In order to use SDS EFI &/or ignition products, it is necessary to modify the ring gear to insert magnets for timing sense. This is a straightforward procedure, but not commonly done for dual pulley ring gear as used when an A/C compressor is fitted to the engine.
My ring gear (flywheel) was supplied by Airflow Systems as part of the A/C kit, and uses a serpentine belt for the A/C compressor drive. The magnet insertion points wind up in the middle of the serpentine belt area, which changes how things need to be done compared with the standard procedure published by SDS. Here’s what I did.
My flywheel had timing marks on the front side only. This confused me for a long time because the timing marks didn’t line up with the tooling holes as described in the SDS procedure. It turns out this was all because of my ignorance about Lycoming engines. When timing marks are on the front side of the flywheel, the TDC #1 mark lines up with the small hole at about the 2 o’clock position (viewed from the front) of the starter motor. When timing marks are on the aft side of the flywheel, the TDC #1 mark lines up with the vertical split at the top of the casing. So, with the flywheel lined up at TDC #1 (which I confirmed by looking at the #1 piston through the top plug hole), I marked the rear TDC #1 mark, and the tooling holes then matched up as described in the SDS instructions.
There is a difference though – the tooling holes in the Airflow Systems flywheel are 3/16″, whereas those in the standard flywheel are 1/4″. Ross at SDS kindly made me a drilling jig to suite the flywheel I have.
I then went ahead and drilled the flywheel exactly as per the SDS instructions, using a new #29 cobalt split point drill and plenty of cutting fluid. By sheer good luck, the holes wind up exactly centered in one of the grooves for the serpentine belt – see the pictures. This is lucky because it means it isn’t necessary to rebuild the “crest” of the grooves on the flywheel, just fill in material that has been removed inside the groove.
I tapped the holes #8-32, again as per the standard instructions, and carefully de-burred the sides of the holes in the micro-V groove using needle files, sandpaper and a magnifier. I then cleaned out the holes thoroughly with acetone. At this point though, it is necessary to deviate from the SDS procedure.
There isn’t enough depth to insert two grub screws as well as the magnet, only one grub screw will fit. Here’s what I did:
- Each hole contains the magnet, and one grub screw. I soaked the grub screws in acetone to remove any grease or oil, then set the four grub screws aside on a clean, dry tray.
- For each hole, I picked up a grub screw with an Allen key, applied red loctite (these aren’t ever coming out) to the grub screw threads, and inserted the grub screw to a bit short of the right position from the outside, leaving the Allen key still in place.
- I then mixed up some 5 minute epoxy, filled the hole per the standard instructions (the plastic shaft of a Q-tip with the head cut off works well for this), and inserted the magnet from the inside. The magnet gets pulled in and smacks against the grub screw.
- Now carefully wind the grub screw further in, until the magnet is at the desired position, just inside the inner surface of the flywheel. Remove the Allen key. Wipe off the excess epoxy with some acetone on a clean rag.
- Clean off the red loctite from the serpentine pulley groove with acetone. I used a piece of paper towel with acetone, and a clean feeler gauge to get right down to the bottom of the groove. Then I used a Q-tip dipped in Acetone to get down and clean the loctite out of the threads in the hole, above the grub screw, while holding the flywheel so that the acetone would run out of the hole rather than down around the grub screw – so as not to displace any loctite around the threads.
- Do one hole at a time – go back and repeat steps 2-5 another three times, making sure to get the magnet orientation right. Then set everything aside for a few hours or overnight for the epoxy and loctite to cure.
Now it is necessary to “rebuild” the damaged groove. I used a product called Devcon Titanium Putty to do this. It’s an expensive product and you only need a small amount, I was able to borrow some from a friend that is a commercial user. There are other metal repair products around that would also be suitable.
Mix a small amount and force it down each hole so that there is a continuous layer of putty around the top of the grub screw out to the bottom of the groove, and use a fine knife to roughly shape the two sides of the groove, ensuring the threaded hole is fully filled up to the top of each side of the groove. I then let the Titanium putty cure for 3 hours. Don’t let it go much longer – after about 6 hours it gets a lot harder to work.
Three hours after applying, I used a combination of tiny needle files and various fine grades of sandpaper to rebuild the groove in each of the four hole positions. I started with the files, then switched to 120 grit paper to get each position nearly done. I wore a magnifying headset to do this, so that I had good vision for this micro-surgery. When I got close to the final result, I set the flywheel aside for another few hours for the putty to harden a bit more.
After about five hours total, I did some further sanding using 240, 400 and finally 800 grit paper to polish each side and the rim of the groove to complete the rebuild. The Titanium putty is well secured by the threads of each hole – see the final picture. The job is done when each side of the groove is smooth and flat, and you can run your fingers across the top of the groove with your eyes closed and not be able to tell when you run across where the hole was.
