The bristle material originally was Nylon 660 which was chosen for its resistance to many chemicals and solvents used for surface cleaning however for cleaning aluminium alloy surfaces of motorcycle motors the best and safest is simply warm water and detergents.
Some years ago polypropylene bristles replaced the Nylon. All brushes are now using polypropylene bristles. Some brushes with Nylon are still in stock and will be supplied on request until sold out.
Polypropylene bristles are slightly stiffer than Nylon but both soften considerably with hot water.
See the following links for chemical compatabilities of the two materials.
Polypropylene chemical resistance
Nylon chemical resistance
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