1979 BMW R100S

Here is some information on my highly modified R100S, with links to the story, the parts, and the resources that helped me figure out how to do the work. This bike started life as a very heavily worn R100RT, and converting her to an S model was not difficult.

• Completely rebuilt at 88,000 miles
• 336 cam, 10.2:1 compression, dual plugs
• Ported and polished heads
• Balanced pistons and conrods
• Boyer-Bransden Microdigital II ignition
• Dyna coils
• Upgraded pushrod tubes and seals
• Lightweight Nippondenso starter
• Deep oil sump, no oil cooler
• Progressive springs
• Ikon fully-adjustable shocks
• Late model brake cylinder conversion
• Stainless steel brake lines
• Billet aluminum triple clamp
• Late model (1983) transmission, clutch and flywheel
• Brown side stand, with rebuilt center stand
• Napoleon Bar-End mirrors
• Custom blue and silver paint
• Rev-Pack tank bag

This bike is smooth and quick, and very fun to ride. Using recommendations from experienced tuners like Jim Roche and Kari Prager, I rebuilt her in 1996 to be a “street rod”, a highly-reliable and powerful sport bike. She has been my weekend warrior since 1997, with about 30,000 miles since the rebuild.

The power band is wide and very torquey, all the way up to red line. I built her out of standard BMW parts for reliability, including factory valves, rockers, pushrods and wrist pins, re-bored cylinders with 1st oversize pistons, and the famous 336 racing cam. These parts have given me a 70-hp motor that should last as long as stock. 40mm Bing carbs are re-jetted (170 mains) to give excellent starting and idling, and the mixture is rich enough throughout the power band so that the engine does not ping. Even with these modifications, she delivers 40 to 48 miles per gallon.

Originally configured with a Staintune racing exhaust system, she had a big, deep voice like a Norton Combat Commando. I’ve since installed this mysterious pair of nice, throaty (and somewhat quieter) mufflers that I bought used. If anyone can identify these, I’d appreciate it!

The 1970’s airhead engine works well with large carbs, a richer mixture, and no oil cooler. With an aftermarket deep oil sump and pickup extension, the bike runs quite cool, and oil changes are easy. A silicone oil pan gasket lets me run synthetic oil without leaks.

Parts from later R100’s help. The pushrod tubes from the 1980’s have brazed stops for the seals, keeping the seals on the engine block from weeping oil.  1981-on transmissions have a spider flywheel and improved clutch mechanism, and they bolt right onto the earlier engine, giving a light clutch pull and quick revs (and lighten the bike by 15 lbs!) The handlebar-mounted front brake master cylinder with a conversion to stainless steel brake lines improves the original ATE brakes. I also switched to a drum rear brake, which honestly works better than the late-1970’s disk brakes.

— Tom Childers, tom (at) fairfaxjourney.com