122 and 149 ring gear

I'm wanting to swap to an SDS ignition on my RV-4. My plane currently has a 122 tooth ring gear on the flywheel with the matching starter. The SDS flywheel comes with a 149 tooth ring gear. If I bought their flywheel with no ring gear, could I just install a 122 tooth ring gear and everything line up right with my starter?

Yes, I already asked Ross. He's not sure.

122 to 149 tooth ring gear

Unfortunately no. The widths are different. You should be able to find a 149 tooth flywheel ( support assembly) used here on VAF or Ebay for around $300.
Sorry reread your post and see your getting Ross’s flywheel. Your going to have to bite the bullet and get a new starter.
See SI 1141A for correct ring gear to support assembly application.

The 122 tooth ring gear is .375” wide while the 149 tooth ring gear is .440” +/- wide. The mating lip on the 149 tooth support assy. Is thinner so if you put a 122 tooth ring gear on a 149 tooth support assy. the back face of the gear will not flush up with the support assy. But will be recessed aprox .060”-.070”.

Is it necessary to change flywheels to use the SDS ignition?

I was trying to avoid changing to a 149 tooth spec. That will require a new starter to match.

When you say width of the gear, is that just the thickness in the vertical plane or the width of the horizontal plane?

Is it possible to change the existing gear on the starter to match the new 149 tooth flywheel?

scsmith said:

Is it necessary to change flywheels to use the SDS ignition?

Click to expand...

In my case, yes. The hall sensor floats inside the pulley. My existing flywheel is a 7-1/2" pulley...it's not big enough. The newer style has a 9-3/4" pulley.

Thinking about it...I'm not sure a bigger pulley diameter will fit on my plane. I'll have to measure clearances this afternoon.

If you have a flywheel that has the 9 3/4" OD alternator drive pulley, why not just drill the flywheel for magnets? Basically 3 magnet holes & Ross supplies the jig to do it.

If you have a 7 1/2" pulley on the flywheel, the magnets could be drilled in just below the ring gear but would require a different SDS pickup sensor and a custom bracket fabricated for it (like in the good old days).

As for the ring gear, I just compared a flywheel (149 teeth) & the flywheel off a o-235 (7 1/2" pulley & 112 teeth) and ring gears appear to be interchangeable, so I suspect the 122 tooth is same ID.

If in fact you are changing out a 7 1/2" pulley flywheel for the 9 3/4" (149 tooth) flywheel, take into consideration your alternator pulley size as the bigger pulley will speed things up significantly.

Ralph Inkster said:

If you have a flywheel that has the 9 3/4" OD alternator drive pulley, why not just drill the flywheel for magnets? Basically 3 magnet holes & Ross supplies the jig to do it.

If you have a 7 1/2" pulley on the flywheel, the magnets could be drilled in just below the ring gear but would require a different SDS pickup sensor and a custom bracket fabricated for it (like in the good old days).

As for the ring gear, I just compared a flywheel (149 teeth) & the flywheel off a o-235 (7 1/2" pulley & 112 teeth) and ring gears appear to be interchangeable, so I suspect the 122 tooth is same ID.

If in fact you are changing out a 7 1/2" pulley flywheel for the 9 3/4" (149 tooth) flywheel, take into consideration your alternator pulley size as the bigger pulley will speed things up significantly.

Click to expand...

I do have the 7-1/2" pulley on mine. I didn't even think of the speed difference.

Seems with everyone's input (the great thing about VAF), making my own mount is the way to go and will eliminate all problems. I've just emailed Ross about a different magnet mount location.

PM sent to you.

149 / 122 teeth ring gear swap

FlyFastJP said:

I'm wanting to swap to an SDS ignition on my RV-4. My plane currently has a 122 tooth ring gear on the flywheel with the matching starter. The SDS flywheel comes with a 149 tooth ring gear. If I bought their flywheel with no ring gear, could I just install a 122 tooth ring gear and everything line up right with my starter?

Yes, I already asked Ross. He's not sure.

Click to expand...

Yes you can swap ring gears on an EXPERIMENTAL BUT it may or may not work.

I swapped and installed 149 teeth ring gear on a 122 support. I had no issues with the swap till I switched to a SkyTec NL starter. There is very little clearance between the back of the gear on the support and the front of the gear on the starter. IT works on my airplane but may not work in all cases. SkyTec (before they were sold to Hartzell Engine Tech) had a warning posted on their website.

I just so happen to have a 122 tooth gear on a 122 tooth support and a 149 tooth on a 149 tooth support in my hangar. Here are some photos of the two next to each other.

This image is 122 Tooth left with 149 Tooth right with FRONT up.


Next image is 122 Tooth left with 149 Tooth right front up ZOOM in for close up.


This is 122 Tooth Left and 149 Tooth Right front DOWN.


Final image is 122 Tooth Left and 149 Tooth Right front down ZOOM in for close up.


Hope this helps. My experience has been that the gears will interchange on the support BUT there MAY be a starter gear engagement issue.

Email sent with drawings & description.