Connecting Rod stud w galled threads

[Diastole]

Please advise.
I have a 1930 Model A engine that was Rebuilt 10 years ago and was put on a test stand and ran well for eight hours. I put it on a crate in my garage as a back up. I was told by the rebuilder that one of the connecting rods studs did not torque. The engine has been well cared for on the crate with routine installation of lubricant and turning the engine over almost monthly by hand.

The engine in my car cracked, between cylinders three and four. Common problem.

Therefore, I’m giving the engine I have as a spare a once over and I’m looking for advice how to fix the galling of the threads of one of the studs of my connecting rod. It will not torque and the nut spins.

As you can see from the picture the Babbitt is in great condition. The crankshaft is ground 0.030”. The valve seats are hardened and the valves themselves are new. Going over every part of the engine. The only thing I can find wrong with it is this one set of galled threads on the stud of the number three connecting Rod.

I’m looking for advice how to deal with this situation
1) order a new connecting rod. Accepting the fact that it may not be balanced with the other three.
2) braze up the threads and cut new threads, ensuring that I keep the Babbitt from receiving too much heat
3) weld up new threads to cut new threads protecting the Babbitt from heat.
4) try to remove the stud and replace
5) cut all new threads and use a smaller castle nut 1/2” -> 7/16”. Add a sleeve to cover gap
6) send the engine out and start over

Any advice and help is greatly appreciated

Diastole in the Smokey’s

[Flathead]

Another rod would be the best answer, it can be balanced to match the old rod which we are assuming was balanced with the original rebuild. The stud can't be replaced as it is part of the rod forging. You have it out of the car and taken apart, now is the time to make it right not just patch it up.

[nkaminar]

I would go for option #5. Not sure what the thread is but say it is 1/2-20 then you could go to 7/16-20. Use a grade 8 nut.

[old ugly]

i would guess that stud is 7/16nf that is the only size i have seen.

other than changing the rod the only cheat i can think of is. find the metric size die that is a wee bit smaller that will cut nice threads onto those studs. i don't know what is just a bit smaller than 7/16 but there has to be one. cost nothing to try.
if you do that you will need to find castleated nuts or at least some way to lock it.
i would not worry about balance.

[Synchro909]

Originally Posted by old ugly

i would guess that stud is 7/16nf that is the only size i have seen.

other than changing the rod the only cheat i can think of is. find the metric size die that is a wee bit smaller that will cut nice threads onto those studs. i don't know what is just a bit smaller than 7/16 but there has to be one. cost nothing to try.
if you do that you will need to find castleated nuts or at least some way to lock it.
i would not worry about balance.

7/16" is 11.11 mm. A 10 mm thread might work but I very much doubt the thread pitch will work out for you.
I'd go for another rod - in fact, I have just done that when I found a rod stud was stretched. When I found there was a 25 gram difference between the replacement rod and the others, it had to be balanced again. I am happy that when that engine fires a shot, it will be acceptably smooth.
FWIW, 25 grams is very nearly 1 ounce. So much for close tolerances during production!

[Diastole]

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Thank you for all the great advice

The original studs are 1/2“ x 20 tpi

According to Les Andrews, all I need to do is get to 35 to 40 foot pounds of torque

Each side of the bearing cap had two shims
I doubt the studs were stretched out
I will plasti-gauge when I put it back together

Diastole in the Smokies

[Bob C]

Originally Posted by Diastole

Thank you for all the great advice

The original studs are 1/2“ x 20 tpi

According to Les Andrews, all I need to do is get to 35 to 40 foot pounds of torque

Each side of the bearing cap had two shims
I doubt the studs were stretched out
I will plasti-gauge when I put it back together
Diastole in the Smokies

Are you sure? Snyders shows then as 7/16-20.
https://www.snydersantiqueauto.com/p...8702&cat=41684

[Diastole]

U r correct
7/16”-20

[Kurt in NJ]

unless you are prepared to suffer the consequences— the rod escaping the engine
Replace the rod.

[Big hammer]

Windows in engine blocks won’t hold oil !

[CWPASADENA]

Originally Posted by Flathead

Another rod would be the best answer, it can be balanced to match the old rod which we are assuming was balanced with the original rebuild. The stud can't be replaced as it is part of the rod forging. You have it out of the car and taken apart, now is the time to make it right not just patch it up.

