Sunday, February 16, 2020

Flywheel

(Posted on FB 1/21/20)

PROJECT 51: Flywheel. The most obvious problem with the flywheel was the condition of the teeth on the ring gear around its periphery. These teeth usually get chewed up when the pinion gear on the starter-motor fails to engaged the ring gear all the way before trying to turn the engine over. This is generally caused by the "Bendix" mechanism on the starter sticking and not pulling the pinion all the way in. When I rebuilt the starter, the Bendix seemed to work freely and the pinion gear was not chewed up, so Brutus must have replaced it when he could no longer get the car started without a push by friends or by parking on a hill. By that time, of course, the damage to the flywheel was done.
Fortunately, the ring gear is a separate piece from the rest of the flywheel so it's far less expensive to replace just the gear than the whole flywheel. The tricky part is that the gear is a tight shrink-fit onto the outer edge of the flywheel.
By concentrating heat on the gear with an acetylene torch, I was able to get it to expand enough that I could rap it with a small hammer and knock it off. (They sometimes have to be cut off.) I then cleaned up the mating surface on the flywheel in preparation for receiving the new ring.
Always the engineer, I measured the ID of the gear and the OD of the flywheel so that I could calculate what temperature the gear would need to be heated to in order to expand enough to drop over the flywheel. That's when this "relatively straightforward" project went off the tracks!
The flywheel was .038" larger than the gear, which meant that I would have to get the gear up to around 740-deg F to make it about .050" larger to be able to easily drop on. But having engineered shrink-fits before, .038" seemed like WAY too much interference. I did the math, and found that that much of a shrink fit would have stretched the steel gear to near to its breaking point when it cooled down. I envisioned the gear snapping like a taut rubber band the first time the starter pinion hit it.
I measured the old ring, and it had only .014" interference fit. Much more reasonable!
Without a lathe large enough for the job, I had to set up a borrowed rotary table on my mill, and machine the diameter of the flywheel to get the right fit. (The calipers in the photo are older than the car (and me), but I can still measure to less than .001" on the Vernier scale.)
Checking with a pyrometer, I used the "rosebud" tip on the torch to heat up the new ring gear to expand it, now only needing to raise the temperature 350-deg F to get the gear to expand enough to drop onto the flywheel. That temperature was apparently a little more than the first layer of my "hi-temp" gloves were rated for.
(I could have used Diane's oven to heat up the gear, but her next batch of cookies might have tasted like a crankcase, so I stayed in the garage.)
With the new ring gear securely shrunk in place, the next issue was a "fingernail" chip of steel that had been shaved off of one of the dowel pins when it was pressed into the flywheel. This chip was .030" thick, and would have prevented the flywheel from bolting flat against the end of the crankshaft. Again, doing the math, I calculated that that chip would have caused 1/16" wobble at the outer edge of the flywheel. The engine probably vibrated so much that Brutus could have made milkshakes on the dashboard at forty miles an hour!
I got rid of the chip, stoned the surface to make sure it was flat, and put in new dowel pins.

Chewed up teeth on the ring gear.

Heating gear with acetylene torch.

Rapping gear free after heating.

Machining OD of flywheel for correct fit of gear.

Vernier scale on vintage calipers.

Checking temperature of ring gear before heating.

Heating ring gear with "rosebud" torch tip.

Burn marks on "hi-temp" gloves.

"Fingernail" chip around dowel pin.

Ring gears old and new.

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