1977 BMW R100RS Rebuild: Project Index


This bike is what I call a “10 Foot” bike; it looks nice from 10 feet away, but upon closer inspection, it’s clear it needs some love and attention.

"10 Foot" View

“10 Foot” View

10 Foot View

10 Foot View



Please be advised that there is no representation of the accuracy of any of the information presented on these web pages relative to BMW motorcycle maintenance or modification and that the material is presented for information purposes only. In no case will I be held liable for injury or damage (consequential or otherwise) resulting from or arising out of alterations you make to your motorcycle. The reader should recognize that motorcycling is a dangerous activity that can result in injury or death, and that the alterations portrayed on these web pages can and will change the behavior and performance of your motorcycle, possibly with fatal results. You are encouraged to seek qualified assistance before undertaking any of the procedures outlined here, and are here by notified that, should you decide to proceed, you do so at your own risk.



This is a first year R100RS that was built in December 1976, VIN 6180422. It’s the 422nd R100RS imported into the United States. It has the “CFO” version of the engine that was configured to meet the pollution and noise requirements in California, Florida and Oregon, hence “CFO”. The primary difference is the exhaust header and muffler; the CFO engines have 38 mm exhaust headers and mufflers while the standard engines had 40 mm exhaust headers and mufflers. Two different models of the same sized Bing carburetor were used on the 1977 standard and CFO engines. In later years, BMW again changed the model of Bing carburetor used likely to avoid pinging with no-lead gas and to meet pollution requirements.

R100 Cylinder Heads

Both versions use the same heads with 44 mm intake and 40 mm exhaust valves, the so called “Big Valve” heads. By the start of the 1979 model year, all R100RS engines used the 38 mm header pipes and mufflers. At that point, the CFO version of the engines was not needed as all engines now met the EPA requirements with the smaller headers and mufflers. Below is what I learned from the MAX BMW parts fiche about the Big Valve heads and header pipes.

NOTE: *FG = UP TO & INCluding; FG* = AFTER

         Valves From MAX BMW Fiche
Model   Ex (mm)    In (mm) Header (mm) Chassis Notes                                                Model Yrs*
R100RS 40 44 40 UP TO&INC 1978 *FG 09/78 R100RS(USA) MODELL 79 1977-1978
R100RS 40 44 38 AFTER 1978 FG* 09/78 1979-1984

* Starts in 09 of preceeding year. So, 09/78 is start of 1979 model year.

R100 Carburetors

The Bing carburetor version is slightly different for the CFO engines than those used on the 1977-1978 standard engines. The CFO engine uses the 94/40/103-104 version (odd last three digits is left side and even last three digits is right side) while the standard engines used the 94/40/107-108 version until the start of the 1979 model year. The CFO engines were not available in 1979 as BMW changed all the R100RS engines to use 38 mm headers and exhaust, again to meet EPA requirements in all states and used a third version of the Bing carburetor, the 94/40/109-110 and then changed them again starting in the 1981 model year to the 94/40/113-114 version. Below is information I found in the MAX BMW parts fiche about the Bing Carburetor models BMW used on the R100 engines from introduction in 1977 through 1984..

NOTE: *FG = UP TO & INCluding; FG* = AFTER

Bing Model Chassis # Notes Model Years*
94/40/103-104 Various
CFO Models
(to 09/77)
NOTE: *FG indicates this would include model year 1978, but MAX shows this only for model year 1977. I show 1977-1978 since I assume the CFO was made until the 38 mm header R100RS was provided which started in 1979 model year.
94/40/107-108 AFTER 6183254
UP TO&INC 6185421
FG* R100RS(USA) 6 183 254
*FG R100RS(USA) 6 185 421
94/40/109-110 AFTER 6185421 *FG R100RS(USA) 6 185 422
NOTE: I believe *FG is a typo. It should be FG* and the number should be 6185421, “AFTER 6185421”.
94/40/113-114 (From 09/80) 1981-1984

* Starts in 09 of preceeding year. So, 09/78 is start of 1979 model year.

Modifications Made By Previous Owners

This bike was modified by its previous owners. Here is a list of what I found.

  • The heads were machined for dual spark plugs;
    (a common solution to pinging with lower octane gasoline.)
  • The cylinders, pistons, rings and gudgeon pin were replaced with later Nikasil cylinders and the lower compression 8.2:1 pistons;
  • A Dyna III electronic ignition replaced the original points;
  • A deep 3 quart oil pan replaced the stock 2.3 quart pan;
  • A set of VDO oil pressure and oil temperature gauges were installed;
    (they have been removed but I have them and may install them again);
  • Heated grips;
  • a Telefix fork brace; and
  • Cast “snowflake” wheels with the added front wheel reinforcement at the midpoint.
    (the 1977 bikes came with spoke wheels and a blue pin stripe along the circumference. Many folks replaced these with the later cast snowflake wheels).
  • The rear snowflake had a steel sleeve insert in the hub likely due to a spun wheel bearing at some point.

Build Plan

Due to the number of upgrades and modifications made to the bike, my plan is to keep them as part of the project rather than attempt to restore the bike to original showroom condition. These upgrades reflect contemporary OEM and after market upgrades that were often installed to “improve” the R100RS.

Project Pictures

You can find the all the pictures for the project here on my Flickr account:
–> 1977 BMW R100RS Rebuild Pictures-Flickr

I’ll add to this collection as I document the project.

Rebuild Procedures Index

These are links to pages that document how I did the work organized by the main sections used in the BMW parts fiche. In each I list the parts used and resources I found helpful.



12-Engine Electrics

13-Fuel Preparation


31-Front Suspension

33-Rear Axle, Suspension



46-Frame, Fairing, Cases


61-Electrical System

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