O-320-D2B Flywheel

[Doug Rohrer]

I recently purchased an O-320-D2B narrow deck engine less cylinders for my RV-9A project. I wanted to use new cylinders anyway, so I was glad to find a deal that did not include reworked cylinders. My question concerns the flywheel that came with this engine. It is a 122 tooth flywheel with a pulley about 6.5 inch diameter. I know I will need a case mount alternator, but I am looking for confirmation I can use this flywheel and not have unacceptable alternator speeds. I hope to use this flywheel, as it came with a SkyTech starter. Any advice would be appreciated.

[eaglen92ce]

I had just such a flywheel on my IO-360 with a B and C 40 amp alternator. Worked just fine! When I had the engine rebuilt we found that the flywheel had a crack in it. I ended up with one with a flywheel with the larger pulley and 149 tooth flywheel (had to change the starter drive). I did not notice any difference in alternator performance. The small pulley flywheels are lighter, I would keep it!

[eaglen92ce]

Maybe one caveat though! If you are planning to use Electronic Ignition the small pulley flywheel may not work with the crank mounted crank position sensor/magnet set up. I have the Electroair with crank mounted sensor and that worked with the small pulley. I don't think that Lightspeed or SDS (magnets installed in the flywheel) would work with the small pulley.

[N941WR]

Originally Posted by eaglen92ce

Maybe one caveat though! If you are planning to use Electronic Ignition the small pulley flywheel may not work with the crank mounted crank position sensor/magnet set up. I have the Electroair with crank mounted sensor and that worked with the small pulley. I don't think that Lightspeed or SDS (magnets installed in the flywheel) would work with the small pulley.

Good advice. However, the P-mag electronic ignition doesn't care about the flywheel. So you can run an electronic ignition. (Not starting another EI war, just pointing out there are other options.)

[tim2542]

Bill is right on the EI sensors, you will definently need the large dia pulley for those, and if your buying one anyway the 149 tooth ring gear/starter is more common. Either size pulley will drive your alternator with no problem, but I?d suggest the larger pulley so you have more EI options later.
I believe e SDS is selling CNC ring gear supports now.
Tim Andres

[johnbright]

Edit: I stand corrected, ref post #7.

BTW it's easy to change the ring gear from 122 to 149T or vice versa.

https://www.lycoming.com/sites/defau...Ring_Gears.pdf

[tim2542]

Originally Posted by johnbright

BTW it's easy to change the ring gear from 122 to 149T or vice versa.

https://www.lycoming.com/sites/defau...Ring_Gears.pdf

I don?t think that?s 100% correct, The supports may have the same dia but don?t nessearily index to the same longitudinal position.
http://flytexas1.com/RingGearTech.htm
Tim Andres

[Doug Rohrer]

Thanks for all the responses. I am feeling better about what I have. But I don't understand why I would want to change out the ring gear if I have a perfectly good 122T starter. Is there some gear ratio advantage for the starter going with the 149T gear?

[eaglen92ce]

Nothing wrong with the 122 tooth setup. I noticed no difference in operation between the 122 tooth an the 149 tooth (same starter, just the drive was swapped). I would keep it unless the EI thing is a concern. PMAG/EMAG does not use a crank sensor (so no issue with small pulley) and Lightspeed and Electroair crank sensor would work with the small pulley, it is just Lightspeed, SDS, and probably EFII that would conflict with the small pulley. Lightspeed also offers a Mag hole mounted sensor.

[Doug Rohrer]

EI will not be an issue as I would probably go with one or two pmags rather the crank sensor systems. Looks like I am good to go with what I have. Thanks to all.