blow out. I had no way of knowing how long ago it had happened.
Knurling a BMW motorcycle piston skirt
I realize that most of you know the physics of knurling a piston, but just in case there is one person who doesn’t know, here is a simple explanation. There is nothing wrong with knurling a piston. A piston may rattle when cold, and that is often called “piston slap” by many. The piston is undersized and needs to be larger. At first glance, it appears that some metal is squeezed upwards, and that makes it larger. So, the same old problem is there when the “new” metal wears off. The piston would be again too small. Nothing could be further from the truth. The piston is squeezed in one plane, bulging out in the plane where we desire it to be larger. We relieve the stress by mashing grooves into it to make it stay larger. When the original squeeze is removed, it only partially goes back to the older “too small” size. One could file off the raised metal from the grooves, and the piston would still be larger and work just fine. The problem is that why did it get too small in the first place? Maybe the metal isn’t up to the job of maintaining its size under heat stress. In that case, it won’t matter what is done, and it will fail. The metallurgy is so very important in a piston, as it undergoes huge stress in its heating up. Had I not started by knowing that pistons do work, I would never believe it could possibly survive for more than a few explosions.
BMW motorcycle piston and cylinder trivia
I have little information on an aftermarket piston made for the /2 series made under the name Meteor. The few reports that I have heard are all bad. I understand that the pistons sold by Vech are modern and work well.
Some owners would sleeve a cylinder to save it. I saw a lot of failures with this process. It seemed that they developed hot spots and would seize again. Far better bonding procedures are available today, and the sleeves are OK. I do not have any experience with this and am apprehensive. I would investigate thoroughly before spending my money.
BMW made one year of one model with pistons that had four rings. It was the 1960 R60, and while I didn’t have enough samples to make a definitive statement, I did notice that I found seizures. It could have been a coincidence.
A problem with the R90 new series in 1974 led to some very destructive advice. Butler and Smith, not BMW, suggested washing the cylinders in soapy water, letting them air dry, and getting a coating of rust on them. Then assemble, with no oil on the piston, and run it up in rpm for a minute or two to really “seat the rings.” To start with, this wasn’t even the problem, and secondly, one never assembles an engine without proper lubrication. This urban legend has evolved, and now all or parts have been applied to even the /2. One must use common sense in these things. When something seems far-fetched, ask around carefully. BMW is made of normal materials and responds to science and physics, not mirrors, magic, and snake oil.
Updated 14 July 2022