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

[felix37]

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

[rotorwrench]

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.

[GOSFAST]

Originally Posted by rotorwrench

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 & 21A rods either.

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.

[JWL]

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.

[KiWinUS]

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