21A Rods in 36 Engine

[33rds]

21A rods in a -36 engine, is it working?
I'm thinking of how the balance will be?
Must it be balanced?

[FlatheadTed]

91A is a common conversion Lock in bearings .cheaper more assessable ,all flathead engines should be balanced ' if you can .,I am sure others will add, Ted

[flatjack9]

91A rods use full floating bearings, same as 21A.

[FlatheadTed]

I believe your right ,does he mean spit flanged or 8ba .Ted

[JM 35 Sedan]

Originally Posted by 33rds

21A rods in a -36 engine, is it working?
I'm thinking of how the balance will be?
Must it be balanced?

21A rods can be used in a 36 engine. Did you mean 29A or maybe 8ba?

[tubman]

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The last engine I did (last year) cost $125 to balance. I had to spend some time shuffling parts betwwen shops, but I think it's worth it. Several years ago, I was at a show and had my original '51 next to a guy who had a '50 two door sedan with a fresh rebuild. He was amazed with how much smoother my engine was than his. After a couple of minutes of conversation, he told me that he didn't bother to have his engine balanced because "it was too much trouble". nuff said!

[Brian]

21A rods were not original equipment in 36 Fords. They measure up the same but are beefier at the bottom end. All my 21 stud rebuilds get them. I also get every engine balanced.

[Barn Junk]

Just went out in the shop and threw a few on a gram scale for reference. There are some variances but here are some general numbers without rod nuts.
1932-38 (less metal at big end than 91A and 21A) about 440 grams
1939-41, 91A about 455 grams
1942 up 21A about 467 grams

So looks like balancing would be a good idea.

[Lawrie]

I know the 21A rods are the go ,But the smoothest and nicest engine I have is the original 33 babbit engine with those real spindly looking early rods,went like a rocket too,shame its just wasting away on the engine stand,but soon will go into the 34 3w as its engine is a real droner.
Lawrie

[Bored&Stroked]

You should be able to use the 91A or 21A rods - they are much thicker/beefier around the 'big ends' than the early rods . . . but, do you need the strength?

With that said, some use the early rods because they are so light. I recently took apart a 284 cubic inch race engine (back from the 50's) that had an Isky 404-A radius lifter cam, 4 1/8 stroker Merc crank . . . and ran the early rods. I don't know the history, but my guess is that it was probably in a circle/dirt track car - probably accelerated quite nicely . . .

[33rds]

I probely dont need the extra strengt but i have a NOS set 21a but maybe better/easiest
To restore my old rods?
I' m building a stock engine with a mcculloch supercharger.

[flatheadmurre]

It depends most on if you´re going to balance the engine as i see it.
If you have a nice 21A set ready to go and are balancing the engine i can´t see any downsides to it.
And with the blower you add a bit of power to.

[Ol' Ron]

In this neck of the woods, Balancing is over 250 bucks and very few shops know how to balance a flathead rod. I agree that swapping rods, cranks and pistons have to e balanced. A good reason to keep the rods and crank together, then only the pistons have to be brought into weight. However Piston weight isn't as important as the big end of the rod. Another problem is the RPM of the engine. As RPM rises the UN-balance sets up harmonics that can't be balanced out, This gets critical at around 5k in a V8. The 60 deg V12 is the only engine that can be balanced on all plains.
The Flathead as it comes from the factory is probably the best balanced production engine. I doubt that any engine re builders balance their engines as the cost would put them out of competition. I asked thei question when I visited Industrial engine rebuilders in Patterson NJ if they Balanced their engines. The answer was NO. I also asked a NASCAR engine builder if he had a new set of pistons that were 10 G heaver or lighter than the originals would he re balance the engine? the answer "NO" as the formuls for balancing varies when it comes to piston weight. I spent 8 years at the Danbury speed way and only a hand full of engines were ever balanced, because there was only one balancer on the east coast at the time. One balancer no longer in business came up with a "street Balance" One Rod, piston rings etc were brought t his shop and weighed for Bob weights and the crank was spun for balance, then Drilled.I never balanced any of the engines I built if I had the original assembly. But you never KNow. This is one reason I like buying a stroker assy from a reparable dealer.

[GOSFAST]

Originally Posted by Barn Junk

Just went out in the shop and threw a few on a gram scale for reference. There are some variances but here are some general numbers without rod nuts.
1932-38 (less metal at big end than 91A and 21A) about 440 grams
1939-41, 91A about 455 grams
1942 up 21A about 467 grams

So looks like balancing would be a good idea.

If anyone had this weight difference in the rods it would not be entirely necessary to balance or rebalance the unit, even the difference in those numbers you show will have no adverse affect on the end performance.

If in fact it's a "fresh" balance job, as in a brand new build, I would use the given numbers and make up the correct bobweight amount, if it has already been balanced and you are simply changing the rods you will be good to go in most cases!

It's the same scenario with a piston change, as much as a 20 gram spread will not matter. If anyone doesn't buy this simply call Federal-Mogul and get their opinion! We ARE talking sets here, not singles!

Here's a little "tidbit" to think about, over the years we have opened up many Mopar units, B and RB units, and have found as much as a 40 gram spread between the 8 existing connecting rods from Chrysler! Never caused a single vibration complaint (or brg failure) from a single customer from the day the car was produced!

Thanks, Gary in N.Y.

P.S. I would add this also, we have found over time that it is best to NOT try to get all 8 rods "dead-even", a few grams +/- won't matter. It is much safer leaving the rods as-is! If you don't do engine building for a living you can ruin a good set of rods fairly easily, leave them alone if they're close!

