Conrod life - do they fail eventually?

[t20racerman]

Hi all

I've owned my A10 for 36 years, and done at least 50,000 miles on it. No idea how many miles it had done before that. Its got a decent tune on it (gas flowed head, 9:1 pistons, Spitfire cam etc) and does get ridden 'enthusiastically' including plenty of long distance work. I tend to keep it below 5500 rpm all the time these days, except the odd fast 2nd gear overtake...

Anyway, having rebuilt the thing many times, nitided the crank etc, It struck me the other day that the only thing really that could trash it completely now would be a dropped rod. I had the rods polished in 1984, but do they have a finite life I wondered? I'm not racing it and thrashing the nuts off it, but it does get some decent use.

So, are the rods ever likely to fail if I keep the revs sensible, or should I think about replacing them at some point? The cost to do this would be huge, so I'm not minded to, but just wondered what peoples experience was. Incidentally, its a big journal, nitrided crank.

Adrian

[A10 JWO]

Never had to change a con rod since I started in the 60's. EXCEPT when my oil tank split on my Commando and blew the motor. I always change the big end bolts and shells, but that's it. My A10 was 1954 and they were in good order. I am sure someone may contradict me, learn every day.

[Swarfcut]

I suppose the prudent thing to do would be to get a spare engine. Sod's Law is such that it will be completely surplus until the day you sell it on, so as long as its safe and sound your existing rods will continue to perform. Not saying a 100% guarantee, but if the worst happens, you have the parts to hand.
 Rods do go bang, particularly the drive side one, as a consequence of lubrication failure, and the forces will bend the alloy like putty. Other failures are more often to do with conrod bolts and nuts, very rarely fatigue of the rod itself. It depends how much you want to spend. A modern rod is certainly less of a nagging doubt, but whether this expense is justified depends on the budget and the amount of use the bike will get.

 Whenever I pass under the Iron Bridge in Shropshire, Brunel's  Suspension Bridge in Bristol and similar constructions it is amazing to think those little old atoms are still doing the job as good as the day they were put into place. So as long as these materials are used within their design parameters all is well. Like they say, Ignorance is Bliss.

 No doubt the materials scientists among you  think twice about flying in old planes, just as they did before boarding a steam train across the Firth of Tay on a stormy night in December 1879.

Swarfy

[edboy]

racerman, try to ride with the left fingers hovering over the clutch lever. if a rod fails you may salvage some part of the engine if the clutch is pulled in quick enough

[RDfella]

My personal view is replace them. Aluminium work hardens and that doesn't help. I recall when Chevrolet experimented with alloy rods - they only lasted about a million and a half cycles. How long is that at 5,000 rpm? 5 hrs?
And the A series is not unknown for 'throwing a rod'. Mine had at one time, as the crankcases are mismatched and the rods clearly not a pair..

[Sluggo]

Yes of course they have a service life.  Tons of factual engineering papers on this, as well as real world experience.  Triumph knew this in the 1950s and by early sixties had sent out advice that the service life of a Triumph twin rod was ONE 500 mile race at Race RPMS.  Scrap after that.  Top fuel dragsters and Funny cars in NHRA competition is one pass down the dragstrip at full power. (Insane boost and fuels).  Some race cars have one race, others consider themselves lucky for a season before rods are shot.

In aviation, all components have a service life and most are time change items. After XXX amount of hours its scrap. 

This varies on material, and ops conditions but a easier way is talk about Duty cycle.  Its an algorithm based on stresses and loads, with the materials in question.  Titanium, Steel, alloy or whatever.  But lets say your BSA..  If you race it at high RPMS you can assume a duty cycle of 90%-100% and the life of the rod is hours
Plod around Yorkshire at relatively low revs, good servicing, and no over runs we can say 20% and its decades.  But haul around a side car or lug it around off road?  Its not always RPMs that kill a rod. Thats assuming good oil, frequent service and no bodgery by DPOs.

There is a lot of discussion about use of Alloy rods, and they fit a purpose and design at the time in the 40s-50s and 60s but the biggest thing was cost and design.  Back then steel rods were very heavy which is a can of worms with many ways of changing everything.  Modern H beam steel rods (Carillo for instance) now are very close to OEM alloy rods weights and easy to rebalance.. their service life is excellent.   Goldstar owners, for the most part wont reuse stock rods and cranks.. Most (Sensible ones anyway) opt for modern Carillo type rods and new hi quality steel cranks and pins, axles and bearings.

