MGA generator pulleys (for pushrod engines) came in two different sizes. The standard pulleys were two piece stamped sheet metal riveted to an alloy hub. If the rivets work loose the part is usually junk and needs to be replaced. The pulley was reduced in diameter (and belt shortened) at engine number 17061 (February 1957). See Confidential Service Memorandum MG205 (pdf). Early pulley was 3-5/8" diameter. Later pulley about 3" diameter. Crankshaft pulley is 5" diameter. I personally prefer the larger diameter generator pulley, as I don't spend much time at slow engine speed.

Original fans are typically two pieces of sheet metal spot welded together, the front piece being generally flat and the rear piece formed into the blower blades. The center hole may have different offset from the flat plain depending on how wide the fins would be. I have two different fans in hand. One is 4-1/2" diameter and 3/8" thick with 1/32" offset in center. The other is 4-7/8" diameter and 1/2" thick with 7/32" offset in center. Neither one has notched fins.

MGA Twin Cam generator pulleys and fans also came in two different sizes, all different from the pushrod engine cars. The Twin Cam pulleys were cast iron and virtually indestructible. See Confidential Service Memorandum MG237A (pdf). Early pulley was 4-3/4" diameter. Later pulley was 4" diameter. The Twin Cam crankshaft pulley is also different than on the pushrod engines. The T.C. crank pulley is 4.75” diameter. Originally 1:1 drive ratio on the Twin Cam, it was changed to 1.2:1 to allow the generator to run faster for better charging.


These pictures show an aftermarket molded one piece pulley and fan with 3-1/16" diameter pulley and 5" diameter fan. . Material is phenol-fiber, Phenolic or Bakelite. This one was purchased in the early 90's. The pulley size matches the smaller pulley used on the pushrod MGA cars. The fan is about the largest size that can be accommodated by the generator, both in diameter and thickness. The fan blades initially jammed on the bolt head for the adjuster link, so I nipped off the corner of each blade to allow it to clear and turn. This piece was in service for two years while awaiting delivery of an original style replacement front pulley. I was happy to remove it as I never quite trusted the mounting nut to stay tight against the plastic part.

This problem stems from the fact that there were originally two different generators for the pushrod engine MGAs. First one had slightly smaller end plate and used the smaller fan. Second one using the larger fan has a larger end plate with the link bolt hole farther out so the fan doesn't hit the bolt. They are all original pats, just matched up at a different point in time. The adjusting link shown here has been modified to avoid touching the fan fins with the larger fan installed on the generator with the smaller end plate.

Generators for pushrod engines (left) and Twin Cam engines (right).
PR pulley may be 3-5/8" early or 3" diameter later.
TC pulley may be 4-3/4" early or 4" diameter later.
Notice the iron TC pulley sits closer to the fan than the sheet metal PR pulley.

On 7/9/2022, JORG JEMELKA in Boulder, CO, USA wrote:

"This old CCM bicycle repair tool fit the square fan pulley mounting block, and being thin enough to fit between the inside pulley face and the fan, made loosening the front mounting nut a breeze".