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Helping keep British Motorcycles on the road for over 30 years - established 1977

Bunn Crankcase Breather Kits

Part Numbers 635-01 & 635-02

The Bunn Breather Kit (BBK) cuts oil leaks AND reduces engine wear, as it flushes toxic blowby from your crankcase & rockerbox. It is warranted in use for two years with unlimited mileage. The new kit cuts oil leaks by restoring original crankcase air pressure, back to the levels classic designers planned. Over the decades, the OEM breathers malfunction and air pressure increases. Oil leaks are often the first sign. The kit suits all the usual four-stroke makes, whether singles, twins or multi-cylinder engines, i.e. AJS, Ariel, BSA, Indian, Matchless, Norton, Sunbeam, Triumph, Velocette, Vincent. The BBK also cuts engine wear by flushing filtered, fresh air through your crankcase. This controlled airflow forces out corrosive blowby gases that otherwise cause damage to your engine.

Harley owners and owners of motorcycles that are not Pre-1988 British classics who are interested in the Bunn Breather please contact Rex Bunn by e-mail: bunnbreather@gmail.com
Be sure to check out Bunn's Official Website

Alternatively, if you're looking for a specialist in Indian V Twin Bunns, we recommend:

Tom Krise, Cherry City Motors
1615 Court St NE
Salem, OR 97301

The Effects of Blowby

* Blowby contaminates and dilutes oil * Washes oil film off engine internals * Rusts ferrous components and bearings * Forms sludge * Causes oil leaks * Deteriorates rubber * And turns crankcases into fire hazards *

Noted Classic engineer and Norton racer Peter Williams says, in his June 2005 Classic Bike review of engine breathing,"...leaked blow-by gas is nasty, obnoxious, and corrosive stuff and it is best to have as little as possible in the crankcase..." Blowby occurs every time we ride. It forms a corrosive, flammable mixture, that eats away at our engines if left inside. Blowby is ~70% fuel, water, sulphur dioxide, nitrogen oxides, soot and ash. It's like a concentrated acid rain. The BBK is an effective conservation aid, designed to keep classic motorcycles on the road into their second century.

Click Here for a fascinating article on understanding crankcase breathing.

Included in Bunn Breather Kits

Valves, air filter, tubing, cable ties/clip, spare seal and comprehensive instructions. It's a simple, easy-to-install solution for classic engine oil leaks, crankcase pressure problems and blowby wear and corrosion. It can be fitted to nearly any four-stroke engine, to improve engine operation. It involves no permanent or unsightly changes to your engine. Flowing fresh, filtered air through the hot, closed-off rockerboxes and crankcase helps oil return and the engine to run cooler. It also helps prevent any dirt being sucked into the engine via existing oil leaks. Engine power may be enhanced on some bikes, as crankcase air pressure, downforce work-loads and drag reduce. Research shows oil leaks are signs of more serious problems.

Kits Available From British Cycle Supply

635-01 - Classic Universal Kit - Installation Instructions
635-02 - Royal Enfield Classic Kit - Installation Instructions

Disclaimer: As user installation and operation vary widely, this kit is not guaranteed to work on every make and model of classic motorcycle; for instance, some J.A.P. engines.


635-01 Classic Universal Kit



635-01 Universal Kit on a Triumph Single



635-02 Royal Enfield Classic Kit