Farm Equipment - Fairbanks Morse FM-J4A and J4B Magnetos?

Print Page | Close Window

Fairbanks Morse FM-J4A and J4B Magnetos?

Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=17638
Printed Date: 24 Aug 2025 at 5:27pm
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com

Topic: Fairbanks Morse FM-J4A and J4B Magnetos?

Posted By: clovis
Subject: Fairbanks Morse FM-J4A and J4B Magnetos?
Date Posted: 02 Sep 2010 at 9:23pm

I have a Fairbanks Morse manual for FM-J4A and FM-J4B magnetos.

I know these fit some of the Allis tractors. I found some information about which tractor these fit by Googling, but I thought I would ask the experts here at this forum to be sure.

Thanks so much for the help!!!!!!

Replies:

Posted By: steve(ill)
Date Posted: 02 Sep 2010 at 9:44pm

FMJ or FMX of FMK is a model number of the mag. the 4 means it fits a 4 cylinder motor. the "A" means it is base mounted. the "B" means it bolts up with a flange. After the A or B should be another number. That is the manufacturer that FM made the mag for. If the number is "3", then the mag originally came off an AC tractor. The FMJ 4B3 is standard equipement for the B-C-WD-WD45, etc. If there is no "3" after the B, then it might work. Some manufacturers used different advance or timing internals than AC required.

-------------
Like them all, but love the "B"s.

Posted By: GlenninPA
Date Posted: 03 Sep 2010 at 6:51am

In addition, the numbers "not 3" could have different shafts or impulse drives....

Posted By: Steve in NJ
Date Posted: 03 Sep 2010 at 9:03am

Lag angles are important. AC machines used a 30 degree lag angle. Usually stamped on the flange side of the Magneto, back near the impulse. Adding to Steve's post, the RV4 series of Mags started off on some of the early AC machines. The FM4 or "K" (FMK4) series replaced the RV, and then the popular "J" series came along replacing the "K". The "J" series Mag was the most popular, but not without its quirks through the years. By the time F/M realized the quirks that unit was having, there were over a million "J" units built. Hence, the "X" series came along, moving the timing gears to the breaker plate, and improving from their failing tar taped coil, to the epoxy (red colored) coil eliminating some major problems that the "J" unit experienced. The "J" was also offered in another configuration. The FMJ4B3-A was a unit designed for the WC which had the grounding lug moved to the engine block side of the unit. These were usually operated by a rod connected to a spring steel lever which mounted to the Mag unit. The lever could be pushed against its grounding lug shutting down the engine via the long rod from the drivers area. Getting back to the "X" series, the "X" (FMX4B3) unit was eventually replaced by the Delco Distributor system or Battery Ignition line as it was referred to, but the "X" was still available from F/M as a replacement Magneto for any of the above series Mags mentioned for many years afterward. HTH
Steve@B&B

Posted By: clovis
Date Posted: 03 Sep 2010 at 10:19pm

Thank you so much!!!!!! I do appreciate it!!!!

Posted By: bobola
Date Posted: 05 Aug 2011 at 1:50pm

what are you askng for the manual? if still available