I was trying to buy a 540 cc single cylinder Briggs, Came across a 33R877 motor. But right next to it on the OPEengines site was a 540 cc 33S877 with the word " Professional " in the title, and only $ 10 more . So I ordered one of the 33S877 because of that " Professional " word. I figured it would be more durable. Very misleading , For I found out within 5 hrs. of placing my order that the 33R motor is the better sturdier motor. Tried to change my order but was told , to late. So do any of the Briggs fans on here know what is the difference between the 33R877 and the 33S877 engine ? Thank you for any help in advance.