Skip to main content

I have read many posts about the suspect nature of the nuts used on the standard connecting rod bolts and i am now suffering considerable paranoia.

Does the issue with the connecting rod nuts only concern engines built in the U.S. or are the nuts used on Australian produced rotating assemblies suspect as well ??

Or is that unknown and all oem nuts regardless of their source should be regarded as being prone to failure ??

Can the moderator shed any light on this ?
Original Post

Replies sorted oldest to newest

replacing the connecting rod bolts has always been a standard thing to do here in the US, ever since the early 1970s. This is because every once in a while a connecting rod cap comes loose when retained by the OEM bolts & nuts. Everyone blamed the bolts. In reality it was only the threads stripping out of the nuts. This is true even of the premium nuts used on the 351 HO motors (Boss 351).

I have no way of knowing if the 351's manufactured in Australia have the same issue. You'll have to cautiously ask that question of one of the seasoned (old as the hills) 351C enthusiasts over there. I'm sure you are familiar with the unfounded rumors and opinions that certain 351C parts manufactured in OZ are superior to the parts made in the US. I wouldn't want to base the durability of my 351C upon a rumor. The ARP nuts are inexpensive, they are far better quality than the OEM nuts, and dropping the pan & replacing the nuts is relatively easy to do.

-G
quote:
Originally posted by jb1490:
quote:
Why would anyone still be using the original Ford valves and rod bolts and nuts now?

Those items should be changed at every rebuild.

There are quite a few Panteras that have never had the engine rebuilt...like mine, with 124k (although I did replace the valves at 106k).
John


Sure, original everything until you take it apart.

The rod bolts and nuts have never been particularly suspect. The Ford valves have though.

All I am saying is why would you reuse those components on any rebuild

If you ask, why should I rebuild it now with 124K on the engine, maybe you should look at some of the pictures of engines that were posted here, where the valves came apart, took out the piston, block and head all in one shot.

Forewarned is forearmed, or as Winston Churchill once said, "those that refuse to learn from history are doomed to repeat it."

Big Grin
Originally posted by PanteraDoug:
quote:
All I am saying is why would you reuse those components on any rebuild

>>>Agreed, they should be replaced at any rebuild.

Originally posted by PanteraDoug:
quote:
If you ask, why should I rebuild it now with 124K on the engine, maybe you should look at some of the pictures of engines that were posted here, where the valves came apart, took out the piston, block and head all in one shot.


>>>You missed the part where I said that the valves had been replaced. Smiler

John
quote:
Originally posted by jb1490:
Originally posted by PanteraDoug:
quote:
All I am saying is why would you reuse those components on any rebuild

>>>Agreed, they should be replaced at any rebuild.

Originally posted by PanteraDoug:
quote:
If you ask, why should I rebuild it now with 124K on the engine, maybe you should look at some of the pictures of engines that were posted here, where the valves came apart, took out the piston, block and head all in one shot.


>>>You missed the part where I said that the valves had been replaced. Smiler

John


I guess so, but then again it really isn't original 124,000 miles is it? Wink
quote:
Originally posted by 4V & Proud:
quote:
Originally posted by PanteraDoug:
The rod bolts and nuts have never been particularly suspect...


the stock rod nuts have been determined to be worse than suspect, they're junk

more than likely you can see fractures in the threads without much 'help'


I think using ARP bolts on a rebuild is a very good idea. I personally change head bolts and rod bolts every rebuild.

Years ago, 1967 ish, Chevy was having problems with the bolts cracking right on the shelf. Brand new.

They would just develop these hairline cracks.

At one time it was recommended to magnaflux the rods as well to pick up otherwise invisible hairline cracks.

All things considered though you need to go by track record and few engine builders report problems with the rod nuts.
The carnage in the picture below has been happening to 351C lower ends since day 1. Most "mechanics" interpret it as bolt failure because rod bolt #1 is snapped in two! This is why it has been common practice to replace the rod bolts for the last 4 decades.

However, observe there is less thread protruding beyond the end of the nuts of the busted rod (1 & 2) compared to the nuts of the good rod (3 & 4).

The threads in one or both nuts gave way, the bolts pulled out of the nuts, the loose cap hammered or sheared the bolt in half and a moment later this motor came to a stop. There is no evidence of a spun bearing, so it is safe to assume this happened very quickly and the nuts were the source of the failure.

Yes I still believe replacing the rod bolts with 180,000 psi ARP bolts is a good idea, the OEM bolts are only rated for 150,000 psi, so the ARP hardware makes the lower end 12% tougher ... but the simple fact is replacement of the nuts alone has proven to prevent a street motor from failing in this manner. It is an easy to perform, cost effective way to fix one of the Cleveland's weaknesses. Replacing the bolts requires re-sizing the big ends of the connecting rods, therefore the motor must come completely apart. Replacing the nuts alone can be done in an afternoon simply by dropping the oil pan.

-G

Attachments

Images (1)
  • rod_nuts_fail
quote:
Originally posted by SoCalCat:
why must the rod end be re-sized if the bolts are replaced?


In my experience it doesn't but what you need to do is clamp the cap to the rod in a vise as you insert the new bolt.

If you don't, the cap will misalign to the rod. You can feel even .001" with your fingers.

