Rod strength and folklore

[Quickstop [UK]]

The connecting rods have the tinsel(sic) strength of a wet noodle.

Tensile strength doesn't seem to be the factor most important - it refers to the metal being pulled whereas the rods are designed to withstand the much larger compressive force. That's apparently when the rods let go. This article from hotrod suggests that by changing the fasteners, you can double the horsepower of an engine that the rods could handle. That will depend on torque and seems more relevant to higher RPM motors than ours but still...

http://www.hotrod.com/techarticles/general/choosing_the_right_connecting_rods/

With very few exceptions, the weakest link in a stock forged rod is the fastener system. The rod bolt is usually the most marginal component. Simply upgrading from stock bolts to quality aftermarket replacements can improve durability by 50 percent. Just be sure to have the big end re-sized to restore concentricity any time the bolts are removed. Stock forged steel rods are an economical choice that should be able to handle one horsepower per cubic inch with quality fasteners, and as much as twice the factory-rated output if the beams are polished.

Read more: http://www.hotrod.com/techarticles/general/choosing_the_right_connecting_rods/#ixzz2JpyO3Y7K

So 500hp on stock rods?

When was the last time anyone had a rod let go without there being some other major reason? Am genuinely curious where this folklore comes from.

Most of the time I read about bearing failures but everyone talks about the stock rods being weak and crappy. Yes they are weaker than forged but when was the last time someone bent a rod or similar? It obviously happens but usually there is some other major failure that a forged rod wouldn't cope with in any case.

I've only been on the forum for 7 years so maybe things went on before but I am curious to know why this belief persists.

Why do people rate the stock rods so poorly?

:2cents:

 

[ziembic]

Re: Rod strength and folklore

My very humble opinion on this is that it is hard to know what caused what. Unless your data logging when your motor decides it time to check out, you can only go with speculation and a careful dis-assembly.
Sure it is easy to see that a head gasket or water jacket let go and water ended up in the cylinder and wammy we have the end. But what about a situation where the bearings let go in a catastrophic manner. Could it have been the rod bolts loosening that led to larger clearances that led to wobbling that led to wammy. Or could it have been an oiling situation that led to bearings becoming welded to the rods which leads to more heat and wammy. Or could it be rod failure where it just snaps, which puts the engine way out of balance and wammy.
My long winded point here is what comes first the X or the rod. In the end it is an engine that's service life has come to an end. Unless an autopsy is very revealing, I think it is easier to blame a mechanical component than ones in-ability to tune or over zealousness with the components at hand.

 

[Quickstop [UK]]

Re: Rod strength and folklore

I totally agree with you.

I just see people slating the stock rods all the time when actually I think there was another root cause of the failure and a forged rod wouldn't have made an iota of difference.

 

[johne30m3]

Re: Rod strength and folklore

I know what you mean that there always something that lead to the rod failing. Everything does have its limits though, barring any major catastrophe with good tune and supporting mods is there a power level where the rod bolts would just quit on you. I'm talking about a proper running truck with no knock issues.

 

[Turbotoyz]

Re: Rod strength and folklore

Some dbag was giving me crap, not for grammar, but about a question I had asked about nitrous. The member told me the rods were crap in these trucks. This was at least 8 or 9 years ago. I never looked into the rod strength, just found his reply amusing.

 

[TYTILIDIE]

Re: Rod strength and folklore

I spent a solid 100,000 miles beating the living crap out of my TY. 85,000 of those miles were on the stock rods. I can assure that I was not making huge power since the biggest I ever went on those rods was a 20g but never the less, they took severe abuse and went in to 3 different engine failures. I am not cheer leading for the stock rods but on a mild build situation I don't see why people should steer away from them.

On that note, the nice aftermarket forged rods sure are gorgeous even though no one ever sees them:lol:

 

[wildphil]

Re: Rod strength and folklore

That is very interesting info about the stock rods.

I also wonder about the strength on the stock untreated crankshaft.

 

[Icarus-54]

Re: Rod strength and folklore

cyro treat a set of l-35 rods w/good bolts and cyro the stock crank and youre good for anything that still streetable.

 

[Quickstop [UK]]

Re: Rod strength and folklore

I know what you mean that there always something that lead to the rod failing. Everything does have its limits though, barring any major catastrophe with good tune and supporting mods is there a power level where the rod bolts would just quit on you. I'm talking about a proper running truck with no knock issues.

