Building a Lycoming O-320A2B narrow deck

[Mark Moyle]

I picked up a set of narrow deck O-320 Millenium cylinder assemblies with the stock 7 to one Lycoming pistons. The assemblies are from a salvage aircraft with 258 hours on the prop strike engine. Has anyone looked into using the TSIO360 Lycoming dished pistons in a straight valve O-320 engine? Compression should be 8.3 to one. Piston pin height, diameter and bore diameter are identical to the 320. I'm told dished/concave pistons run cooler and will decrease detonation tendencies. I wonder if a hemispheric combustion chamber is why they run cooler? I'd like to bump up the compression a bit. 80 octane Avgas doesn't exists so why run 7 to 1 pistons? 87 octane unleaded price off the fuel barge is $3.24 a gallon. Want to build the engine to use 87 octane. Mark M. Sent from my iPad using Tapatalk Hd Sent from my iPad using Tapatalk HD

[alaskabearhawk]

Don't have a clue Mark. Did you call Bob? He seems to do a lot with Lycoming cylinder/case/piston combinations.

[Mark Moyle]

Thanks Paul. Just spoke with The Bob about the compression ratio. Hung up bob called back to talk about the cylinder base studs. Great guy! Lycoming 180/160 hp pistons at 8.5 to 1. Part number 75089. For every 1/32 machined off the face of the piston the compression ratio is reduced 1/2 point. I live at sea level which means I'll not benefit from high compression pistons in the 8.1/8.3 to one range. The other issue is the cylinder base studs in the case are the short ones, the cylinder base plates on the 160 hp narrow deck will not work with out allot of effort and money to install the longer studs. So machining .049" off the piston face will give 7.5 to 1. Taking off .045" will give 7.8 to 1 compression ratio. Even after machining off the metal the piston deck will still be thicker than the 7 to 1 pistons...... Will balance the pistons after using a triple beam balance scale that'll measure down to a tenth of one gram... One gram is more than close enough since your allowed one ounce between a posing piston rod assemblies. Mark M. Sent from my iPad using Tapatalk HD Sent from my iPad using Tapatalk HD

[marcusofcotton]

Mark,
Thanks posting back with the details from your conversation with Bob, great info!

Mark J