The 8 cylinder flat (crab) cap sold by Mallory is simply a red version of the stock 1942-48 flat head Ford V8 distributor cap. Mallory was the OEM supplier of the distributor to Ford during that time. (Mr. Mallory and Henry Ford played a lot of golf together I hear).
Mallory is again making flat head Ford V8 distributors for the aftermarket and the same cap that was designed in the early 1940's is still in use (may even be the same mold).
In addition to using them on flat head Ford's, Mallory found that the cap can be adapted to many of the 1970's and later Mallory aftermarket distributors (except the Comp9000 versions) by machining the top of the bowl a little smaller and using a special custom rotor. The most common uses of the flat cap at Mallory (other than flat head Fords) are marine engines and engines with blowers because both applications commonly have clearance problems with the standard Mallory cap. The new Mallory catalog still lists several "blower" distributors and many marine distributors that use the flat cap. Unfortunately the FE Ford versions are no longer listed but any standard bowl mallory distributor can be sent to Mallory and refitted with a flat cap. (the service department does it all the time).
The basic part number for the flat cap is 221A. But there are several versions such as 221, 221A, 221B etc. The standard 221A is the one designed for the flat head. Most of the others are just modified versions of the standard cap. For instance, the standard 221A cap has the cylinder numbers molded into the cap which doesn't work out correctly if you install the cap on an engine that has a different firing order than a flat head. So 221 is the same as 221A but with the numbers milled off and a vent hole added. 221B has the numbers milled off but no vent hole as it is used for marine applications.
The 221A was designed in the early 1940's when compression ratios were lucky to be 7 to 1 and octane was around 76. Therefore cylinder pressure and ignition voltage was low. The cap is made out of "bakelite" which was good in it's day but not too impressive today. There are no arc prevention ribs inside the cap like most modern caps.
Will you have crossfire if you use a 221A style flat cap? Maybe not but since they are made from an inferior material which cracks very easy, and there are no ribs inside, you are much more likely to have crossfire than if you were using a more modern cap.
There are alternatives. Mallory also sells a flat cap part number 221F which is listed as the flat cap for magnetos. It will fit in place of the other 221 caps but is a more modern design with ribs and made of modern materials. It is also harder to find and costs twice as much. MSD now also makes a flat cap of modern design and modern materials. The MSD version even has male spark plug terminals. It will fit in place of any of the Mallory 221 caps. It too is pricey but if I had to use a flat cap on a high compression or blown motor, I would only use Mallory 221F or the MSD version. I've seen too many crossfire and cracking problems with the original flat head 221 design.
Just another history lesson from your humble Mallory engineer.
I copied this from the FE Web Sight
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