/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, R69US. Some info on the 1974 R90/6 and the R90S that leaked oil from the rear main seal. Just added, BMW piston seizure, collapse and holing.
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 upon model, they changed to 3 rings. In 1960 only, the R60 had 4 rings, 3 compression and one oil ring. In 1961 they were only 3 rings, 2 compression and one oil ring. As far as I can remember all of the other models went from 5 directly 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 to see a lot more pistons seizing on quite new bikes with low mileage, than on older bikes with 75-100 k miles. It was usually on R60/2 models, but then most of the bikes being sold were the R60/2.
At least two, of four, common factors always accompanied the seizure.
1. Non stock mufflers, often Dixie mufflers or Hoske.
2. The flat advance limit spring on the advance mechanism was broken.
3. 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 pistons that I replaced. I have no idea of why, I just kept most of the bad parts.
The Marusho was a sort of a Japanese copy of the R50 and had been around in 65 or 66, but had quickly gone out of business. While it was a copy, it’s history goes back to BMW. An enthusiast bought up all of the USA distributors 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 Marusho’s of his customers. In order to sell him all of my remaining old junk pistons from the R50’s I had to separate them. The 500 and 600 in different piles and then pulled the “S” pistons out. To see the separated piles of pistons was alarming. About 75-80% were R60 pistons, about 10% R69S, about 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 an 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 till 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.
I picked out a set that seized in the first 250 miles, after boring, 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 and spread to below the oil ring and widened out to the whole skirt at the bottom. His evaluation was that the Nural pistons were made incorrectly and couldn’t work. Just another example of the advantage of using the expert.
This was rather shocking to me. He suggested that we just turn off about .004″-.005″ and use them as before. Since I didn’t have a lathe he suggested that we just remove the rings and use a file to take off 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.
We bored many more sets and never had a single piston seize up after the modification. If I owned a R60/2 with good original Nural pistons, I would modify them to be sure.
Technical drawing by Chris, thanks
This shows the correct orientation of 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 of them 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 sales person 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 specs provided by the customer (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 were always accommodating to me. They even provided me with a list of known inventory of prewar BMW pistons in various warehouses around Europe and even other continents. It was interesting to see many pistons for late 20’s and early 30’s BMW’s, in stock, but nobody knew about it. I was able to acquire some for 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. I have seen another brand of piston, but don’t remember it’s name, but it was also German.
My experience with the 3rd and 4th over pistons wasn’t good and I quit boring them out that far. I also saw lots of problems with putting 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 own R69. I have also seen them work. I have never seen a BMW circlip break and cause damage. You decide.
A problem occurred with the R90 new series in 1974 that led to some very destructive advice. Butler and Smith, not BMW, suggested to wash the cylinders in soapy water and let them air dry and get 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 kind of 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 just 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 just fine again and all seemed to be 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 usually have some piston slap. More riding would show that the bike was also burning oil.
The engine didn’t make any strange noise. 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. This is 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 relax into the new larger skirt size permanently.
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. It has gone past the collapsed stage of heat mentioned above. The piston(s) on a BMW don’t stop moving. Nothing breaks or bends. The cylinder walls are badly scored and now the rings won’t seal. The piston will also be collapsed and slap badly, even when hot. The only solution is to bore it out and replace the piston. One of the most common reasons for /2 BMW motorcycle piston seizure is poor design.
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 the design is poor. Photo by Jim Shaw, thanks.
A rare find. The exhaust valve head broke off and beat a hole into the 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 and fill 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 to know how long ago it had happened.
I am really excited to get this photo from Ed. I have seen so many of these over the years. It is from a R69S and the hole was caused by ignition timing that was too advanced. That was caused by the advance limit spring breaking. It was the old style that often broke. There is no way to predict the spring failure. I have seen them break in less than 25k or 3 years. If I owned one today, I would replace the spring with a solid small plate. That won’t break. The last few degrees of slower advance would be lost, but that isn’t important compared to losing a lower end.
Knurling a BMW motorcycle piston skirt
I fully realize that most/all of you know the physics of knurling a piston, but just in case that there is one person who doesn’t know, here is a simple explanation, maybe too simple. There is little wrong with knurling a piston. A piston may rattle when cold and that is often called “piston slap” by many. The piston is undersize and needs to be larger. At first glance it appears that some metal is squeezed upwards and that makes it larger. So, when the “new” metal wears off the same old problem is there. The piston would be again too small. Nothing could be farther from the truth. The piston is squeezed in one plane and it bulges out in the plane in which we desire it to be larger. To make it stay larger, we relieve the stress by mashing grooves into it. 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 the heat stress. In that case, it won’t matter what is done, it will fail. The metallurgy is so very important in a piston, as it really undergoes huge stress in its heating up cycle. 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.
Some owners would sleeve a cylinder in order to save it. I saw a lot of failures with this process. It seemed that they developed hot spots and would seize again. Apparently, 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 on year of one model with pistons that had 4 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 coincidence.