Old connecting rods...
 

Hi,
Trying to get a feeling for the viability of re-using original connecting rods.
Not a 911 guy, but all the concepts still apply to the water-cooled world...

You guys seem pretty content revving the nuts off your engines.
Some of you have con rods that are approaching (or even past!) 50 years old.

I know that the fancier builds get aftermarket rods but what are the real limits of the stock pieces, if you keep the revs down? Is age a factor?

Anything to consider besides new bolts, re-sizing the big ends and pin bushings? You guys shot-peening the rods to stress-relieve the surfaces? Anything more?

The ads which state 'shot peened to relieve stresses' clearly demonstrate a lack of understanding of this process and I think originates from China.

Shot peening is a process which, if carried out correctly, can significantly improve the fatigue performance of many metallic components

By bombarding the surface of a metal with a smooth round shot in a controlled manner it is possible to introduce a significant level of compressive stress and this is extremely beneficial in terms of fatigue endurance.

Fatigue is responsible for around 95% of mechanical failures and at a basic level is easy to describe.

In simple terms most fatigue failures can be described as a two-stage process, Stage 1 is fatigue crack initiation and Stage 2 is fatigue crack growth.

Fatigue crack initiation generally accounts for 90% of the life of a component.

Fatigue cracks normally develop on the surface of a component and initially form due to what is referred to as 'slip band intrusion and extrusion' which are crystallographic defects that are virtually impossible to detect.

The development of these defects always require the presence of traction vectors which are usually caused by shear stresses and commonly form in specific crystallographic directions.

The presence of residual compressive stresses means that the surface tensile stresses must be of a much greater magnitude to cause the damage that would occur if the surface were unstressed.

Nitriding has a similar influence on fatigue life as it also introduces surface compressive stresses but Shot Peening avoids the need for heat treatment.

If is carried out correctly shot peening is very beneficial to components such as Turbine Blades, Con Rods and many other critical parts. If it is carried out badly it can have a negative impact on fatigue life and can introduce defects.

The quality of the shot, the screening of shot to remove broken particles as well as the velocity and the direction of application all have a significant influence on performance.

With used components it is difficult to make a decision with regard to the amount of fatigue damage that has been accumulated unless the part has been used in very controlled conditions and monitored in terms of the stresses that have been generated in service. Many Aircraft components are monitored in this manner but the costs of producing the base line data to make this approach work is very high.

Once a detectable defect has developed any component affected should be discarded.
With a used con rod it is possible that the part could have used the majority of its 'fatigue initiation' life and may be ready to crack.

By careful shot peening it is possible to remove the accumulated damage and effectively re-life the part.

By using companies that are FAA/CAA Approved it is very likely that excellent results will be obtained at a very reasonable cost.

I would suggest that Con Rods are shot peened in the following manner.

Shot Peen 200% Coverage 6-10 ALMEN "A" WITH #230 Austenitic Stainless Steel Shot.

Would there be any benefit to cyro treat them as well? What is best, cryo then shot peen or vice versa?

Cryo Treatments are interesting and IMHO they should be considered with a great deal of cynicism.

There is no doubt that many tool steels, particularly those used for the manufacture of high precision parts benefit from this type of treatment as it generally assist with dimensional stability.

The main benefit of this process is for the treatment of steels that suffer with problems of retained Austenite following heat treatment.

Retained Austenite is a metastable phase and there is a thermodynamic driving force which wants to allow its transformation to a Martensitic type of structure. The activation energy to allow this transformation can be supplied from either a mechanical force or can be thermally activated either by low temperatures of increased temperatures.

This transformation involves a small expansion in the volume of the component and this can lead to distortion and subtle changes in shape.

When considering close fitting parts or transducer materials for the manufacture of load cells these very small variations in size can cause problems and it has been common practice to cryogenically treat parts prior to final grinding.

Once the steel has been cooled to typically Liquid N2 temperatures full transformation will occur and the parts will be stable.

