I am a big fan of the 1970s small block (2-4hp) Briggs and Statton flathead engines, likely from spending most of my youth behind the handle of various lawn mowers. A few years ago I came across a couple of 1970s mini bike frames (Byrd Wren and an Arco), both of which needed a lot of work. The guy I bought them from eventually let go of the engines he had, which were three 2hp flatheads in pieces.
I got the first frame put together and assembled the first engine as a stock 2hp to see what I could get out of it. surprisingly it carries my 175 butt up to about 20mph with 11/60 sprockets and 10" tall tires (bike is in my avatar). (I am using unmodified vacujet carbs only on these engines)
I sold the Wren frame in a moment of weakness (which I regretted). Later I managed to get ahold of a later model Muskin Cat frame in about the same condition and set about to put it together. I wanted to see what kind of power I could get out of the 2hp so I sourced a 3hp crank and rod (adding 1/4" stroke over the 2hp), with a ball bearing on the PTO side. This also meant I had to find a crankcase cover to house the bearing. Ebay vigilance paid off and I was in business. I did some port filling and other minor mods and got the bike rolling on 12" tires with 12/60 sprockets. The power difference between these two bikes is significant. The stroked engine pulls harder from a dead stop and reaches 30+ mph with no problem (mostly from the taller sprocket ratio).
So I have this third engine setting on the shelf and began to wonder how a 2hp with the ball bearing crank would run. While the parts books show the crank I wanted was out there, I could not find one. So I measured up and cut the bearing area of the crank down on the lathe to fit the factory briggs ball bearing. While fitting the bearing I got to thinking, what if there were a bearing out there that could be installed on an unmodified small block crank???? Searching the web with the measurements in hand I was finally able to locate one on Bearings Plus that will fit. Now all that is needed for the conversion is a BB crankcase cover (which is a lot easier to find).
Note, this is a double rubber seal bearing, I would recommend removing the seals and cleaning out the grease before installing it so it can be splash lubed by the crankcase.
I got the first frame put together and assembled the first engine as a stock 2hp to see what I could get out of it. surprisingly it carries my 175 butt up to about 20mph with 11/60 sprockets and 10" tall tires (bike is in my avatar). (I am using unmodified vacujet carbs only on these engines)
I sold the Wren frame in a moment of weakness (which I regretted). Later I managed to get ahold of a later model Muskin Cat frame in about the same condition and set about to put it together. I wanted to see what kind of power I could get out of the 2hp so I sourced a 3hp crank and rod (adding 1/4" stroke over the 2hp), with a ball bearing on the PTO side. This also meant I had to find a crankcase cover to house the bearing. Ebay vigilance paid off and I was in business. I did some port filling and other minor mods and got the bike rolling on 12" tires with 12/60 sprockets. The power difference between these two bikes is significant. The stroked engine pulls harder from a dead stop and reaches 30+ mph with no problem (mostly from the taller sprocket ratio).
So I have this third engine setting on the shelf and began to wonder how a 2hp with the ball bearing crank would run. While the parts books show the crank I wanted was out there, I could not find one. So I measured up and cut the bearing area of the crank down on the lathe to fit the factory briggs ball bearing. While fitting the bearing I got to thinking, what if there were a bearing out there that could be installed on an unmodified small block crank???? Searching the web with the measurements in hand I was finally able to locate one on Bearings Plus that will fit. Now all that is needed for the conversion is a BB crankcase cover (which is a lot easier to find).
Note, this is a double rubber seal bearing, I would recommend removing the seals and cleaning out the grease before installing it so it can be splash lubed by the crankcase.
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