Outdoor Power Equipment and Small Engines - Piston in briggs

I have a engine for a lawnmower that is a 3.5 hp and is very nice but the guy that gave it to me took the piston out to use it as a ornament hanging from his rear view mirrow. I also have a nice engine, 3.75hp but its missing the carburator. The carb from the one missing the piston wont fit on the 3.75 so I was wondering if the piston from the 3.75 will fit in the 3.5? Thanks for all the help.

How about asking him for the piston, and in return give him the one from the 3.75 hp engine?

Without the model type and code numbers from each engine it will be near impossible to advise you. Do you have these numbers? If so, what are they?

How about asking him for the piston, and in return give him the one from the 3.75 hp engine?

Thats a good idea, but the 3.75 is in such good shape I hate to destroy it. As for the numbers I can probably find them on the 3.75 but the 3.5 is missing all the names, tags I couldnt find anything. I may just tear them all apart and try to build something or today I was looking at it and think I may be able to get the nice carb to fit the 3 3/4 hp. What is the difference between the 3.5 and the 3.75? I would think a bigger cylinder bore.

You know the horsepower of both engines, right? That indicates you are reading those numbers on the engines cowling...the top cover over the flywheel. Briggs engines have all the info you need stamped into this top cowling. Extremely old engines that have been sanded and painted several times will still have those numbers and they can be read! Briggs stamps a string of three sets of numbers in a row and together except for spaces. 1st numbers is the MODEL number, space, TYPE , space, and finally the engines CODE. You'll probably find them on the flat portion of the cowling just above the small kick out for the spark plug wire although Briggs does stamp each engine in a location around the cowling that is usually visible based on the engines typical use. You may have to put the engine in different angles and lighting to find them but I have never not found those numbers. I can often feel the indention's and in extreme cases I've had to put a piece of paper over the area and rub it with a pencil. Those numbers will print perfectly. The meaning of each number is explained in one of the very first and permanent articles that appear just above this forum's list of new post. You can also go to the Briggs and Stratton website and find it. This is a one pager that tells you what each number represents. Almost every question you need answered, including the question you've posted here can be answered by referencing these numbers. For example the first numbers in the last set, the CODE, happens to be the year of manufacture, then month and day, followed by plant ID where it was manufactured etc.. Hope this helps a bit. Good luck, Ron