The Ford Barn

Andy · 09-07-2023, 09:54 PM

I am building my 32 engine. I have NOS 32 connecting rods and also NOS 21A rods. The bearing placement and dimensions look the same. The later rods are much beefier. The engine will be completely stock so I really don't need better rods but I have them. Any comments?

Brian · 09-07-2023, 10:56 PM

Yes, totally interchangeable. The 21A rods are beefier as you've noticed.

fordscript · 09-08-2023, 05:49 PM

Quote:

Originally Posted by Andy

I am building my 32 engine. I have NOS 32 connecting rods and also NOS 21A rods. The bearing placement and dimensions look the same. The later rods are much beefier. The engine will be completely stock so I really don't need better rods but I have them. Any comments?

I would be going with the 18 rods. The 21A rods will fit but are heavier and will cause balance issues.

Lawrie · 09-08-2023, 06:12 PM

You need to check, I am pretty sure the 18 rods are heavier,,funny enough,
they dont look it, But I have a memory of weighing them and the 21 a year or two back.
Lawrie

38 coupe · 09-08-2023, 06:36 PM

I though the 18 rods used flanged bearings where the 21A rods do not. I think the rods can be swapped as long as you use matching bearings.

Andy · 09-08-2023, 06:55 PM

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The rod end widths seem the same. I have flanged bearing to fit the crank. I have good scale. I think I will weigh both ends of both rods. That may be interesting. I need to measure both rods very carefully.
A weight difference would push me to the 18 rods. The engine will have the low compression heads and the stock cam and carb. I would like it to run smooth. It will not see high rpm.

Brian · 09-08-2023, 08:35 PM

I've always used 21A rods in my 21 stud builds...however, I always get engine balanced.

38 coupe · 09-09-2023, 08:08 AM

At the risk of side tracking Andy's discussion, I have a question. I have not assembled the bottom end of a 21 stud motor with flanged rod bearings.

Question: Is the crank pin for the rod journals wider on the early cranks to fit the flanged bearings?

I looked in the Green Book for interchange guidance but didn't find what I was looking for.

18-6200 rods are used with 32-36 engines, have an overall length of 9.25", use a piston pin retainer in the center of the pin, and are to be used with 40-6110 pistons.
48-6200 rods are used with 35-38 engines, have an overall length of 9.125", and are replaced by 21A rods.

From the parts description in the green book if you are using stock flat top pistons you need to use the 18 rods. No clarification is provided for rod journal dimensions.

Kurt in NJ · 09-09-2023, 09:39 AM

The 21A rods with original wrist pin bushings are machined for the center clip wristpins.
The clip can be removed from center lock rods to use end lock pistons
The flanged bearings have wider journal.
Center to center length is same

Jack E/NJ · 09-09-2023, 02:24 PM

>>> am building my 32 engine. I have NOS 32 connecting rods >>>I really don't need better rods>>

Agreed. 8^)

Andy · 09-13-2023, 12:20 PM

My new concern is the crank. It do not know what year it is. It may be balanced for the later rods. It has three large holes in both large counter weights. Maybe it is a later production replacement crank that was balanced for the earlier rods? Sure looks like a lot of metal was removed.

Seth Swoboda · 09-13-2023, 12:30 PM

If you are building and engine, would you not have your machine shop balance the entire rotating assembly? Why worry about if your crank and rods are balanced when you can have all the components balanced as you should do anyway? To not do so is just guessing.

Ol' Ron · 09-13-2023, 01:13 PM

At my age I have to say that I've had very few crank assys balance. For one thing, nobody did it. here was a place in New Jersey that did it for the Offys and other race enginrs. but the cost was out of sight as well as the wait time. Built sllot of engines in the 80's thru thr early 2000. None were balanced. Fortunately, some are still running todayYes balancing is important, but most street engines neve hit 4k and cruise in the 2k reagion. I just had My 258 balanced $300 bucks. When building a flathead the cran and rods are balaced at the factory and the pistond don't need to be balanced. Some people will disagree with me here, BUT! !!

09-07-2023, 09:54 PM	#1
Andy Senior Member Join Date: May 2010 Location: Kerrville, Tx Posts: 2,771	18-and 21A rods interchange? I am building my 32 engine. I have NOS 32 connecting rods and also NOS 21A rods. The bearing placement and dimensions look the same. The later rods are much beefier. The engine will be completely stock so I really don't need better rods but I have them. Any comments?