[John R]

Originally Posted by Ol' Ron

In this neck of the woods, Balancing is over 250 bucks and very few shops know how to balance a flathead rod......

Ron, What is special about balancing a flathead rod versus balancing any other rod?

[Ol' Ron]

The Chevy rod had a balance Lug on the cap, which can be ground to match the weight of the big end. The Flathead rod has ribs that strenghten the cap. When these are ground they weaken the cap, os the weight has to be removed from the sides of the ribs. Tricky to say the least. To speed things up a special fixture in a lathe or mill makes it easier and quicker. The big ends are weighed at the factoruy and matched in sets, for this reason I've always kept these rods together, that way when I have to have a crank balanced they don;t have to do anything to the rods. Unfortunately the last "street balance" I had done cost $220.

[Bored&Stroked]

Originally Posted by John R

Ron, What is special about balancing a flathead rod versus balancing any other rod?

Here is from a long thread on the HAMB that I wrote . . .

========

Full Floating Rods - Old School, With Correct Balance Procedures:
Seems that just about everybody these days either runs late 49-53 style rods or new H-Beams with insert bearings. Well, these didn't exist in 1942 and even in the 50's a LOT of racers still ran the full-floater 91A/21A rods from 39-42. These rods were part of the 'hot combo' that you hear about - where you take a 4" late Merc crankshaft and offset grind the rods to the early 2.00" 221 Ford flathead size and then run the 221 rods (91A/21A) and that gets you a 3/8" stroke increase ( 4 1/8" stroke). Also a lot of the guys building top-end race motors swore by the full-floater bearings - they are a bit harder to setup, but they have a lot bigger bearing surface and can take a heck of a lot of abuse. The key is getting the bearing shells to correctly fit the rod bores and to ensure you have the correct clearances on both the journal as well as the rod side (along with enough side clearances to get the oil out!). I prefer to run them 'loose' - and keep a lot of oil in them. This is a very proven setup, just not that many guys doing it today.

Now - just to set the record straight - by the time you find a NOS set of 21A rods, then the correct full-floater bearings (like the Cadmium Silver - 'Truck and Bus' versions), then have a machine shop set them up . . . you've spent more than a new set of H-Beams, with Buick bearings. Why did I do this on this engine??? Because I'm building a pre-war example . . . so I'm using pre-war parts and procedures. LOL

I looked around and found a NOS set of 21A rods - packed in cosmoline from Ford - in a nice sticky ass box. Man, I love the look of a new set of these rods - such nice workmanship from Ford, wonderful steel, highly polished big-end bores (remember, these are bearing surfaces).

21ARods-BigEnd copy.jpg

Correct Way to Balance Flathead Rods - Like Henry Did! (Important!):
These rods do not have big lumps of steel (known as 'balance pads') like modern rods - see below:

BalancePads1.jpg

BalancePads2.jpg

So, what most machine shops do is just grind away on the bottom of the caps for the big ends and the tops of the pin areas for the pin ends. Yes - it gets them balanced, but this is the WRONG way to do it! Ford balanced their rods by cutting away material in a lathe of some sort - around the big ends on both sides and on both side of the pin ends. This is NOT what most machine shops have a clue about. If they just grind away on the bottoms and tops like they plan to, then you weaken the rod - don't let them do it!

CapGrindingExample.jpg

Notice these two pictures - I've denoted the areas where material is removed to correctly balance the rods - notice how on this complete set that you can easily see the machine work and differences. This is how you need to instruct your machine shop to balance the rods - if they won't do it, then find another machine shop!

21ARods-BigEnd copy.jpg

21ARods-PinEnd copy.jpg

Note: I've made my own rod balancing scale fixtures and machine fixtures - here is an example of how I do the caps in my lathe:

2015-02-02 21.34.42 copy.jpg

2015-02-07 19.33.56 copy.jpg

[John R]

Very elegant method of shaving weight from big and little rod ends. However, strength is reduced somewhat. Some random thoughts: maybe using a heavy nut on the big end of the lightest rods would add a few grams and reduce the amount to be removed from the heavy ones. For a heavy small end, an option might be to remove weight from the corresponding piston.

[Ol' Ron]

I also increas the size of the rod bore a .001 or 2. I try to use "USED" floaters because they have proven them selves. NOS bearings have a tendency to Flake, which made it very important to run a full flow filter. I learned allot the few years I spent in the "Flathead racing era" All the tricks that mad the engines last lap after lap. My Lodun engine will be a 276 with 21A rods on a 4" crank. I just might have you balance it. Have to think about that, don't have anyone around here I'd trust. Weve had your 294 well over 5k several times, engine now has about 5K on it runs good. Not happy with the fuel economy, getting a new AF meter. Later.

[Lawrie]

One of the things I did on my A engine was to use a variety of ARP nuts on the rods,all different weights and by mixing and matching them got the rod weights right,
Now on my v8s I have a real nice set of rod weighing jigs and its easy to weigh each end of the rod,
I also take the weight from the rib on the cap.
Lawrie

[33rds]

Wow! Thats was a lot of good information! Thanks everyone!
I think I use the 21a rods, and balances the engine

[JM 35 Sedan]

Originally Posted by 33rds

Wow! Thats was a lot of good information! Thanks everyone!
I think I use the 21a rods, and balances the engine

You are making the best choice in my opinion, and I plan to go the same way on a 36LB engine rebuild.