D4 7U broken Magneto shaft

[cubpilot]

While starting my 7u today to plow snow the pony motor quit suddenly. Troubleshooting revealed no spark. I pulled the mag off and the shaft is broke just behind the front bearing. Clean break and looks like sudden truama and not fatigue. I need a new mag for sure but.....

What would cause the shaft to break? The motor runs strong ( I cleaned the fuel system and rebuilt the carb recently) It did have quite a clatter noise when running (and has had as long as I can remember). The mag gear teeth dont show any sign of damage but are quite worn. Im wondering if the teeth didnt mesh and broke the shaft. The engine turns over fine and makes compression etc so it didnt sieze up suddenly and break the shaft. I dont want to buy a new mag and have the same thing happen again! Id sure like to stay with the pony motor as opposed to a direct start conversion. I commonly start this thing at below zero to plow snow.

Anybody else ever had this happen? What should I look for as a cause?

[wimmera farmer]

hi cubpilot.
You need to measure with a dial indicator the bearing clearanceof the main bearings ie vertical and horizontal movement of the flywheel, Next check the longitudinal movement of the crankshaft. I am told it is a fairly common occurance to break the magneto drive when crankshaft bearings are out of tolerance on pilot motors.
cheers wimmera farmer

[bob]

Make sure the nut holding the mag gear on it tight when you install the mag. I have seen one back off and ruin the casting behind it.
Later Bob

[8C 361]

It did have quite a clatter noise when running (and has had as long as I can remember).

Check to see if the pony flywheel is tight on the shaft. The nut can be tight but if the keyway is wallowed a bit the flywheel will rotate a bit causing a clatter and shock that will take out the mag.

Tom

[gemdozer]

Its should be the front bearing loose up and dan or the flywhell is loose and am still have some mags.

[Old Magnet]

I'm surprised that no one has mentioned that gear mesh is one of the main causes of mag shaft failure......as it has been discussed many times.
Here's the service information again.

Seems to be only a few solutions for gear mesh.
1. Try another gear and recheck mesh.
2. Change thickness of the manifold (top) gasket.
3. Alter the Mag mounting flange.

[edb]

Hi Team,
here is possibly a better scan of the August 14 1953 S/Mag article as posted by OM above.
Have not found the other to rescan as yet.
Seasons Greetings to ALL.
Cheers,
Eddie B.

[cubpilot]

Last night after my post I remembered that a few years ago I noticed that the pony flywheel was loose. Tried to tighten it- no luck and just sort of forgot about it ( Ive got lots of old tractors with stuff I try to forget about).

Went out today and checked the play in the gears and when I grabbed the flywheel it was flopping. The nut showed lots of damage from previous attempts to tighten it. I got it moving with heat but couldnt back it off and ended up having to split it. One of the threads on the nut was embedded in the crank threads. Turning the engine over without the flywheel all the clanking noise is gone and the engine actually seems in pretty good shape.

Looking for a new nut, woodruff key and a complete mag now.Thanks a lot for all the posts and they were VERY helpful!

[Willie]

EMail me I have all the parts you need LazyGFarms2@aol.com

[Willie]

Heres what your gettin anything else??