Opposed 2 crankshaft engine construction- Briggs & Stratton





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Published on Sep 25, 2009

Construction details of my Opposed type engine with 2 pistons and crankshafts based on Briggs & Stratton 3-1/2 hp lawnmower engines model 92502.
This engine required some careful thinking and designing to figure out the simplest method to build it. I went thru about 6 designs on paper before I decided this layout was easiest.
Each cylinder head is 1/4" aluminum plate, bolted to the block with countersunk head bolts. The 2 heads are then bolted to each other by a ring of bolts that hold the whole assembly together. The intake manifold using just 1 carburator got a bit interesting also. So the engine is now pretty much an overhead valve design, as the valves from one block feed the opposite cylinder, with a small passage connecting the 2 combustion chambers. I messed up in one respect, the chamber volumes are too small, so compression is in the 9 to 10:1 range, which makes starting this monster a REAL chore, especially as it is now 18 cubic inch, or like a 7 hp engine with high compression. It weighs in at about 50 lb.

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