Forum bmwbike.com

Does anybody know when (year or number) BMW abandoned the R26 aluminium piston rod
and reintroduced the steel rod? And a second question: nearly every advertisement of R26 I see
offers the bike with a big rear light (like the one of the R27) and not the small one. When did
BMW introduce big rear lights for the R26?

Thanks for answer
Stefan

Hi Stefan,

a friend of mine wrote a "summary" about the rear lights from R25-R27.
There´s everything you need to know.

Have a look under:

www.r25-bike.de > INDEX > RUECKLICHTER R25-R27

(or the direct link: http://members.friendfactory.com/ravenl ... klmenu.htm)

Hope I could help you

Best regards,

Karl

Hello Stefan, I read with interest your enquiry about the R26 aluminium con-rod. I purchased an R26 in Papua New Guinea in 1982, which had a mid grey paint scheme, and chrome inlays on the side of the tank. It belonged to a Swiss person, and I found out later that it was a "Swiss Special ", which came with the features as described. ( By then I had "restored "it to the usual Black with white stripes paint job). I am most interested to find out if you have an R26 with aluminium con-rod,as mine did have when I purchased it. As there was a bearing rumble in it, I removed the crankshaft assembly, pressed the big end pin out, and found that the alloy con-rod had a plain whitemetal bearing cast into it. I sent the assembly to a BMW mechanic in Sydney,Australia, and he replaced the aluminium rod with a cast iron one, possibly one from an R50/2. I assembled it with new piston,etc,and have not been happy with the vibration when it runs. The vibration has been so bad that it has broken the horn mounting bracket, and one of the petrol tank mounting bolts has vibrated a hole in the petrol tank. I think this must be due to the iron rod being heavier than the aluminium one, and putting the motor out of balance. The only other modification I did was to instal the camchain tensioner from an R27, which went straight in. I would be very interested in your observations and advice on this, as the only solution I can think of is to strip the engine down again, and have the assembly balanced. I will have a scanner soon, and can email you a picture of the R26 as it was when I brought it. I will also dig up my BMW book with engine number references in it, and tell you what year it is. From memory, I think it was 1956 or 1958.
I am currently riding a 1985 K100RS, which I have had for 4 years, and before that, a 1960 R60, which I owned from 1976 to 1996. I do regret having sold it, even though the K is a great long distance high speed tourer.
Hoping to hear from you soon, regards, Tony.

Hi Tony

Its a crying shame that you restored your original "Swiss Blue" to the conventional black. There were over 500 of these machines produced solely for the Swiss market, and unfortunately even in Switzerland these are restored in black mainly due to the new owner not knowing that the "Swiss Blue" with chromed tank and white pinstripes, was an original special limited edition. The other reason is cost. It can cost you CHF 2000.-- less to restore it in black than in its original trim with chrome. This R26 was also the first production BMW to come on the market with a Schorsch Meier bench seat manufactured by Denfeld, and a chrom sports or grab handle on the rear fender.

Don't get confused by the blue bit. It is more of a grey, and fades to only grey over the years. Protected areas like inside the headlamp reveal the original colour.

Regarding the crank shaft story, this is the famous R26 disease of shaking bikes and broken R26 frames. There was a small series of R67/3 service motorcycles with side cars produced for the ADAC (German Automobile Association) for the autobahn patrol in 1955 with aluminium conrods running directly on the crankpin with no rollers. These were very successful, and BMW adopted this system in a series of Isetta as well as the R26. Due to the very light conrods, the crank webs had very small counter weights and the motors went like hell, reved well and were as smooth as sewing machines. However, the rods suffered from metal fatigue, and the old slinger story fed all the sludge to the bearing. The steel metalic parts in the sludge embedded themselves in the aluminium, and wore away the crankpin surface.

This affected almost all the R26's, so BMW reluctantly went back to the forged steel conrod and roller bearing principal. The extra rod weight meant new crank webs with larger counter weights to achieve the 55% balance factor. All BMW workshops world wide were instructed (some time in 1958 I think) to replace the complete crankshaft in the case of a failure of the old type. The 250cc clientelle of BMW were the low budget customers....the could just afford to buy a BMW, but could'nt afford the official BMW upkeep, and so these were usually repaired by the local village mechanic. When somebody ordered a new R26 conrod, they automatically received the new forged steel conrod as the Aluminium rod had been discontinued. The machine shop just rebuilt the crank without giving two hoots about the balance factor. Balancing is just an added expense, and the customer is not prepared to pay much any way. Believe you me, this was the attitude in 1958, and is still the case and attitude today.

As the old R26 crank shafts are more common then the newer replacement type, we cut out triangular blocks of metal and weld them onto the small webs and remachine them to the size and spec of the later crankshaft. It is a tedious and expensive process. It would be cheaper to just drill the webs and insert wolfram heavy metal weights, but we cannot get up enough weight with this method. When a customer send us an R26 crankshaft for an overhauled replacement, we send hime the same type that he sent us i.e. if he sends us an old type, we send him an old type in return with built up counter weights. If he send us a new type, we supply him with the new type in return. If any of you out there want to ask if you can buy new aluminium con rods, forget it! I also do not know of any workshop who has successfully repaired an alu rod.

Attached is a jpeg of the newer type R26 conrod were I have drawn in the size and shape of the old type R26 crank with small webs. I couldn't take a picture of the old type R26 crank as we don't have any in stock as they are all at our machine shop at present.

Best regards,

Mark
Mark Huggett GmbH

Mark thank you for your prompt reply to my query on the R26. I am not sure if the crank is the new or old type, but am fairly sure it will be the old light type, as it vibrates quite a lot with the iron con rod. Can you give me a price on replacing this with one of your rebuilt exchange units, and do you need the piston and piston pin sent as well ( these were new when I rebuilt the engine,) and because of the vibration, the engine has only done a few hundred Km.
I will have to disassemble the engine to be sure of what type of crank it is.
Cheers, Tony.

Regarding the aluminium conrod, I have only been able to ascertain that the R26 BMW parts catalogue edition 1958 only mentions the duraluminium conrod part number 11 24 0 017 050. The R26 parts catalogue edition February 1959 mentions both 11 24 0 017 050 conrod duraluminium and 11 24 0 017 052 conrod forged steel.
We are unable to find any further info eg. as of which motor number did the change take place. This may be noted on the original works drawings, but I have no possibility of researching this at this moment.

Best regards,

Mark
Mark Huggett GmbH

Hi Mark,

I have found this Isetta 300 crank on eBay. http://cgi.ebay.com/ebaymotors/ws/eBayI ... 2461300106
Seller claims that it is in good condition and has an aluminium rod (to me it looks like a steel rod). After reading your message on the subject I´m a bit suspicious about this item. So is this a collectors item or should cranks with aluminium rod in general be avoided?

Best regards

Bent Gammelby