I'm not sure - if it were the case of the rods being crappy then you'd expect there to be people saying that at a given speed or torque rating, for example, an 11 second 1/4 mile, the rods always let got because the metal wasn't up to it. The compression strength of the rods equates to their torque limit and thus their likelihood of failure. However, the tension they are under won't vary as most people don't spin their motors more than about 5k RPM. I accept that a few are going with higher RPM due to code59 but they are not the majority and presumably have other rods. My guess for a limit would be 500hp as mentioned in the article (2x stock hp if the rod bolts are left alone). Who has been in the 10s with a stock lower end? If the rods are so bad, I would expect they wouldn't be able to perform at that level.

Some dbag was giving me crap, not for grammar, but about a question I had asked about nitrous. The member told me the rods were crap in these trucks. This was at least 8 or 9 years ago. I never looked into the rod strength, just found his reply amusing.

Lol, those grammar Nazis are annoying

See - this is what I am curious about - I feel like it is something that never gets queried and is accepted without question whereas other people running well tuned trucks have had great luck with the stock rods. Maybe it's easier to blame a "crappy" OEM component than accept that someone dialled it in or assembled it wrong.

I spent a solid 100,000 miles beating the living crap out of my TY. 85,000 of those miles were on the stock rods. I can assure that I was not making huge power since the biggest I ever went on those rods was a 20g but never the less, they took severe abuse and went in to 3 different engine failures. I am not cheer leading for the stock rods but on a mild build situation I don't see why people should steer away from them.

On that note, the nice aftermarket forged rods sure are gorgeous even though no one ever sees them:lol:

Mike, you are a perfect example of what I am talking about. The stock rods get a bad rap but you got a lot of use from them. That sounds like a lot of abuse and reliable as well. What were the causes of the engine failures if you don't mind saying? I think the stock rods would be fine for the majority of people. I wonder if I could have gotten away on my recent build with a set of stock rods instead of the Eagles. I am not sure the price difference is significant however.

That is very interesting info about the stock rods.

I also wonder about the strength on the stock untreated crankshaft.

I'm glad you found it interesting. I was looking up the different steel strengths but couldn't find the numbers for OEM and aftermarket.

Regarding the crank, anecdotally and from the limited reading I have seen on here, people report 750hp being the upper limit of what the crank can take.

 

[2kwik4u]

Re: Rod strength and folklore

I have seen pictures of rods both bent and broken. I've only seen piston ring lands broken on failed motors personally.

I think Big Willie (what the crap was his username again), cracked off a couple of low 11 second, high 10 second passes with what was essentially a stock bottom end. Both I would guess to be due to detonation more so than an increase in boost pressure.

If I was rebuilding a motor, it seems that rods/pistons are a logical "while you're in there" mod if money is available. I would also think that measuring the stock rods to be dimensionally accurate might be just as costly as getting a new set of billet rods. Just some thoughts.

 

[bezerk]

Re: Rod strength and folklore

have seen 12 syty engines in my life, and 4 of them had a rod through the block.

 

[TYTILIDIE]

Re: Rod strength and folklore

What were the causes of the engine failures if you don't mind saying?

First failure-Ex was cruising around with no oil and locked it up.
Second-Oil pump pick up tube fell off.
Third-Oil pump tube weld broke and tube fell off.
Hence why I now only run an oil pump pick up tube with a bracket or strap now. FWIW, I bought the TY with 120,000 on it, so those rods really went 205,000 miles, just so happens that my 85 or so I put on were rough.

 

[tnsyty]

Re: Rod strength and folklore

I bought my Ty with a rod sticking through the block. Previous owner had made several 20psi passes on race fuel for months/ years and he said one day while just driving on the highway, the rod went through the side of the block. Made some nice holes in the oil pan as well.

 

[nallj92]

Re: Rod strength and folklore

I am on my 3rd motor currently.

1st- wrist pin let go and broke rod in half, trashed block
2nd- headgasket/rings/blowby
3rd- detonation- killed some SRP's

If you're rebuilding and doing forged pistons, eagles would be the natural progression. However I think pistons are more of a necessity over rods. I wouldn't be afraid of stock rods with good bolts and a set of good pistons at all. Rods usually bend/break from another issue that took out the rod.

However by the time you do machine work and resize rods for arp hardware, you may want to think about some h beams.

 

[Quickstop [UK]]

Re: Rod strength and folklore

I get that it makes sense to step up when you're in there but so far, it seems that the rods aren't the problem, it's other components failing or the truck being badly tuned etc. I think it is a poor excuse for someone to say "the rods are junk" etc.

I am curious about having to machine the rods to accept ARP hardware. I didn't know that was necessary - do they not make stock replacement bolts but in ARP material (180,000 psi tensile strength.)

I imagine the same applies to the pistons - they're fine and good but only get holes in them when the truck has another failure, usually owner related.

