Since you are building a 9, one presumes you are talking O-320? If so, the D1A is an A1A with Constant Speed prop provision, 25 degree spark advance, Bendix mags, high compression, 7/16 prop attach bolts, straight riser in oil sump, and -32 carb.

A D2A is a D1A with fixed pitch prop and 3/8 prop attach bolts.

This info from a certified Lycoming engine list, which if I recall correctly I got from Dan C.'s website.
 
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O-320-D1A, IO-320-D1A, O-360-D1A, IO-360-D1A
Some have variable prop provisions, some don't.
Some have Dynafocal mounts. Some have Conical.

If you're talking O-320, James has it right.

This information is found in the Lycoming Engine specification list. The list can be found in the Sky Ranch Engineering Manual. Everyone who owns a Lycoming engine should have this publication.
 
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prkaye said:
what's the difference between a Lyc D1A and D2A?
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If you got it from Aerosport (as I believe you did), put most simply, the D1A is for a constant speed prop, the D2A is for a fixed pitch prop but can be converted to a constant speed (essentially making it a D1A) if you buy a couple extra parts. Call Bart or Sue at Aerosport and they can give you the exact part numbers and cost.
 
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