The type of material used for the manufacture of con rod, which would typically be a 4340 type steel and normally these steels are fairly stable due to the influence of carbon and tempering effects which make the retained Austenite relatively stable and immune to the effects of mechanical stresses. The volume of retained Austenite in 4340 type steels is typically in the region of 2% and tempering above 600 degF will cause it fully transform.

In terms of metallurgy there is no benefit to be obtained in cryo treating this type of steel in terms of either mechanical properties or dimensional stability.

There has recently been some evidence to suggest that the cryo treatment of chrome-silicon wire springs can result in an improvement in fatigue performance compared to an untreated spring and this is due to the development of compressive residual stresses at the surface of the wire.

The basic measurements of the residual stresses suggest that this influence is less than would be expected in a shot-peened spring but no comparative data has been produced.

Frankly I wouldn't bother but it will do no harm and if you are going to cryo treat them do it first.

I carried out a significant amount of work on the fatigue behaviour of shot peened gears more years ago than I like to admit and presented a paper on this subject in Amsterdam and have followed this subject with interest for many years.

Some work more recently published in Sweden on behalf of Scania reported the following observations:

In order to find out how the fatigue limit is affected by the material
hardness, a number of gears were tempered whereas other remained untempered.
Similarly the retained austenite content was altered in some of the gears with a
cryotreatment.

The shot peening of these gears was performed by dual shot peening (200% Coverage)

The results show that the fatigue limit is enhanced when the shot peening was
performed with hard media.

The amount of retained austenite does not seem to affect the fatigue limit for samples shot peened with this hard media.

Ollies Machine Shop, inspected for straightness, resized for ARP torque spec, re-bushed the wrist pin holes and Honed both.

I feel good about my 32 yo Rods :D, the New ARP kit will keep her together just fine.

I'm no expert, but considering chris _ seven' s response. I would take his advice. But then, if one has a " budget " .. I know most don't think we do with our hobby and will simply use it as a daily driver. Then rebuilding with new rod bolts like ARP is good. Otherwise, start fresh with some of the higher price point rods from the start. ..how big is your budget for your hobby?

The rods in a 911 are HUGE and ROBUST.

I think these things are barely stressed given their mass and being really, really short.

Chris: "With used components it is not (?) difficult to make a decision with regard to the amount of fatigue damage that has been accumulated unless the part has been used in very controlled conditions and monitored in terms of the stresses that have been generated in service. Many Aircraft components are monitored in this manner but the costs of producing the base line data to make this approach work is very high."

This is the crux of the matter, right? I wonder about "not"? As in, it's hard to determine whether to reuse a component unless the stress has been tracked in some manner? It seems you're discussing damage at a much smaller level than can be measured especially by DIYer's. Are there any tests that don't involve a lab to see stage 2 damage, something like a more sensitive magnaflux of some sort? Or, in your opinion, does Tippy have a point and that, say, a "T' engine's rods most probably won't have seen stresses that rise to stage 1/state 2 damage because the part is over-engineered for lower HP engines?

Here is an aftermarket 8000 RPM capable big-block Chevrolet connecting rod used in a 7.4 L (454 ci) engine that uses a MONSTROUS 101.60 mm (4 in.) stroke:


http://forums.pelicanparts.com/uploa...1489161179.jpg

Now, here are our beloved, stumpy little girthy connecting rods used in the, don't exceed 7000 RPM (in the bigger 911 motors it seems the concensus is) engines at a teeny stroke of say, only 74.4mm for a 3.2 (of course the smaller engines have smaller strokes yet down to 66mm!!!!):


http://forums.pelicanparts.com/uploa...1489161365.jpg

I just can't see how we are stressing these things IMO?. Maybe I am missing something, but a stock 911 rod is a beastly little thing!

Well that was a 'dumb' typo on my part - Thank you for pointing out I have now edited this mistake.

The problem with metal fatigue is that up to the point at which the crack physically initiates there is no real way to test the part to understand how much damage has accumulated.

The rate at which cracks propogate once they have initiated is very important and the conservative decision is always to assume that crack initiation is effectively the failure point.

With some components it is possible to develop a test programme to decide when failure will occur and simply replace the part at some safe proportion of a components life.

