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Allis Chalmers B magneto type

E

extremewhitetail

Member
Guys
I would like to rebuild my magneto but unsure what I got. It says 4 on the side with another number. Can't seem to find the correct type.
Thanks
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From what I see it is a Fairbanks Morse type 4. I use to have the manuals for it but that computer went belly up so I lost that info
 
I've never seen a rebuild kit for them but have seen where you can buy all the parts to rebuild it. But doing so is not cheap. I also think this site lists a number of parts for it
 
(quoted from post at 14:17:59 11/25/21) I have been looking for an hour and found nothing on this magneto. Am I missing something?
Click to expand...


It is a Fairbanks Morse "Type FM" magneto, the "4" means it is for four cylinder engines.
 
The original magneto would have been an an FMJ4b3 which looks like the photo you posted. Not a type FM or type 4 but a FMJ4b3. Most parts for an FMJ series will fit but you need at least the FMJ4 parts to be specific for the 4 cylinder version. The last number, 3 in this case, is just to specify the specific OEM manufacturer, i.e., Allis Chalmers. The "b" just means face mount.

A replacement magneto for the FMJ and FMK was the FMX. Both the FMJ and FMX look the same from the outside but use different points and a few other things such as the reduction or distributor gear. If you remove the cover, the FMJ has points with a large 90 degree bend in the rub arm and the FMX has a straight rub arm. The impulse pawls are different and the gear shaft is also different but easiest to just look at the points to tell if you have an FMJ or an FMX magneto.
 
(quoted from post at 21:53:32 11/25/21) The original magneto would have been an an FMJ4b3 which looks like the photo you posted. Not a type FM or type 4 but a FMJ4b3. Most parts for an FMJ series will fit but you need at least the FMJ4 parts to be specific for the 4 cylinder version. The last number, 3 in this case, is just to specify the specific OEM manufacturer, i.e., Allis Chalmers. The "b" just means face mount.

A replacement magneto for the FMJ and FMK was the FMX. Both the FMJ and FMX look the same from the outside but use different points and a few other things such as the reduction or distributor gear. If you remove the cover, the FMJ has points with a large 90 degree bend in the rub arm and the FMX has a straight rub arm. The impulse pawls are different and the gear shaft is also different but easiest to just look at the points to tell if you have an FMJ or an FMX magneto.
Click to expand...

The FM is shown as a spec'd mag, along with the FMJ, in the Allis Chalmers Service manual for the B. It is not shown in the parts book as I believe the FM was replaced by the FMJ for all service needs. The parts book shows the FMJ4B3 as superseding another part number mag. Just what I see written, not saying I'm right, but there is evidence the FM4 was used on early Bs.

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Thanks for posting all the detailed diagrams.

This post was edited by Nordic on 11/25/2021 at 09:00 pm.
 
I don't know how important it is for you to have your tractor original but if you wish to save on cost you could convert your existing magneto to function as a distributor with an external coil. The cost of doing this is very reasonable and it works great. I have done it to two.
 
I haven't seen a parts book listing for the FM4 series, only commonly for the FMJ4 and, very early on, the FMK4 although the FMX4 was used as a replacement much later. It would be useful to know the year of the tractor. I cannot tell from the photo if the stamp is just a 4 or is it missing the K or J. The logo seems more inline with a later FMJ or FMX rather than the earlier FM4 or FMK4 but logo designs may have changed . I also cannot tell from the photo if the cap is a two piece cap or a one piece. The FM4 and FMK4 will have a one piece cap with the coil pin near the top center. The points should have a 90 degree rub block.

If it is a FM4 then the good news is most of the parts for a FMK4 will fit the FM4 and are available. The easiest way to identify is to remove the cap and look at the insides.
 
(quoted from post at 22:31:04 11/25/21) I haven't seen a parts book listing for the FM4 series, only commonly for the FMJ4 and, very early on, the FMK4 although the FMX4 was used as a replacement much later. It would be useful to know the year of the tractor. I cannot tell from the photo if the stamp is just a 4 or is it missing the K or J. The logo seems more inline with a later FMJ or FMX rather than the earlier FM4 or FMK4 but logo designs may have changed . I also cannot tell from the photo if the cap is a two piece cap or a one piece. The FM4 and FMK4 will have a one piece cap with the coil pin near the top center. The points should have a 90 degree rub block.

If it is a FM4 then the good news is most of the parts for a FMK4 will fit the FM4 and are available. The easiest way to identify is to remove the cap and look at the insides.
Click to expand...

"I also cannot tell from the photo if the cap is a two piece cap or a one piece. "

I sure don't see a "parting line" where the small "distributor cap" would attach to the main cap, therefore making it a one-piece cap and an "FM4" just as it is labeled.
 
(quoted from post at 01:31:04 11/26/21) I haven't seen a parts book listing for the FM4 series, only commonly for the FMJ4 and, very early on, the FMK4 although the FMX4 was used as a replacement much later. It would be useful to know the year of the tractor. I cannot tell from the photo if the stamp is just a 4 or is it missing the K or J. The logo seems more inline with a later FMJ or FMX rather than the earlier FM4 or FMK4 but logo designs may have changed . I also cannot tell from the photo if the cap is a two piece cap or a one piece. The FM4 and FMK4 will have a one piece cap with the coil pin near the top center. The points should have a 90 degree rub block.

If it is a FM4 then the good news is most of the parts for a FMK4 will fit the FM4 and are available. The easiest way to identify is to remove the cap and look at the insides.
Click to expand...


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mvphoto85081.jpg
 
The larger photos helped. It does look like a FM4. It should have a disk rotor rather than a normal looking rotor and the cap should have 5 carbon brushes and a coil post at the top.

Some photos of one of my FM4B parts magneto will show what I mean.

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A search for FM4 parts may turn up what the original poster needs. If he searched for "FM" only or "4" only they that may explain why he was not able to get any results. FMK4 parts should fit at least the common ones such as rotors, points, condensor, and coils, and may be easier to find at least for a while.
 
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