Balancing Model A Rods

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C.A.R.E.
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Balancing Model A Rods

Post by C.A.R.E. »

After having rods poured and machined customer decided to have me balance the rotating assembly on a stock model A engine. Three of the rods were within range to balance normally, but one the big end was 25 grams light. No way could I take that much metal from the heavy three. So I made two new rod nuts from mallory metal. They were 12 grams heavier each ! This made it so I could balance them the normal way. The second picture is balancing the rods in a lathe the right way. The mandrel is made of nylon to protect the babbit
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miniv8
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Re: Balancing Model A Rods

Post by miniv8 »

While tightening them up, will you be lining up the slot in the nut with the hole in the bolt to slide the cotter pin in ?
I have tried searching for a weight comparison list of 12point nuts vs hexagonal to no avail to add to this reply. The 12point nut is drastically lighter, but 12 grams is a lot.

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Re: Balancing Model A Rods

Post by C.A.R.E. »

I had to add 12 grams each = 24 total. Yes you torque to 40 foot lbs, then tighten up to align the cotter pin up.
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Re: Balancing Model A Rods

Post by Dave Koehler »

I cringe any time the old fords, including the flathead v8 show up after 47 different rebuilds over the last 100+ years.
The rods often have unnecessarily long threads extending out beyond the nut. I have trimmed some of them down when it was possible.
The HM rod nut is a great idea. I will have to try that when the time comes.
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Re: Balancing Model A Rods

Post by PackardV8 »

Dave Koehler wrote: Mon Nov 29, 2021 9:11 am The rods often have unnecessarily long threads extending out beyond the nut.
Hmmm - long threads and additional weight needed. How about a case-hardened coupling nut?

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Re: Balancing Model A Rods

Post by C.A.R.E. »

The long nut would work for weight. but I need the castle for use with a cotter pin
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Re: Balancing Model A Rods

Post by lefty o »

C.A.R.E. wrote: Mon Nov 29, 2021 12:27 pm The long nut would work for weight. but I need the castle for use with a cotter pin
yup, everything in a model A has cotter pins in it.
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Re: Balancing Model A Rods

Post by PackardV8 »

lefty o wrote: Mon Nov 29, 2021 4:41 pm
C.A.R.E. wrote: Mon Nov 29, 2021 12:27 pm The long nut would work for weight. but I need the castle for use with a cotter pin
yup, everything in a model A has cotter pins in it.
Only if the guy is a stickler for authenticity in a part no one will ever see.
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Re: Balancing Model A Rods

Post by C.A.R.E. »

I have always used the cotter pins on the Model A rods. With only 40lbs of torque on a 7/16 bolt I was afraid the stretch on the bolt was not enough to keep it from loosing. No way to measure the stretch on these rods.
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Re: Balancing Model A Rods

Post by SchmidtMotorWorks »

If only one rod was out, why not just find another rod?
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Re: Balancing Model A Rods

Post by PackardV8 »

SchmidtMotorWorks wrote: Sat Dec 04, 2021 12:32 pm If only one rod was out, why not just find another rod?
OEM Ford A rods have poured babbit bearings. Today, just finding the guy to pour and machine a new one is not easy. My guy is backed up months.
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Re: Balancing Model A Rods

Post by C.A.R.E. »

This customer had the crank ground the minimum on the worst journal and the other three ground to the same size. Then the babbit in the rods bored to size w shims for the right clearance. This makes replacement of the rod a custom piece.
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Re: Balancing Model A Rods

Post by SchmidtMotorWorks »

C.A.R.E. wrote: Sat Dec 04, 2021 3:48 pm This customer had the crank ground the minimum on the worst journal and the other three ground to the same size. Then the babbit in the rods bored to size w shims for the right clearance. This makes replacement of the rod a custom piece.
Interesting, I wonder how much of the 25gs came off the rod pin.
Maybe it it were spun up with bws, it would have been close.
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Re: Balancing Model A Rods

Post by C.A.R.E. »

Remember a 4 cylinder crank is a flat plane crank. The crank is balanced neutral without any bob weights. Then the rods and pistons are balance to be a matched set. This means that changing the weight of the rods and pistons has no bearing on the balance of the crankshaft. PS: the rod journals are all the same size, just not a normal .010 - .020 - .030 undersized
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Re: Balancing Model A Rods

Post by C.A.R.E. »

SchmidtMotorWorks
I just realized what the bws stood for bobweights ! And if the journals were different sizes you are right. But note they finished all the same size.
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