Steve Barr with his 8-10 hp Challenge engine. The engine has a 7-by-11-inch bore and stroke. Steve says the engine is somewhere between an 8 hp and a 10 hp because the bore and stroke are equivalent to those on an 8 hp engine, but the flywheel is what would typically be found on a 10 hp engine. “And the crank is somewhere in between,” he says with a laugh. The engine was built between 1910 and 1915.