Small Block Briggs Ball Bearing Crankshaft

#1
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.
 

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Harquebus

Well-Known Member
#2
Would it be better to remove only the bearing seal that faces the inside of the crankcase?

Reason being, the outer seal might assist in keeping the oil inside the engine. Not sure though because I don't know the ball bearing engines all that well.
 
#4
Good question, I posted a few pics, if you note the ball cover has a seal installed and a drain to allow the oil that makes it through the bearing to drain back to the sump. So no benefit to leaving the outer seal in. I suppose both seals could be left in and the bearing to operate on the grease within but I felt that the sump oil would bring the bearing up to engine operating temp sooner.

As for fitting the bearing in the cover, it is a close slip fit. The bearing is installed on the crankshaft by heating it up in oil until it reaches about 250degF, then it slides into place outboard the timing gear. Upon installing the cover a few light taps with a rubber mallet will push the bearing into the housing.

Note: the bearing covers the timing mark on the crankshaft gear so outboard marks have to be added. I found this out the hard way. The marks a drilled divot in the crankshaft counterweight which aligns with a second mark on the cam gear. If neither one came from a BB engine then you will have to add these marks. Third pic shows the mark on a factory BB 3hp crank, fourth shows added marks to my 2hp setup.

These parts are clearly not cleaned up for final assembly yet.
 

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Bird Brain

Active Member
#5
You would be correct, it would operate with both seals in place on grease fine, personally i would remove them letting it use the oil bath it was designed with even though its a filterless engine and aluminum chards would pass through the rolling moments of the bearing but for that application the oil would be plenty sufficent in my thinking. Good luck and longevity to you with it.
 
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