I agree. Looks like a nice engine. Replace the rod. Get it balanced and you will have a good engine. Trying to do something with rethreading is not a good idea.

My opinion after working on these engines for more than 70 years.

Chris W.

[400A]

Another vote for a new rod.

[old ugly]

Originally Posted by old ugly

i would guess that stud is 7/16nf that is the only size i have seen.

other than changing the rod the only cheat i can think of is. find the metric size die that is a wee bit smaller that will cut nice threads onto those studs. i don't know what is just a bit smaller than 7/16 but there has to be one. cost nothing to try.
if you do that you will need to find castleated nuts or at least some way to lock it.
i would not worry about balance.

maybe misleading. i am not saying rethreading is the best answer. i will add to it.
if an attempt was made to rethread the rod studs there would not be any need to balance the rod. that would lighten the repaired rod by so little and all the other rods would be the ones getting balanced.
i suggest this as a CHEAT. the OP could thread the studs just to see if it can be done, then torque to spec while it is out of the engine to make sure it will accept the proper torque. if it doesn't then throw it away.

one more thing re the balance. i have taken so many of these engines apart and found mismatched nuts on the rods. different dimensions from rod to rod. makes me wonder how many years they ran for without perfect balance.

we know the best solution is a different rod, (used or new). if a different rod is to be installed then the rods and pistons need to be balanced.

[old ugly]

one more issue

what do the other rod studs look like? pull them all off

[nkaminar]

How about welding the nut on? No no, that is not a good idea.

Would the thread restorer that rolls the threads work?

[Joe K]

Google AI: Nearest Metric size to 7/16-20


Closest Metric Equivalents

• Best Diameter Match (M11 x 1.25): At \(11.11\text{mm}\) (\(7/16"\)), the diameter is almost identical to an M11 bolt. However, M11 is not a standard size in many applications.
• Most Common Alternative (M12 x 1.25): This is the typical practical alternative, offering a close pitch (\(1.25\text{mm}\) vs \(1.27\text{mm}\)), but it is slightly larger in diameter (\(12\text{mm}\) vs \(11.11\text{mm}\)).
• Thread Pitch Comparison: 7/16-20 has a pitch of \(1.27\text{mm}\) (\(25.4 \div 20\)), which is very close to the standard metric fine pitch of \(1.25\text{mm}\). [1, 2, 3, 4]

Summary Table


Feature [1, 2, 3, 4]7/16-20 UNFClosest Metric (Approx)Diameter\(11.11\text{mm}\) (\(0.4375"\))M11 (\(11\text{mm}\))Pitch\(1.27\text{mm}\) (\(20\text{ TPI}\))\(1.25\text{mm}\) (Fine)

The weight difference would be negligible.


A die in M11x1.25 is readily available on Ebay.


Joe K

[Dodge]

New rod!!!

[Diastole]

No chance to repair the threads. They are not rolled over. They are gone. Only way to rethread is to weld it up or braze it up and cut new matching threads by putting the die on the stud before brazing.

All other studs look great. Torque’d to 40 ft. lbs. All nuts match.
Babbitt is perfect. Brazing or welding threatens to melt/damage Babbitt. Even if submerged in oil to heat sink brazing/welding.

Snyder’s sell new rods for $123.
This rod weighs 763 grams. Snyders weighs 755 grms or 765 grms.

Thanks for all the advice.

Diastole

[old ugly]

measure your crank pin journal exact. because for babbitt, not all machine shops turn cranks to the exact undersize specs or it maybe stamped wrong.
Then order the appropriate pre babitted rod from snyders (or whom ever).
your galled up rod will not be accepted as a core so you will need to send a rod that can be rebuilt.
once you get your new rod and you know it will fit the crank pull all off the other rods, pull off the pistons and make sure all the rods are close to balanced.

this is the best option.

p.s
wrist pin bushing may need to be fitted aswell

p.p.s.
make sure you install the pistons onto the rods the correct way around.

[J and M Machine]

It would be best to change the rod/s . Those are lead babbitt as it's not shiny and it's dull like a lead weight. Do yourself a favor and get a replacement set of rods.

I wouldn't go back to that rebuilder if he put faulty parts in and then told you after rebuilding it , wouldn't torque up.??

Tin base babbitt looks like picture supplied.
https://www.jandm-machine.com/rebabbiting.html