StoneRoad2013's photos with the keyword: side-valve

BSA M20. 126,000 [approx] of these side-valve 650s saw service in WW2 The saddle was remarkably comfortable. en.wikipedia.org/wiki/BSA_M20

That's a 170cc "scooter" on the left, which contrasts with the size of the two flat-tankers to the right. A 1913 example of a Kynock with the JAP side-valve engine developing 4hp, and further to the right, a 1913 Lea Francis V-twin developing 3 1/2 hp.