On most of those engines it is 1in" But it could also be 7/8"
Most likely 3/4" diameter, though it's possible it's 5/8". Although, I have never even once heard of a 3.5 hp mower with a horizontal shaft.
Yes! The motor says its 12.5 hp but that's b.s it's the exact same thing as an 11hp Briggs and Stratton, cept they have a different style carb
5/8 inch
I am afraid not Depending on the size of the shaft on the thing you are putting on it is possible to get a bushing made to increase the size of the shaft. Go to your local machine shop and ask if they can make one.)
The torque sequence for a single cylinder Briggs and Stratton engine horizontal shaft head is increments of 2's for hp and 1's for head bolts. That makes 11 head bolts for an 18 hp.
Clean the carburetor with special attention to the high speed jet and fuel passage.
On a horizontal crankshaft engine it is done by loosening the pinch bolt and rotating the shaft in a clockwise direction with the throttle in the high speed position and retightening
Unless the type # is off the wall strange, it has a 1" diameter shaft, 1/4" keyway, the end is tapped to thread size 7/16 - 20, and length from mount to tip is 3 and 5/32".
You loosen the nut own the governor arm turn the arm clockwise and then turn the governor shaft clock wise. Tighten nut That should do it.
If it back fires or it gets jammed up it may have shear the key on the shaft under the fly wheel that would put it out of time.
There are many parts at are used on multiple models of engines, so its like a 50/50 chance. The 1/2 horsepower difference may use a completely different one though... Post back the model and code numbers of each of the engines, and I will look up if they are compatible or not.
Check your timing if you have sheared a key on the fly wheel it might : 1.just work if the plug is out because it might just have enough friction to turn. 2.Your valve seals are letting oil threw and it is dead heading on compression stroke. The list goes on. Hope this helps.