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Attachment 4, Flywheel Magnets

Updated 18 March 06

( Rather than delete this page in the 2012 update, I made it an attachment.)

7.1 It is hard by just looking at the magnets on flywheels to tell the NORTH pole from the SOUTH pole or the differences between similar looking flywheels. This page provides some help in determining the magnetic pole configuration and some actual measurements of the magnet field strength on selected flywheels. The field strength is the peak at the pole and in measured in Gauss.

Old Tecumseh

7.2 This flywheel is typical of the older H or V 50, 60, and 70 series engines. The South pole is on the left and North pole is on the right.


7.3 This is the typical Tecumseh ignition the is under the flywheel.


7.4 This flywheel, part number 610758, is from a Tecumseh HH120 engine with an under the flywheel Solid State Ignition system, 610759. The other parts of this ignition are shown on the pages titled Pulse Transformer and Solid State ignition.





7.5 The flywheel, part number 611000, on the right is from an above the flywheel Solid State Ignition. The magnets here are ceramic.

OH140/OH160 Flywheel

7.6 The magnets in the OH 140 / OH 160 flywheel are numbered in red. The even numbered have the NORTH pole in the center while the odd numbered have the SOUTH pole in the center. The important thing here is that there is a pole transition between each magnet since this is what generates the voltage.


7.7 I use a Gauss Meter to locate the NORTH and SOUTH poles since there is no ambiguity this way. ( The Gauss meter output increases above quiescent with a SOUTH pole and decreases with a NORTH pole.) But I doubt if any body else has a Gauss meter, so I have devised a way here of using an inexpensive compass and a large nail that works fine. The first thing to do is put masking tape on the magnets for the labels. Next, put a 90 degree bend in a spike, #12 or larger, about inch from the head. Place the head of the spike on the pole to be determined and position the cheap compass behind and close to the point of the spike. For the automotive type compass shown, if the spike is between the compass and your eye, then the letter you see on the compass indicates which pole it is. Figure 7.7 MagnetMeas.jpg .

A word of caution here. On one occasion, I put the compass near a strong magnet and reversed to polarity of the needle of the compass. If one used this compass to return on a hike, the result could be a disaster. This is why I recommend a cheap, disposable compass.

Figure 7.7

Added; 22 March,

Ed the welder has a better idea to map the magnets. He used a small magnet to feel if it was attracted to a pole or was repelled. To illustrate on the 20 amp fly wheel, I labeled one pole of a inch ceramic magnet A and called the other pole B. If the A on the test magnet was attracted to a pole on the flywheel, then I labeled that pole B. If the A on the test magnet was repelled, then I labeled that pole A. I did not bother to determine if A was a NORTH pole or a SOUTH pole since it doesn’t matter. The poles just have to alternate as one goes around the flywheel. See Figure 7-7 B, TestMag.jpg

Figure 7-7B

7.8 There seems to be a problem obtaining magnets for OH 140 flywheels. I have been looking for suitable replacements with a mechanical form factor and comparable magnet field strength. The best I have come up with are some Rear Earth disk magnets ( Item 1, $4 each, www,wondermagnet.com ) that are 1 inch in diameter and 1/4 inch thick. The original ceramic magnets are .3 inches thick. The disk magnets have a flat surface that is mounted on the curved inside diameter of the flywheel so there being a little thinner provides a comfortable mechanical clearance. Also, since the Rear Earth magnets are stronger, the increased air gap helps off set this.

For the OH140, 20 Amp stator and flywheel, three disk magnets fit nicely where one ceramic magnet was. To install them, one has to first map the poles of the adjacent ceramic magnet using the technique in paragraph 7.7 above. Then three disk magnets will need there poles marked for either two SOUTH poles and one NORTH pole or two NORTH and one SOUTH pole. Thus the total of three magnets puts the magnetic poles where we need them. They must be installed so the magnetic poles alternate as one moves around the flywheel. Warren gave me an un-serviceable flywheel with one magnet missing so I used that to test this repair. See Figure 7.8, NewMag.jpg

Figure 7.8

I used E6000 Industrial Adhesive to fasten the magnets since the surface of the Rear Earth Magnets is very polished. I have to admit that the magnets were so strong that they wanted to go there own way. I recommend a few dry runs before applying the adhesive and a non magnetic tool like a popsicle stick to help control the magnet.

I weighted some of the original pieces of the ceramic magnet and estimated there total weight to be about the same as the three disk magnets. So I don’t think balance is an issue.

At this point, need to run the repair on the simulator where I could compare the output voltages with those from Seb’s flywheel. To be continued:

Also, I have yet to get my hands on a 10 Amp stator and flywheel, but from what I have learned form Jacques, there will be differences from the above.