Technical Connecting Rod Nuts

One of the things that has amazed me since I began working on engines some sixty years ago, is how the con rod nuts stay on! On most engines they are just nuts! Not lock nuts, no lock washers, no jamb nut ( most of the time) just plain old nuts! And they are in probably the most violent area around. Heavy up and down forces, add centrifugal ,then add vibration .......how do they stay on so good? I don’t think I’ve ever had a rod nut back off........but I had a ton of other nuts come loose, in what seems to be a less harsh environment! Any thoughts on this?








Bones

 

I think maybe because they are fine thread,,,and torqued good.
My old Daddy always taught me that any bolts that were fine thread,,,were harder to shake off,,,he worked on a lot of old dozers and equipment.
I am certain some experts will explain the physics involved in this equation.

“I’m just a caveman,,,,- and your world,,it frightens me “,,,,sorry,,,that was a line from an old Saturday night Live skit,,,Caveman lawyer,,,,but,,,it seems appropriate for me sometimes,,LoL

Tommy

 

The physics is gear ratio. the fine thread is the same as a lower gear. A fine thread bolt torqued at 35 ft pounds is much tighter than a coarse thread at 35 ft pounds. Rods are machined to be round with the bolts in place and torqued to proper spec. Many old engines had cotter pins or bend over locks on the rod bolts. The letter series Farmall tractors and Caterpillar dozers had cotter pins and Bend over metal tab style locks. Nowadays myself I put a small amount of Loctite on the rod bolts. Probably not necessary but it don't hurt anything.

 

Bolt stretch plus fine thread

 

Yep! What he^^^^^sez......

 

Interesting question. I wondered the same thing. I rebuilt an old LawnBoy 2-stroke 600 series engine a couple years back and even that had tabs that bent over 1 flat on the con rod nuts.

 

It's almost as if some engineer years ago did an analysis of the clamping load, the coefficient of friction, and the thread angle.

 

Interesting. Most 2 strokes have a built up crank, one piece rod and a roller or ball bearing, since there is no oil system. But there are some, I have heard, that do have a pressurized oil system. What was it on?

 

A 7266 series lawnmower. OT, from ~ 1976. They use needle bearings on both ends of the rod.

 

Lol, Squirrel! I sure they did, but it still amazes me! They’re 16 of those nuts in most engines and they never seem to let go. I have seen bolts and nuts, fine threaded,loosen up else where on vehicles. I realize , these are good bolts, fine thread, and torque close to max, and on machined surfaces, but it will never cease to amaze me! With all the forces in envolved , power impulse, vibrations due to detonation, ping, high rpm, they seem to hang on!






Bones

 

I've heard of split rods and needles. The needles have to ride over the seams.

 

The one thing that will let them get loose, I think, is if you over rev the engine, and stretch the bolts enough that they momentarily lose the clamping force holding the cap tight to the body of the rod.

 

Famous last words "I thought you were torquing the rod nuts"!

 

Bones , I’m sure you have attacked a Cummins or two in your day , maybe a Harley or two also . I sometimes scratch my head at may things also . I don’t understand how one damn bolt stays tight on either of those engines . The torque and stretch is the answer , not much stretch on the old Harley connecting rod bolts ! But the crank pin nuts really amaze me , also . I have had the damndest things shake off them through the years . I really think at low speed the Cummins is the worst vibrating mechanical rotating engine I have ever been witness to operating . I had a rear sliding glass explode sitting at a traffic light , I’m sure it was the harmonics of the old BT Cummins .

 

If I have a hard time understanding how rod nuts stay on, what about the pal nuts sometimes used to lock the main nut? How they live there is something I just accept.

 

Our fleet was almost exclusively Cummins engines. We had a ladder truck that had a 350 Cummins engine in it and while trying to back it into the firehouse the images would disappear in the mirrors due to vibration. I told the guys to “ live with it, reach out and hold the mirror with your hand! “ Well that wasn’t good enough of an answer for them, they went to the Chief. The Chief called me in a told me to “ fix that Cummins vibration at an idle” ! Really! I told him I’ll see what I can do!










Bones

 

C'mon man, don't leave us hangin'!! What did you do?

 

so does a MC 91A........

 

seems some fine thread nuts are actually lock nuts in themselves....usually a little dimple on the side.......

 

Nothin, just went back to my shop, shaking my head! Sometimes it’s best to just leave dumb shit alone...... don’t want to get none of that shit on you!






Bones

 

I bet they do. There are a few strange little non-intuitive quirks to the LawnBoy engine disassembly/assembly. Toro (who owns them now) has the manuals available in .pdf which are a real help, then it's a piece of cake. The rod bearing halves are silver, or silver plated anyway, can tell by the color of the tarnish. Silver makes a good bearing material for some reason. I do snow-blower maintenance in July and lawn mower maintenance in January. They thought I was nuts at the hardware store when I was rounding up parts for the lawnmower. "Yeah well we're backed up right now with snow-blowers". "Oh, sorry."

 

Paying attention to specs I'm sure is a good thing.
Just wildass guessing on plenty of the less critical stuff explains a lot, I'm also sure.

 

Had a 67 Mustang engine once that one of the rod nuts did NOT stay on....bad result..

 

I doubt you or anyone else has seen connecting rod bolts/nuts on a Harley twin, because there ain't none...

 

That reminds me of a time a friend of mine had a mutual friend overhaul his 289. The man doing the overhaul was a Chevy man and was going through a divorce, that’s kinda why he got the job , needed money! After he got the engine back in the car , it would not start, for a while. So they called me, knowing I was a Ford guy. Well, you guessed it, he wired it like a Chevy, in ten minutes I got it started, but it had a horrible noise. Our friend that did the work reved it up and a rod went through the pan/ block! The owner said” man, that engine threw a rod! “ Then being the friend that I was, I uttered my famous statement “ did you want it to last forever? “ That went over like a lead ballon! Seems the mechanic forgot to tighten any of the rod bolts at all!






Bones

 

Shit happens!.....

 

When you torque a nut or bolt properly it makes the threads bind (distort) this happens just before you strip it. LOL

That is the only thing I can think of.

I actually had the con rod nuts come off on one rod on a ford Fairmont banger of all things and it had those washers that you fold the ear over the flat on the nut. I screwed it together using the suggested torque, new washers folded over. The nuts came off of one rod anyway. I ate the cost and repaired it. I think I probably should have replaced the nuts.

 

Yes those fine thread nits that hold the torque converter to the flex plate on a ford are one example.

 

Actually when an engine has a catastrophic rod failure, it is not unusual to find (usually) one connecting rod nut, loose in the oil pan. I went through a spate of engine failures in a moderately loaded industrial application. As the Service and warranty Manager for the manufacturer of the end product, I personally tore down about a dozen engines over a 5 month period. Consulting with Federal Mogul Bearing company, I learned that they had seen this same thing on occasion and they had determined soft rod nuts to be the culprit.

I requested an engineering print of the nut from the engine manufacturer and the specified Rockwell C hardness was 28-32.

We pulled 100 nuts from our service parts stock and sent them through our incoming inspection department. (They were not normally inspected.) We had a large number of the sample below specified hardness, some off the bottom of the C scale of 20!

The engine manufacturer quit blaming our oil filtering system we applied to the engines and started paying our warranty claims for these out of warranty engines.

 

Exactly and they forgot to do it for metric....oh, those weren't engineers, just thread counters.....won't mention Whitworthless that is a wave thread that is designed to unscrew. Then there is the tri-lobular thread that is designed to not unscrew or is at least vibration resistant