Ti rods used to be systematically replaced after around 50 racing hours and many military aircraft parts used to be treated in a similar manner.

This is because for many components the life between crack initiation and failure is very short and the risk of failure is too great to tolerate.

If parts can support reasonably long cracks it is possible to use an inspection based technique as long as the crack length does to reach a critical length between inspection periods. In this way if parts were crack detected and found to be defect free they could be re-used.

I would generally agree that 911 rods are conservatively designed and under most circumstances will never suffer from fatigue failures.

The decision is a simple risk/reward analysis.

If I were building a 2.0 litre engine to run at 8500rpm I would almost certainly shot peen the rods.

The cost is small compared to the damage that could occur if a rod failed.

If I rebuilt a standard 911T engine I wouldn't generally bother.

That's been my point. I think nearly all rods out the side of a case were from oil starvation. :)

Stock Porsche rods, with the exception of the 993 ones, are plenty strong and do not fail unless the bearing has an issue.

The main problem with OEM rods is they are heavy and that exponentially loads the bearings above 7K RPM. Porsche did use a nitrided OEM rod in the 2.8 & 3.0 RSR engines, but that was due to FIA Gp 3 homologation. The factory usually used a Ti rod in almost all race engines.

We prefer Pauter or Carrillo 4340 rods in all of our race engines for weight reduction and additional strength. (Arrow also makes an excellent product). Special apps get Ti rods, however these are life-limited so they are not for anyone on a budget.

Has anyone actually seen a Porsche Titanium rod have a fatigue failure? I know PMNA doesn't always replace them in 3.6L Cup motor rebuilds and with people running 996 Cup motors 200+ hours without rod failure and GT3s accumulating 100+k miles (slightly different rod to Cups but still Ti) with the rods intact, are we sure that they are actually operating beyond the fatigue limit?

What signs does the rod bearings exhibit?

Porsche Titanium rods are usually replaced by PMNA at 60 hours yet several set are running at 100+ i and the street version of the GT3 is not pulled into the Dealer at any preconceived time and replaced Why?

Some great advice given. Like all advice, it is only as good as the person receiving it wants to use it.

If you are talking about 911/930 Rods, I have never seen one fail if it was used as designed. Same for 944 Turbo Rods.

If they are rebuild correctly with care, they will do a great job. Its my opinion that all too often aftermarket rods are used when not required. I see Carrillo H section rods with Carr bolts in engines that make 250 HP and run at factory RPM limits.

Any part of an engine should be used based upon the engines use, performance and the parts used if its design falls within the engine use criteria. If there is a fitment factor, pump clearance, length or other design features that are required, then an aftermarket rod has to be considered. Weight is another factor. However, many aftermarket rods are as heavy as factory Rods.

If you are wondering about re using 944 Turbo Rods, have them rebuilt correctly, including all typical checks, handle them with care, and as suggested have them shot peened.

I am amazed at the mishandling of engine parts, I often see. Parts thrown down on the bench, put inside boxes without separations etc. This is one of my pet peeves, as is calling air cooled cylinders "jugs"!You put milk in jugs and pistons in cylinders!!

Just make sure that you do not induce the stress areas with nicks etc by mishandling. The engine will make you pay for it. I can assure you.

If the rods aren't breaking they simply haven't accumulated sufficient damage to cause failure, the issue is when will this happen? :)

This is a very difficult question to answer and I would reiterate that a decision to use a part beyond the life recommended by the manufacturer is a simple risk/reward judgement.

As we have no data in terms of levels of stress and no real fatigue test results you simply pay your money and make your choice.

The entire problem is based on the stochastic nature of the fatigue process, the variability of the material we are considering and the loading spectrum that creates the stresses that drive the fatigue process.

The concept of a Fatigue Endurance Limit is interesting and has been well accepted for many years and in practical terms seems to work - at least for the majority of conventional engineering steels.

The main reason for the development of a Fatigue Endurance Limit is concerned with a metallurgical process known as strain aging. In this process the dislocations present in the material's crystal structure are pinned into place by solute atoms which produces a very localised increase in yield stress and this effect is activated by the cyclic loading that causes fatigue. By using Electron Microscopy the development of dislocation 'tangles' around these solute atoms can be observed and this would suggest that this explanation is valid.

