| Since many of us are interested in small engine for driving alternators, I thought I would post some general info on engines less than 7HP. Push mower engines are verticle engines of about 2 to 6.75HP (so far) and all have the same mounting dimensions and shaft sizes (3 lengths). The shaft size is 7/8" with a 3/8"x24 tapped hole and 3/16" woodruff key slot.
The standard shaft length is measured from the bottom of the mounting ring to the end of the shaft. The bottom of the mounting ring is above the deck, you won't get the right measurement just measuring the exposed shaft that extends below the oil sump. The standard lengths are 3-5/32" (most common), 2-7/16", and 1-13/16". Self propelled mowers have a 1" shaft section and a second key slot for a drive pulley for the wheels.
The "flywheel" on a lawnmower is the blade. The aluminum "flywheel" on the top is the FAN. There were some verticle engines made for other applications that actually had flywheels, usually cast iron (or steel) fans. So if you think the flywheel is too light it's most likely that you do not have a flywheel, so of course it will be too light. Use a very heavy pulley or add something on your 3-5/32" shaft to act as a flywheel. It's easy to scrounge something, but they do make actual flywheels in this size. Horizontal motors often have one. If nothing else, try a go-kart supply house.
You can often buy a 5 or 6HP motor for the same or less than a 3 or 4HP motor, so look for deals. In the more expensive models, the engines are often swapped out on new mowers to get in the "right" performance or price range. Sometimes they simply don't sell one year and a more up-to-date engine is required next year. A couple of years ago EPA required emission carbureators, and there are a lot of pre 2000 6HP motors on the shelf. Before that it was "compliance" (the kill switch). The same with 6.5HP motors, new ones are 6.75HP. Try here (I don't work there, I'm just a happy customer).
http://www.jackssmallengines.com/Special.cfm
My old Murray that came with a 3.75HP now has a 6HP with a quiet muffler & cast iron sleeves and cuts much more easily and faster.
Most manufacturers have horizontal engines corresponding to above verticles. There is more variation among snowblowers, roto-tillers, generator drives and go-karts or other models. Much of the info is the same however. See manufacturer sites for model number ID and more info. HTH -RussH
Links to manufacturers:
http://www.briggsandstratton.com/
http://www.tecumsehpower.com/
http://www.jackssmallengines.com/phone.cfm for others
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