ffkiwi

The OS Pet first appeared on the market around 1957, replacing an earlier OS '099' engine (a twinstack similar to the larger OS 20's of the early 50's). The Pet was intended as a 'bottom of the range' beginners engine. Production continued well into the 70's.There were 4 distinct models (though OS only acknowledge 3 by model type!) The very first used a drop in liner,slip on aluminium fins and a plain unfinned head retained by only 2 bolts. The next version had the more familiar integral finned steel liner, a plain unfinned head and this was held down by 4 head bolts. The series-II Pet appeared around 1960 and was made in huge quantities-in fact the Pet-III only appeared because the dies for the Pet-II were worn out! The Pet-II had the blued steel finned cylinder, and a rounded cast-finned head, plus an R/C option was available where the throttle assembly plugged into the intake of the standard model.
I concur with other comments that this model was quite hard to start (certainly in the pre-electric starter days) from cold-and almost impossible when hot! They were very common in NZ when I was a junior-and still turn up from time to time.
The Pet-III appeared around 1972 and was a complete redesign, using a full depth crankcase casting, drop in liner, and a deeper finned casthead. There was no provision for an integral backplate mounted fuel tank, unlike the earlier models.In appearance not dissimilar to a series 100 Veco 19. This model too had an R/C throttle option, but with a square, rather than a round thottle body-and unlike all previous Pets, came with a silencer-the OS 701 model. Undoubtedly the best made Pet by far=and superior in performance to the earlier OS Max-10! [but by now of course OS had the Max -10 FSR under development if not in production!]
Pet-II and Pet-III's seem to crop up reasonably frequently on Ebay.................

'ffkiwi'