I am not sure if there is an automotive equiivalent but in aviation, when there is an accident, investigators look at the chain of events that led to the incident. In about 80% of cases, it's Human Factors rather than mechanical failure.

It would be interesting to see if that is the same in our vehicles.

I guess when someone says the rods are junk in our applications, I'll refer them to this thread.

 

[nallj92]

Re: Rod strength and folklore

when adding bolts/studs, the roundness of the rod hole is not correct when torqued. The need to be torqued down and then resized so the brearings are correctly round once installed.

search around for resizing rods, should come up with good info.

 

[DaveP]

Re: Rod strength and folklore

The big end of a properly-prepared connecting rod is not round. It is longer across the parting line than top to bottom. This so that when the rod is pulled in tensile at the top of the stroke, the parting lines of the bearings don't get 'pulled' into the crank and 'chisel' the oil off the crank.

I have never seen a bent Romulus 4.3 (SyTy OEM equipment) connecting rod. I have seen numerous bent Tonawanda (ANY 4.3 built in that plant, including L35, and some "replacement" SyTy engines from Tonawanda) connecting rods. I have disassembled over a dozen SyTy engines from various trucks and projects. I've seen a lot of SyTy connecting rods. I am CONVINCED that the "L35 rod" (which is the same rod as any other LB4 assembled at Tonawanda or Flint Engine) is weaker than the stock Romulus rod used in SyTy LD engines.

I remember discussing bent rods with Jerry McCoy at KC Nats 99. I asked him "what bends them?" He replied "detonation". I have no idea. I've replaced bent rods in engines I've rebuilt for my projects. But because these engines have then lasted, I've never had the opportunity to dis-assemble a SyTy engine that I built. It probably has nothing to do with my experience, and I'm only lucky that my engines last for years and never require bearings and re-work like so many I've seen on here. I have no idea if my engines bend rods, as I've never had to take one back apart. Yet.

Our engines don't rev high enough for the bolts to be an issue. The rod failures I've seen (not just "bent") where the big end came apart at the parting line had the bearing fail first. Or the main failed and spun cutting off the oil to the adjacent rod, and it then spun and failed. In the interim, a LOT of heat is generated, loosening the bolt. I've seen rods with one nut off, and the big end pulled apart, came off the journal, and the crank throw came around and blasted the big end of the rod through the block. Once again, the failure mode I've seen most have been main failed first, cutting off oil to the rod, then IT failed.

So far, I have not seen an on-its-own connecting rod failure in a street-driven SyTy. And I don't work on modded builds, so I'm no help there.

Keep it out of detonation so the mains live, use Romulus rods, and you'll be fine for a street-driven SyTy. It's worked for me for 16+ years.

 

[tnsyty]

Re: Rod strength and folklore

I have never seen a bent Romulus 4.3 (SyTy OEM equipment) connecting rod. I have seen bent "replacement" SyTy engines from Tonawanda) connecting rods.
.

That potentially sucks for me...

On my stock TY engine, the rod wasn't bent, it was broke, almost like sheered, close to where it connected to the piston. I bent a couple of rods in my W-code 4.3 though.

 

[DaveP]

Re: Rod strength and folklore

On my stock TY engine, the rod wasn't bent, it was broke, almost like sheered, close to where it connected to the piston.

Seems like detonation over-load. I've only seen a couple of little-end failures in the engines I've done over the years. My reading is that stress (including detonation) can deform, or 'squash' the piston pin in the little end bushing (or the piston pin bores in a non-floating rod) and it then binds and puts stress on, or the pin seizes in the pin bore and rips the little end of the rod off. Same deal. It's not the rod itself that fails, but is caused by another source. In this case the pin is not strong enough and deforms.

I bent a couple of rods in my W-code 4.3 though.

W-code is an L35. You've collaborate my findings regarding the strength of the "L35 (W-code) 4.3 rod". Thanks.

 

[2kwik4u]

Re: Rod strength and folklore

I have access to UTM's (Universal Test Machines) at school that come complete with load cells, and extensometers. If someone has a stock rod, and a forged rod that they want tested I'll take the time to setup the machines and get some emperical data. It is destructive testing though, so you can't get them back (well at least not in the same shape they arrive in). I can also test both directions (tensile, and compression). We also have an Xray material identifier I can use to identifiy the material precisely (alloy changes can significantly change tensile and compression strength).

My guess is that the stock rods are not forged, and are actaully precision cast, then machined to spec. Cast materials are significantly more brittle than forged, and as such are more sensitive to compressive shock loading (like detonation), than the more ductile forged materials.