21A vs 91A rods

[Ken/Alabama]

I’ve always read that the 21A rods were the preferred choice when using a stroker crankshaft. What makes it superior to the 91A rod ? I just weighed some of each on my digital scales. The 21A weighs 462 vs 464 for the 91A. Other than that I can’t see any physical differences in the two.

[Pete]

Originally Posted by Ken/Alabama

I’ve always read that the 21A rods were the preferred choice when using a stroker crankshaft. What makes it superior to the 91A rod ? I just weighed some of each on my digital scales. The 21A weighs 462 vs 464 for the 91A. Other than that I can’t see any physical differences in the two.

I used both in the days before H beam rods and they worked the same. I didn't use any of them in stock form though.

[Ken/Alabama]

I’m building a new engine using a C69 block with 4 1/8 stroke crankshaft with 3 3/8 bore. I have a really nice set of 91A rods that have been resized and rebushed on the small end and I have enough 21A rods but they need resizing and rebushed. I have the machine to perform this but was wondering if it’s really worth it.

[Pete]

Originally Posted by Ken/Alabama

I’m building a new engine using a C69 block with 4 1/8 stroke crankshaft with 3 3/8 bore. I have a really nice set of 91A rods that have been resized and rebushed on the small end and I have enough 21A rods but they need resizing and rebushed. I have the machine to perform this but was wondering if it’s really worth it.

If this is a race engine, I would spend the money and shot peen the rods before resizing.
I would magnaflux them first before anything, even if they are new.
Check alignment after all machine work is done or bore the bushing with the rod mounted on a big end fixture. The advantage to this is all rods end up the same length.
Use Marsden grade 8 nuts.

[Ken/Alabama]

Not a race engine. Street engine only.

[Pete]

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Originally Posted by Ken/Alabama

Not a race engine. Street engine only.

The more you do, the longer it lasts.

[Ken/Alabama]

Originally Posted by Pete

The more you do, the longer it lasts.

91A rods ok ?

[Pete]

Originally Posted by Ken/Alabama

91A rods ok ?

Yes.

[Ken/Alabama]

Originally Posted by Pete

Yes.

Thanks. I do have the Sunnen tool to check alignment.

[Pete]

Originally Posted by Ken/Alabama

Thanks. I do have the Sunnen tool to check alignment.

By the way, you can say it is just a street engine but you WILL race it somewhere....LOL

[Ken/Alabama]

Originally Posted by Pete

By the way, you can say it is just a street engine but you WILL race it somewhere....LOL

I have one in one of my other 40 Fords but I’ve never abused it. I’ll wind it up sometimes, good torque .

[Pete]

Originally Posted by Ken/Alabama

I have one in one of my other 40 Fords but I’ve never abused it. I’ll wind it up sometimes, good torque .

You are not abusing it till you exceed the limit of the parts you used to build it.

[Ol' Ron]

There is probably a difference between these rods, but nobody seems to know what it is. The 21a were never used in production engines, I was told/

[alchemy]

What’s the difference of a 99 rod?

[Ken/Alabama]

Originally Posted by alchemy

What’s the difference of a 99 rod?

Larger I.D. On the big end.

[alchemy]

Same as a 29A rod?

[Bored&Stroked]

Originally Posted by alchemy

Same as a 29A rod?

Yes - as it was for the first Mercury 239 engines that had the larger rod-journal cranks.