is there a difference between 21-A rods and 91-A?
 

wondering if there is a difference between the 21 and 91 a rods. I have a mix of both in my engine.

Re: is there a difference between 21-A rods and 91-A?
 

The 21A rods are direct replacements for the 91A and are interchangable. I'm unsure of what the difference actually is. You can't really tell the difference between the 99A & 29A rods either.

Re: is there a difference between 21-A rods and 91-A?
 

They are discernably different AND there is weight difference of about 5 or 10 grams between both. (See the photos below)

Ford at some point must have felt they required some add'l strength in the cap halves of the rods so they "stiffened-up" the "rails" on both sides.

Now, having said all that, personally I would have no issues "mixing & matching" them in the same build. The nominal weight difference will have no adverse effect on the overall build. I would NOT try to remove the weight by "sanding" on the cap's rails.

Thanks, Gary in N.Y.

P.S. Here's a shot of both side-by-side, you can see the caps are really different.

Re: is there a difference between 21-A rods and 91-A?
 

No, the picture does not show a difference in rod cap material. The rod on the left has simply received big-end weight removal as done by Ford with a chamfer tool.

Re: is there a difference between 21-A rods and 91-A?
 

I have always noticed "ALL" the caps on either 21a or 91a have 91A on them .