09-07-2023, 10:56 PM	#2
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: 18-and 21A rods interchange? Yes, totally interchangeable. The 21A rods are beefier as you've noticed. __________________ Unfortunately, two half wits don't make a whole wit!

09-08-2023, 06:12 PM	#4
Lawrie Senior Member Join Date: May 2010 Location: Qld, Australia Posts: 4,218	Re: 18-and 21A rods interchange? You need to check, I am pretty sure the 18 rods are heavier,,funny enough, they dont look it, But I have a memory of weighing them and the 21 a year or two back. Lawrie

09-08-2023, 06:36 PM	#5
38 coupe Senior Member Join Date: May 2010 Location: South Texas Posts: 1,631	Re: 18-and 21A rods interchange? I though the 18 rods used flanged bearings where the 21A rods do not. I think the rods can be swapped as long as you use matching bearings.

09-08-2023, 06:55 PM	#6
Andy Senior Member Join Date: May 2010 Location: Kerrville, Tx Posts: 2,771	Re: 18-and 21A rods interchange? Sponsored Links (Register now to hide all advertisements) The rod end widths seem the same. I have flanged bearing to fit the crank. I have good scale. I think I will weigh both ends of both rods. That may be interesting. I need to measure both rods very carefully. A weight difference would push me to the 18 rods. The engine will have the low compression heads and the stock cam and carb. I would like it to run smooth. It will not see high rpm.

09-08-2023, 08:35 PM	#7
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: 18-and 21A rods interchange? I've always used 21A rods in my 21 stud builds...however, I always get engine balanced. __________________ Unfortunately, two half wits don't make a whole wit!

09-09-2023, 08:08 AM	#8
38 coupe Senior Member Join Date: May 2010 Location: South Texas Posts: 1,631	Re: 18-and 21A rods interchange? At the risk of side tracking Andy's discussion, I have a question. I have not assembled the bottom end of a 21 stud motor with flanged rod bearings. Question: Is the crank pin for the rod journals wider on the early cranks to fit the flanged bearings? I looked in the Green Book for interchange guidance but didn't find what I was looking for. 18-6200 rods are used with 32-36 engines, have an overall length of 9.25", use a piston pin retainer in the center of the pin, and are to be used with 40-6110 pistons. 48-6200 rods are used with 35-38 engines, have an overall length of 9.125", and are replaced by 21A rods. From the parts description in the green book if you are using stock flat top pistons you need to use the 18 rods. No clarification is provided for rod journal dimensions.

09-09-2023, 09:39 AM	#9
Kurt in NJ Senior Member Join Date: May 2010 Location: on the Littlefield Posts: 6,159	Re: 18-and 21A rods interchange? The 21A rods with original wrist pin bushings are machined for the center clip wristpins. The clip can be removed from center lock rods to use end lock pistons The flanged bearings have wider journal. Center to center length is same

09-09-2023, 02:24 PM	#10
Jack E/NJ Senior Member Join Date: May 2010 Posts: 2,173	Re: 18-and 21A rods interchange? >>> am building my 32 engine. I have NOS 32 connecting rods >>>I really don't need better rods>> Agreed. 8^)

09-13-2023, 12:20 PM	#11
Andy Senior Member Join Date: May 2010 Location: Kerrville, Tx Posts: 2,771	Re: 18-and 21A rods interchange? My new concern is the crank. It do not know what year it is. It may be balanced for the later rods. It has three large holes in both large counter weights. Maybe it is a later production replacement crank that was balanced for the earlier rods? Sure looks like a lot of metal was removed.

09-13-2023, 12:30 PM	#12
Seth Swoboda Senior Member Join Date: May 2010 Location: Southern Illinois Posts: 3,795	Re: 18-and 21A rods interchange? If you are building and engine, would you not have your machine shop balance the entire rotating assembly? Why worry about if your crank and rods are balanced when you can have all the components balanced as you should do anyway? To not do so is just guessing.

09-13-2023, 01:13 PM	#13
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: 18-and 21A rods interchange? At my age I have to say that I've had very few crank assys balance. For one thing, nobody did it. here was a place in New Jersey that did it for the Offys and other race enginrs. but the cost was out of sight as well as the wait time. Built sllot of engines in the 80's thru thr early 2000. None were balanced. Fortunately, some are still running todayYes balancing is important, but most street engines neve hit 4k and cruise in the 2k reagion. I just had My 258 balanced $300 bucks. When building a flathead the cran and rods are balaced at the factory and the pistond don't need to be balanced. Some people will disagree with me here, BUT! !!