Either way, you still need to take the rod out of the engine. That means taking the crank out also and everything else that is involved.

You are better off pulling the entire engine out to do that.



The real issue that is being avoided is that the rod bearings are held in place very tentatively with a tab that aligns with notches in the rod and rod cap. The tolerance for them is very small.

Changing the rod bolts will sometimes distort the cap out of round by about .001" or so. This increases the possibility of spinning the bearing.

Some of the bearings have dowel holes in them but using dowels in the rods or caps has issues too. They pop out under stress, and if you re-size the rods, they have to be removed to do that, then re-installed which reduces the ability of the knurls on the pin to hold it tightly into the rod.

No solution is perfect!



Most engine builders will tell you, as George has said, to resize the rods when you change the bolts to make sure they are round.

That requires a special connecting rod resizing machine.

This is cheap insurance when the engine is apart since they charge 4 or 5 dollars each rod to do this.



As far as the bolt and nut, I have found that they tend to need to be re-torqued to specs after they have been run.

I've seen this on all engines. Not just the C.

What happens is the rod bolts tend to stretch.

That in itself isn't unusual at all. There are specs now for torquing the rod bolts and measuring the bolt stretch as a spec, rather then reading a torque spec off of a torque wrench.

The bolt manufacturer supplies those specifications to you with the bolts and nuts.



These days ARP rod bolts are the safest and most predictable to use.

Not to contradict anyone but it isn't as simple as just changing the nuts. I honestly don't know if it is wise to go into the engine to change the nuts now?

What you are going to find is that when you remove the existing nuts, you will tend to take some of the threads off of the existing bolts. Then you are in a situation where the bolts should be changed too?

The threads on the connecting rod bolts are not cut as you would with a threading die, they are rolled on a threading machine.

You can't, or shouldn't "chase" the threads on either the bolts or nuts, with a tap and die.

Once you loose parts of the bolt threads, they are no good any more and need to be changed, and you will find that you don't loose chunks of threads necessarily, what will come off is a very fine wire like a human hair. If you get that, the bolt and nut are no longer good, change them both.

THAT RIGHT THERE can be the explanation of why the nuts fail, i.e., the bolt threads were already compromised and reused in a rebuild.



Who can argue with the known disasters but if you want to look at it as an actuary would, how many engines with how many thousands of miles are out there that are still running with no problems vs. the literally a couple or few that have broken?

You just don't know the history of the broken engines.



If you read the old magazine article, "Pro-fecting the 351c" by Jack Rousch, he says to just use the Ford Boss 351c bolts and nuts. PERIOD.

Considering he was THE authority on the 351c in the Pro stock days of the engine, I would go with his knowledge AND EXPERIENCE with the engine right there and close the book on it rather than any other heresay evidence that is presented in an online forum today, BUT THAT'S JUST ME and I can be funny that way as to who I accept as an expert on anything?

TO ME, the #1 on the side of the Gapp and Rousch Pinto counts for a lot. There are few that have better credentials then that?

I consider it as a really good Doctors really good advice. Best to consult with YOUR DOCTOR on YOUR SITUATION first. Wink

Now me, I'm not a real doctor, but I do remember trying to convince Linda Vaughn I was...and that I could help her back problem? Big Grin




The real answer is, "you's pays yours moneys, and you's takes your's chances!" Is it the Lady or the Tiger you will get? I like cats...here kitty kitty! Big Grin
Last edited by panteradoug
FWIW, there are two different stock 351-C rod bolts. One has a pentagonal head and is a ho-hum-std bolt. The higher rated Boss bolt head is football-shaped. Both have their own nuts, but once out of the box, you cannot tell the nuts apart. Race mechanic Carroll Smith once said that the combination of a gr-8 bolt and a gr-5 nut made a gr-5 bolted joint.
As for why its a good idea to resize rod big-ends when changing bolts, it's as Doug said- you CANNOT drive old bolts out and press new ones in consistently enough to NOT affect rod alignment, even when doing one bolt at a time in a rod fastened to a crank journal. Besides, you normally want to go to minimum spec on bearing crush and match crank chamfers, and with the highly variable parts available for rebuilds these days, it's safer to get the rods resized- not just for your new bolts but also for new bearings.
I have not experienced this myself, but I have been told by a very ardent, old time Ford racer (west coast drag racing) that the threads strip out of the premium Boss 351 rod nuts just as easily as they strip out of the non-Boss rod nuts.

Bottom line ... the standard 150,000 psi rod bolt and standard nut, OR the 180,000 psi Boss 351 bolt and Boss 351 nut will fail in the mode exhibited in the picture I posted earlier (the threads stripping out of the nuts). The standard 150,000 psi rod bolt combined with ARP nuts resolves that mode of failure. This has been proven over and over, for the last 3 decades.

I am not claiming the clamping force is any stronger than the 150,000 psi rod bolt, but that's strong enough for high performance street, and it has been used in racing applications too (confession of that same old time Ford drag racer). The simple fact is the nuts are defective, and the strength of the bolt is of no consequence when it is mated to a defective nut.

Have a good weekend everyone!

-G
Last edited by George P
Post
×
×
×
×
Link copied to your clipboard.
×