I have restored an R26 very recently. I find the engine is overheating. To the extent I could visualize I checked all the points that may cause overheating problem. I checked the ignition system with a Bosch tester The timing is correct and the ignition advance is functioning as it should. I have used the old bore and piston only. Advise requested.
Janardhana Rao.
overheating of R26
Re: overheating of R26
Singles do run hot. What makes you think your R26 is overheating beyond what is normal for it? When I first restored my R27 in 1999, Mark Huggett made a remark in this forum about deep sumps being more important for singles because they ran hotter than twins. I asked why, and Mark replied with a forum posting or two. I saved some of his comments:
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Singles Run Hot
The single cylinder BMWs constantly received a power increase with every model from 12hp with the R24 to 18hp with the R27. Compression went up, and cooling problems arose. Main was that the cylinder and cylinder head was out of the main cooling airstream due to the front mudguard. BMW tried to solve this problem by painting the cylinder head black, then improving lubrication by direct oiling of the piston and larger cooling fins with the introduction of the R26.... and all that with the same inadequate small and leaky tin oil sump. That is why we make the cast aluminum oil sumps. They hold more oil, cool better due to the increased surface area and cooling fins, have a machined gasket surface so they are oil tight, and they look just great.
Question : I'm presently restoring a 1952 R25/2. I had such a machine in my student days. Trips to just about everywhere in Europe. I remember that the old girl had some overheating problems, which resulted in a seized piston once, and some dropped valves. In those days I thought that the black paint residue on the cylinder head and tappet cover was someone playing around to make the bike look different. It was a used machine. Today I think it had something to do with the heat dissipation. Have you any suggestions regarding this problem of the so called "Mohrenkopf" or "black head" edition. I would be very grateful as I hate dropped valves
Answer: All the R25 series had heating problems for a number of reasons. Lubrication to the piston and cylinder head where inadequate. The cooling fins on the cylinder head were inadequate, and the cylinder and cylinder head where hidden from the airflow by a large flared front mudguard. The measures that BMW undertook in those days where as follows: As of motor number:
260 322 Ignition timing is changed to 5 degrees before TDC.
261 301 The carburetor main jet is changed from 105 to a 110.
263 051 The cylinder head and valve covers were painted mat black (Customers with older R25 models could get a free tin of lead based heat resistant black paint from their BMW dealer.
263 651 The conventional tappet cover gasket was replaced with an aluminum gasket in order to improve the heat transfer from cylinder head to tappet cover in the hope that the heat dissipation would be improved.
Even the R25/3 had a black painted cylinder head. BMW really solved this problem for the first time with the R26 in two ways. First, the surface area of the cooling fins on the cylinder head was massively increased by about 60 % and the size of the front mudguard decreased allowing a better flow of air to the cylinder head. Second, a major improvement was made in the lubrication by a 3mm hole drilled in the side of the cylinder hole in the aluminum engine block to connect up with the main oil way that leads up into the cylinder and thereafter cylinder head. A 1,6mm hole was then drilled in the corresponding position through the cylinder wall below the cylinder foot in order that oil would be pumped directly into the cylinder and the piston. With the R27, BMW turned an oil groove on the out side of the cylinder under the cylinder foot and drilled a second hole on the opposite side in order that the piston received double lubrication. This is a modification that we make to all of the R24 to R25/3 series. Regarding the dropped valves, excessive heat means excessive wear. The most common is a broken off exhaust valve head. This is due to excessively worn valve guides and valve stem. During operation, the valves can rotate slightly. If the valve guide has too much play the valve can close at a slightly different angle each time. This causes the valve head to be bent backwards and forwards by a minute fraction each time and therefore weakening the point where the valve head and stem come together resulting in a break. Luckily, valve materials have since massively improved since the 1950's and this problem seldom happens with modern valves in these older machines.
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Singles Run Hot
The single cylinder BMWs constantly received a power increase with every model from 12hp with the R24 to 18hp with the R27. Compression went up, and cooling problems arose. Main was that the cylinder and cylinder head was out of the main cooling airstream due to the front mudguard. BMW tried to solve this problem by painting the cylinder head black, then improving lubrication by direct oiling of the piston and larger cooling fins with the introduction of the R26.... and all that with the same inadequate small and leaky tin oil sump. That is why we make the cast aluminum oil sumps. They hold more oil, cool better due to the increased surface area and cooling fins, have a machined gasket surface so they are oil tight, and they look just great.
