Frod 21A vs 91
 

Can anyone tell me the differance. Both rods are interchangeable.

Re: Frod 21A vs 91
 

I am assuming you are talking about connecting rods? To me the most notable difference is the 91a has cassel nuts and cotter keys and the 21a maybe a little more metal on the big end. Eddie

Re: Frod 21A vs 91
 

I don't have a green book so I can't get this info. MYY

Re: Frod 21A vs 91
 

You'd be hard pressed to tell the difference side by side without looking at the casting mark.

Re: Frod 21A vs 91
 

I am no engine guy, but big/small end dimensions and length were the same on 48’s. 92a’s and 21a’s. The 21a became the service replacement for the two former. In a letter sent to Ford certified rebuilders that I have a copy of somewhere I believe the 21a’s were designed to be stronger. However as mentioned 91a’s look identical. Would be interesting to weigh a NOS of each

Re: Frod 21A vs 91
 

I understand all this, and have used them before. However, I was just wondering when they were used originally and why they were replaced. I also used 21A rods in the 4 1/8 strokers of the past. I will weight them and pass on any differences I find.

Re: Frod 21A vs 91
 

Just finished a set of 91A rods last week for my customer, resized (with the correct finish for the "floaters"), all new pin bushings, and balanced! Was for a Merc OEM 4.000" stroker crank offset ground to 4.125" and 1-pc brgs.

He called me Tuesday and informed me he decided not to use them, wants to go with a complete kit instead!

I still have them here and can give you the (accurate) weights on these rods. Total rod weighs 470 grms, pin end weighs 125 grms. This makes the big end 345 grms.

Thanks, Gary in N.Y.

P.S. Wishing all here a safe and "Happy Holiday" season!! "Merry Christmas and Happy New Year"!

Re: Frod 21A vs 91
 

One thing that I found was the 21a rods are lighter than the early 18-6200 skinny rods.
Lawrie

Re: Frod 21A vs 91
 

Let me ask a dumb question then. Can they be interchanged and the motor not know the difference? Thinking of using them on a 4 1/8 stroker motor
Thanks Dave/Green Bay

Re: Frod 21A vs 91
 

Thanks for the numbers, Gary. I'll be balancing these after the holidays. I won't need a scale as I use a two rod balance to find the light one and make all the others the same. But i think I;ll get one weighted, just for kicks. I'll let you make the bob weights.

Re: Frod 21A vs 91
 

Ron where do you prefer to take material away to equalize the weight of the rods?

I'm scared of doing that for the most part. I juggle the rods and pistons to come up with assembly weights as close as I can get them. This works pretty good most of the time for me, but I'm willing to learn a better way.

Re: Frod 21A vs 91
 

I use different weight nuts on the caps,(i have three different style of ARP nuts), and I made a jig to remove metal from the sides of the ribs on the cap.
lawrie

Re: Frod 21A vs 91
 

What Lawrie said. At one time I had a 16" lathe which made it easier. Now I have to do it on the Mill. Unfortunately, The fixture I have is for an 8BA rod. If I can find it. Easy fix. Yes, if you have allot of rods to choos from, you can get pretty close. When I dissemble an engine, I keep the rods with the crank. if I'm not using them for the build I keep them together in a box as a matched set. My knife edge balancer is very sensitive. So you can blow on it to check weight difference. Back in the 60's very few shops balanced crank assy's as there wern't many people that did this work. At industrial engine builders in Patterson NJ rebuilt hundreds of engines a day and not a balancer in site. Had one stock car engine balanced. Took 3 months to get it back. Today everybody wants their engine balanced and the cost of doing it is out of sight, and you don't know how good it was done.
PS very few shops know how to balance a flathead. Very few shops know how to rebuild a flathead connecting Rod.

Re: Frod 21A vs 91
 

So, aside from strength what makes the 21a rod so much better In a engine builders eye over say a 18- or 48- rod?

Re: Frod 21A vs 91
 

Michael, I would be very interested in knowing if there is any significant weight difference between nos 21a and 91a connecting rods. I've always "heard" these rods were interchangeable, but would like to know for sure. If you could weigh maybe six of each and post figures here, that would be super.

Re: Frod 21A vs 91
 

They both have an average weight on the rod end of about 330g and 140g on the pin end from my notes.
The design since 21 is the latest evolution...my thinking is some material from rod and pin end has gone into the beam making the 21 a little better that way.
But total weight and reciprocating weight still same making it interchangeable with 91.

Re: Frod 21A vs 91
 

Thank you

Re: Frod 21A vs 91
 

In checking a few sets, each 21As rod seem to be consistently heavier in overall weights than the 91As. For that reason, I keep them together in sets. Assume, Ford added a little metal to make the 21As a little stronger??

Re: Frod 21A vs 91
 

Kind of an OT question for this thread, but what is the "correct finish for the floaters"? Just asking because I've got a set of rods that were rebuilt years ago and the big end bore is finished in a fairly rough cross-hatch pattern. Rough enough that I've been afraid to use them in an engine...

Just asking.

Re: Frod 21A vs 91
 

The big end bore should be fininhed with a polishing stone. I don't remember the grot (if any) but it was a white one. Might hve been 1200.??? Getting old sucks.