Engineering Steels exhibit high levels of strain aging but this behaviour is not so pronounced in 6AL4V.

It is, however, fair to say that this concept is routinely questioned and recent work published in Acta Metallurgia suggests that when we consider Very High Cycle Fatigue some of the assumptions don't add up. (Cycles of 10exp10 to 10exp12)

Some recent work carried out at UCLA on Ti Turbine Blades subjected to very high levels of vibration suggest that some of the previously applied properties may have been overestimated by at least a factor of 2 .

It is also fair to say that we are unlikely to see this number of cycles within a typical car engine.

Traditionally Ti Alloys were not considered to have a Fatigue Endurance Limit but
it has become customary for the manufacturers of Ti alloys to now quote Fatigue Endurance Limits of around 500MPa at 10exp8 cycles for 6AL4V but I believe if you design components with this level of gross stress you will suffer significant numbers of component failures.

If you read the 'small print' you will see that the 500MPa is an 'estimate' based on 50% of the yield strength of the material and that 'service and geometric' factors must be considered and that this figure should be down-rated between 1.5 and 4 times depending on the specific design.

Very helpful !

I started fatigue testing 6AL4V Ti on a commercial basis many years ago as it was essential to fatigue qualify material used in the manufacture of Sea Harrier components on a batch to batch basis due to material variability.

The thermo-mechanical processing of 6AL4V introduced significant variability which resulted in around 30% or the material supplied being inadequate for the duty cycle

I am sure that modern process control has improved this situation significantly and that by using FEA tools it is relatively straightforward to understand the stress/strain conditions applied to the rod and again using good quality published fatigue data it is possible to design a satisfactory con rod.

The reasons for the 'short' life is most likely due to the inevitable uncertainties and to completely eliminate the risk of a failure.

It is also worth pointing out that improvements in the Fatigue Limit of 6AL4V of up to 10% have been reported by using controlled shot peening and I would consider this to be a worthwhile cost on high value race engines.

Metal Improvements in the UK (part of Curtiss-Wright) have a CAA/FAA approved method for refurbishing Ti helicopter Rotor parts that has been backed up with fatigue test data.

It is also worth noting that it has become a common practice to coat Ti Rods using a Chromium Nitride (CrN) applied using a PVD chamber as an alternative to the plasma moly that used to be applied to the side faces of the big end.

I am not sure I like this process.

Some recent work has been carried out by Bodycote who have investigated the effect of PVD coatings on the fatigue life of 6AL4V Ti landing gear components.

They wanted to apply these coatings to reduce wear and eliminate galling. They evaluated Titanium Nitride, Chromium Nitride and a DLC based coating.

In all cases these coating reduced the axial fatigue life of the components being evaluated and on this basis I would need to see some data before I used this process.

Ti rods will of also be more prone to failure if they are nicked or marked as they are likely to be less damage tolerant than a steel such as 4340.

One day there will be connecting rods in carbon, Lamborghini studies that of meadows! :)

Chris, I got to work with a program off and on for a jet engine manufacturer where the engines were overflown beyond their original intended life cycles. Sometimes, cracked parts were purposely put into the test engine to see if catastrophic failure would occur. Pretty cool project looking back. We also had to gently tear the motor down, but do not touch anything. It was all about allowing the engineers to look at witness marks and possible gasket ot hardware install anamolies. I remember over the course of 15-16 years of working there, engine life cycles were increased by a healthy margin by the engineering group.

Guess my point and you seem to post above, maybe there hasn't been enough testing to fully understand Ti's true life?

I mean, who wouldn't error on the safe side? There's a lot to lose if you get it wrong.

I am sure this will happen but ultimately I think that a nano-platelet Graphene reinforced material will be the final solution and probably use a powder metallurgical manufacturing route.

Recent developments in Graphene manufacture using Soya Beans will impact on the cost of manufacture of the World's strongest material and there are many exciting applications just around the corner.