Connecting rod interchangeability ?

[Special Coupe Frank]

The engine presently in my '28 Coupe has two rods with spent babbit - #1 and # 2.

I have spare engine with two decent rods, # 3 and # 4.

Are all four rods the same?

Assuming the decent rods are straight, and the journal-sizing is the same between the two engines, is there any reason I could not use the decent rods from the spare engine, even though they have been run in different position on the crank ? ( The spare engine has deck-cracks between the exhaust valve seats and cylinder bores, otherwise I would swap the entire engine.)


I know this is a real "Grapes of Wrath" kind of get-by; just trying to get some driving out of my coupe this season, while I save-up for a proper rebuild.

Thanks !

SC Frank

[Mikeinnj]

Frank - "Assuming the decent rods are straight, and the journal-sizing is the same between the two engines"...... Never assume anything !

Check the journal size of each crankshaft throw to be sure the rods will fit. Check the condition of the babbit and if it looks thin or cracked, re-babbited rods are available from Model A suppliers/rebuilders. You also should swap pistons off the rods and have new wrist pins & bushings reamed to size at a machine shop. Good luck.

[Geo. H]

Your experience my vary, but about ten years ago I lost the babbit on one rod (B block) I had another set of used rods. The best looking one seemed to fit on both ends, so I installed it. I'm still running that rod with no problems.

[H. L. Chauvin]

Humble Opinion:

With the millions of Model A's on the highways of yesteryear, if you check things out carefully & find same size cranks & rods, what you are trying to do was about as common in the 1930's as rolling a hand rolled cigarette & lighting it with a match.

[wensum]

I would guess that there's a balance issue too?
When assembling my motor the balancer went to a lot of trouble to ensure accurate weights for the rods both ends
Keith

[Kevin in NJ]

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The rods can throw off balance significantly. Factory specs were+- 1gram at each end. Total weight difference in piston rod assemblies was 4 grams max.

My brother had a rod go bad and slipped in a single rod and list 5 mph off his top end and gained vibration. He has to keep his car to 60 and under or so.

Ford did a lot to balance the engine. If you want any hope of a really good running car you must spend time in restoring the balance.

That being said you can make your own call. Properly fitted any checked to be straight rod will give you a working engine.

[Special Coupe Frank]

Thanks for the replies and cautionary advice !

I'm not expecting a 50,000-mile engine out of this... engine presently in car has nearly 80,000 miles on it, deck cracks, and who-knows what other ills.

As I noted in my original post, this is a get-by while I save-up my money for a proper rebuild.

I was thinking along the lines that w/o a drilled + pressure-oiled crank, the rods should (in theory) all be the same, and therefore, swappable.


I will report back on success / lack there-of.

[H. L. Chauvin]

Hi Frank,

If you go on line, inexpensive gram scales can be found to balance your rod assemblies.

Pistons with rings & pins installed can be weighed first & recorded for comparison. If they are all from the same manufacturer, they will not vary much in weight.

Next, weigh top halves of rods to observe & record total rod top end weight.

Bottom halves of rods usually vary more in weight because of different amounts of babbitt; however, one way to equal the lower end rod weights is:

1. Buy castellated nuts from different Model A vendors, even from NAPA, & local auto parts places not part of a national chain like Autozone, O'Reiley, etc., all of which come from different manufacturers of castellated nuts; hence, different nuts have slightly different weights.

2. Find a company that makes small diameter washers, either regular steel or stainless steel, whereby one can add or subtract washers under rod nuts to change weights of lower ends of rods. (Think Grainger has some of these).

3. One can also slightly grind the back, flat sides of rod nuts to slightly reduce lower rod end weight.

4. Also every now & then check the total assembly of each rod with pistons, pins, & rings installed.

5. With above methods, not difficult to get tops & bottoms plus total weight of all (4) rods & pistons to within (1) gram difference between any two (2) of the (4) rods with pistons, pins, & rings attached.

Hope this helps! Fun, fun fun!

[Special Coupe Frank]

Thanks for the additional suggestions, H.L. !