The 8hp Briggs engines had a 3/4" threaded muffler port, while the 9hp engines on up have 1" muffler ports, except OHV models which do not have internal threads. The Briggs OHV 21**** series engines will replace an L-Head Briggs even if the tank is behind the seat. If you buy an OHV engine to replace a L-Head engine, you will need to buy a muffler also, which may require you to remove the back right post for the normal muffler style to fit. The BS-Muffler-38 works well if you use a bagger. The throttle linkage on the OHV Briggs engines run horizontally instead of vertically, so you may have to get a longer cable for this to work correctly.