FM DRV2B or RV2B Magnetos

Nick

DRV 2, base mount, 2 cylinder, CCW drive, with wire outputs on top.

DRV 2-A, flange-vertical mount, CCW drive, with wire outputs on top. (old style??)

DRV 2-B, flange-horizontal mount, CCW drive with the wire outputs pointed to front when mounted on tractor. (new style??)

There is a difference in the length of the DRV A and B housings and cover.

In September 1941 Fairbanks Morse put a out a bulletin that tells about a new Alnico magnet being used in the RV series mags and was available also to upgrade older mags. The early RV series mags used chrome magnets in the magnetic rotor. Next cobalt magnets were used and then last the Alnico magnets.

Also the impulse assemblies were upgraded to better designs later and the latest new style FM impulse assemblies will still fit a RV series mag. Unfortunately that is the only new parts still available new for the RV series mags from FM factory. New gaskets, oil seal, bearings, rewound coils, NOS and used parts are the only other things available for them.

Most parts from other RV series mags will not really interchange on the 2 cylinder ones. The coils in RV1 are different. The coils in RV 2,4,& 6 were however the same. Later coils were covered in a plastic. Fairbanks Morse made a RV1 base mount in CW & CCW rotations, RV4/2 base mount in CCW, RV4 base mount CW & CCW, RV-4A & RV-4B flange mount vertical CW & CCW, RV-6 base mount CW & CCW and RV-6B flange mount vertical CW & CCW.

One big problem in the RV series mags was corrosion in the sealed aluminum housings. On restoring these you need to drill some ventilation holes on bottom or add some vents someplace. Also the distributor barrels the brushes ran on were prone to come loose and wear bad after many years. The fibre gears also are stripped in many because of corrosion and someone doesn't take them apart to clean before trying to turn them. The brush holders also are prone to cracking. All those parts are not available new so when you buy one of those mags you see advertised for big bucks as rebuilt it can't have much replaced on it. Usually takes several of these mags to really make one good one.

I looked at J.R.'s comment in his table, and indeed that refers to the change in the flywheel on the B so the same magneto could be used on both the A and B. The only difference between the magnetos was in the drive cup and hence the part number. Apparently dealers carried two magnetos (one for the A and one for the B) rather than carry one mag and put the correct drive cup on it - perhaps most weren't set up to do any mag work. With the B flywheel change only one mag was carried for the A and B.

I'm curious as to the reference you mention - if you can provide it I may have it and can give a better answer. There was a significant change at mag s/n 179115 when the endcap was redesigned, but that was in 1942, long after the change to Wico magnetos.

I'm not familiar with a RV2B. FM initially developed the R series of magnetos for use on 1 cylinder gas engines, then a two cylinder version (R2) was used on the GP. Then followed the RV series - the RV4 was used on early AC WC with the Continental engine and the RV6 was used on other tractors. The two cylinder version of this was dubbed DRV2, with a suffix A,B detailing changes. I assume the prefix D implies the version developed particularly for Deere.

The DRV 2-A when mounted has the wire outlets on top and the DRV 2-B has the flange turned 90 degrees so when mounted the wire outlets point forward with one being above and one below then. I figured the reason for this was to clear the later gas tank supports? I've seen some tank supports ground off and we have had to do that to make larger pulling mags fit sometimes on Bs.

The old R series 1 cylinder FM mag for FM engines. Old R series R2 mag used at random on GP,D,O, & W JD in 1927-32 were older than the RV series and parts were discontinued in 1944. Not sure when RV parts were discontinued but some were still made into the 50s as I've had NOS coils for them dated 1955 and 1958 before. Enough old mag history....

I also have some letters exchanged between JD and FM where Deere is clearly unhappy with the FM repair service being provided. Shortly after that the first Wico AP's showed up on the A and B tractors.