Question : I'm presently restoring a 1952 R25/2. I had such a machine in my student days. Trips to just about everywhere in Europe. I remember that the old girl had some overheating problems, which resulted in a seized piston once, and some dropped valves. In those days I thought that the black paint residue on the cylinder head and tappet cover was someone playing around to make the bike look different. It was a used machine. Today I think it had something to do with the heat dissipation. Have you any suggestions regarding this problem of the so called "Mohrenkopf" or "black head" edition. I would be very grateful as I hate dropped valves
Answer: All the R25 series had heating problems for a number of reasons. Lubrication to the piston and cylinder head where inadequate. The cooling fins on the cylinder head were inadequate, and the cylinder and cylinder head where hidden from the airflow by a large flared front mudguard. The measures that BMW undertook in those days where as follows: As of motor number:
260 322 Ignition timing is changed to 5 degrees before TDC.
261 301 The carburetor main jet is changed from 105 to a 110.
263 051 The cylinder head and valve covers were painted mat black (Customers with older R25 models could get a free tin of lead based heat resistant black paint from their BMW dealer.
263 651 The conventional tappet cover gasket was replaced with an aluminum gasket in order to improve the heat transfer from cylinder head to tappet cover in the hope that the heat dissipation would be improved.
Even the R25/3 had a black painted cylinder head. BMW really solved this problem for the first time with the R26 in two ways. First, the surface area of the cooling fins on the cylinder head was massively increased by about 60 % and the size of the front mudguard decreased allowing a better flow of air to the cylinder head. Second, a major improvement was made in the lubrication by a 3mm hole drilled in the side of the cylinder hole in the aluminum engine block to connect up with the main oil way that leads up into the cylinder and thereafter cylinder head. A 1,6mm hole was then drilled in the corresponding position through the cylinder wall below the cylinder foot in order that oil would be pumped directly into the cylinder and the piston. With the R27, BMW turned an oil groove on the out side of the cylinder under the cylinder foot and drilled a second hole on the opposite side in order that the piston received double lubrication. This is a modification that we make to all of the R24 to R25/3 series. Regarding the dropped valves, excessive heat means excessive wear. The most common is a broken off exhaust valve head. This is due to excessively worn valve guides and valve stem. During operation, the valves can rotate slightly. If the valve guide has too much play the valve can close at a slightly different angle each time. This causes the valve head to be bent backwards and forwards by a minute fraction each time and therefore weakening the point where the valve head and stem come together resulting in a break. Luckily, valve materials have since massively improved since the 1950's and this problem seldom happens with modern valves in these older machines.
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Re: overheating of R26
I did not measure the temperature of the engine as such, but I know it is overheating from the smell of the oil and the silencer turning black. Since my machine is manufactured in 1959 end, I presume all the modifications done to this model must have been incorporated. I am located in India where the normal day temperatures are high and run up to 40 º+ C. We do not have a BMW motor cycle dealer here and hence there is no way of getting the black improvised paint. The sump is a conventional one made of tin sheet metal but I feel I can organise for an aluminium cast sump. I have set the ignition timing to 5° before TDC. I have changed the main jet to 110. Black heat resistant paint is not available here and hence I can’t help. I shall provide an aluminium gasket between the cylinder head and tappet covers. I shall try the above modifications to the extent possible and give a feed back.
Re: overheating of R26
"... We do not have a BMW motor cycle dealer here and hence there is no way of getting the black improvised paint... Black heat resistant paint is not available here and hence I can’t help."
Black heat resistant paint is available in ordinary hardware and auto supply stores in the US, so surely it is available in India. The cylinders of Enfield Bullets are painted black, so there is another source of the paint.
A large capacity cast aluminum sump is available here in the Huggett store:
11 13 0 007 000.A Alu Sport oil sump kit complete with gaskets etc., 2 Litre R24 - R27 118.00
It is shown here on my R27:
http://home.insightbb.com/~aatherton/R2 ... in2004.jpg
I suppose the aluminum gasket might transfer more heat from the heat to the valve covers, but it might not seal very well and could allow oil seepage.
Black heat resistant paint is available in ordinary hardware and auto supply stores in the US, so surely it is available in India. The cylinders of Enfield Bullets are painted black, so there is another source of the paint.
A large capacity cast aluminum sump is available here in the Huggett store:
11 13 0 007 000.A Alu Sport oil sump kit complete with gaskets etc., 2 Litre R24 - R27 118.00
It is shown here on my R27:
http://home.insightbb.com/~aatherton/R2 ... in2004.jpg
I suppose the aluminum gasket might transfer more heat from the heat to the valve covers, but it might not seal very well and could allow oil seepage.
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Daniel Andrada
- Posts: 5
- Joined: Sun 18. Feb 2007, 20:37
Re: overheating of R26
[size=small]Is it overheating at idle or when running? The reason I've asked is you might be running on lean mixture. try adjusting the the slide pin up one notch/hole and observe if at full throttle the exhaust pipe don't get red hot[/size]