05-14-2000 Ask Mike! Archive
What is the difference between a narrow and a wide deck engine?

Hi Mike,
 
More questions on engines, narrow deck IO 320 B1A vs. wide deck I guess that Lycoming does not support narrow decks anymore what does this mean? And what's the difference between a wide deck and narrow deck?
 
Chris
 
 
Chris;

Lycoming's first (of the post-war era) cylinder design was what came to be known as narrow-deck engines (Lycoming refers to them as Standard cylinder flange engines). The flange area of the cylinders was thin, and required the use of hold down plates. These plates fit on top of the flange, under the nuts. The nuts were an internal wrenching design, with provisions for safety wire. In an (apparent) effort to eliminate these plates, Lycoming re-designed the flanges to be thicker, and used an ordinary (in appearance) flat nut, with no need for the safety wire (what is known as wide deck). It is noted that the crankcases vary slightly between the narrow and wides, Lycoming SI 1345 discusses what crankcases to use as replacements, as required.
 
As you can see, it is, once again, in the hands of the over hauler to make sure the proper parts are used with the proper engine. As I recall, your engine came from a certificated aircraft, you should have no problem with any mis-matching. As for the engine the kit manufacturer put together for their prototype, well, I believe the previously discussed mount issue tells us they may be in for trouble. 

Lycoming has only (as of July, 1987) discontinued manufacture of new narrow deck engines, but they still support the engine with service spare parts. 
 
Gear Green,
Mike