TD-20 crank and magneto question

[adam1961]

I am working on a TD-20 and installing a brand new powerhead that has been around here waiting for a home. I noticed that the crankshaft that is in the NOS powerhead is different from one in my parts bin. I am not sure what is correct.
The first 2 photos show the crank in the NOS block. It has one keyway and two flat area cut into the crank. The two flats are the same and look to be 180 degrees apart.

Another T series crank has two individual keyways and only one longer cam cut into the crank.

I remember reading about earlier T series motors unsung Maverick spark suppressors and later motors using a different rotor and keeping the points open longer, eliminating the need for the spark suppressor.

*****I got one picture into the text twice and the third file is missing. I have not added anything to this computer to compress images, as this is almost obsolete when uploading in most places, so the crank with two individual keyways is not pictured******

Can anyone tell me which crank is
which?

Thanks
Adam

• This topic was modified 4 years, 3 months ago by adam1961.
• This topic was modified 4 years, 3 months ago by adam1961.

[aquasonic]

The TD-20 has two keys stacked on the crankshaft. The bottom key is for the magnet/rotor, and the top key is for the flywheel. Is there any stamped model number?

[Mumbles]

Well, this is kind of interesting as the ’41- ’42 TD-15 and ’46 – ’49 TD-20 motors along with the TN 25 and 26 motors all used two separate keys. One to align the flywheel and one to drive the round magnet. These magnetos also used the plunger points which had a pushrod riding on the flat area of the crank to open them. Later TN 27 and 28’s used the new universal magneto and had only one key which aligned the flywheel and drove the points cam.

This definitely looks like the earlier style crank but the single keyway and double flats are puzzling. With two opposite flats, each spark plug would fire twice for each revolution and it wouldn’t matter which way the plugs wires were hooked up. I wonder if it isn’t a left over ’51 or early ’52 crank with the old flats machined in it?

• This reply was modified 4 years, 3 months ago by Mumbles.

[2fast4me]

Two flats on the crank, ????? Could this be for a magneto that has the Distributor in the flywheel? (single coil single point setup twin cylinder as in the older DT’s and LT’s). My mind is a little foggy its been a long day. Jim

2Fast4Me

[adam1961]

This may very well be a DT or LT powerhead. The taper goes all the way down to the flats for the point pushrods. The TD crank should have a straight area with the lower keyway for the magneto rotor.
I wish I had noticed that before I stamped the number for my TD-20 in this powerhead. I guess I could just swap out the crank and call it close enough.

[Mumbles]

That’s a good point. Here’s a photo of a single coil/points HS-10 twin mag with two flats on the crank.

Maybe that brand new powerhead isn’t a TD-20 afterall?

[Author]

Posts