ABSAF is common but Phil Pearson in the UK offers these and they are amazing.  Every Goldie guy I know including my gurus say its insane to take chances on the original parts. 

I have seen a few blown up A7s and A10s but not many, but the reason is they are not very common here in the US anymore, But I have seen tons of blown up A6 unit twins and they ALL were fractured/sheared rods, always the drive sides, and usually lubrication related.   I have talked to many engine builders and racers and no one I talked to had any faith in the original BSA Alloy rods.   Now..I cant prove this but many people say the BSA and some Matchless rods are flawed because of the all alloy construction.  they say the distort and expand too much.  Again, have not seen any conclusive proof or studies showing how.. but there is compelling anecdotal evidence.

Nortons and Triumphs use a steel cap and they dont suffer nearly the rod failures.  So one old racer who is a legend (Hall of fame) told me a number of BSA secrets from his decades of racing experience.  In later years he was more known for Yammie XS650/750 race bikes, which some say is just a metric BSA.. (Hah!  ) But ol' Capt Dirt which was what every one knew him as said the BSA made a great race bike but needs certain tweaks to be successful.

One he told me, (This was years before the aftermarket rod business was as flush as it is now)  He said the secret to rod life on a BSA was get some Norton rods and take off the steel caps.. Put them on the BSA rods, resize them to spec, and always use known good quality bolts and nuts every time and check bolt stretch and use that instead of factory torque specs.   I have a set of these rods we did years ago and did several others and none have blown up.   Polishing and stress relief, and always lengthwise.  Do a Liquid penetrant test and if you can, a Eddy current.  (Eddy current runs electrical thru a part and any cracks, voids, casting dross/dirt will show up)

I will note, I cant say for sure on A7-A10 pistons but I have found many pistons vary in weight and construction in A65 unit twins and many are significantly heavier than originals.  They are more robust, but thats more weight on the rod. IE:Stress
 
There is a ton SAE engineering papers studying the modulus of Alloy rods and duty cycles, but no lack of other websites, books and resources.
See:https://www.racingjunk.com/news/2018/04/12/connecting-rods-101-part-1/

"  In a race engine, an aluminum rod stretches and compresses with every completion of the 4-stroke cycle. ... Due to the fact most aluminum alloys used in connecting rods have roughly ½ the tensile strength of a good steel connecting rod, an aluminum rod has to be made larger."

** In other words they only go bouncy bouncy so many times until they dont.  ***

This, more auto and racing but valid points
See:https://www.onallcylinders.com/2015/09/11/rod-school-how-to-choose-the-right-connecting-rods/

" “Aluminum, while not as strong as steel, is often used in very high power blower engines,” he said. “This is due to the fact that aluminum ‘gives’ slightly under violent combustion and acts like a ‘shock absorber’ so that the harmful shock of combustion does not propagate to the rod bearing and cause a bearing failure.”

The downside to aluminum rods is their fatigue life. Aluminum connecting rods have a more limited service life than steel and can begin to stretch, especially when they’ve made many passes down the dragstrip. ."

*** Modern ALLOY race rods now days are typically 7075 T6 Forgings and heat treated.  Which is light years away from 1950s BSA Cast alloy. ***

--------------------------------
My advice?  Most people with a Vintage BSA dont want to be rebuilding repeatedly, most will at best do one engine rebuild in the term of ownership of a vintage bike.  The cost in time, materials and labor do not warrant taking short cuts and "Good enough"  but not warranted to go all out on exotic materials either.

But one area I refuse to cut corners on is good quality bearings and dont reuse unknown bearings (Rare cases is a repeat build and known conditions)  dont risk used rods and pistons and replace any stressed hardware (IE: Rod bolts)  Also optimize oiling system.  Do it once and do it right.  Its well worth the investment for the peace of mind. 

** Fair disclosure, I have several BSA Goldstars and I wont use stock cranks and rods, but I do have an original Goldie scrambler with a racing history rebuilt many years ago by a recognized expert. It runs and I will operate it off road but I dont race it.  Eventually I will replace the crank and rod again,& I believe its got an original type BSA crank and rod built by the local BSA guru circa 1980s. But its lightly stressed and will be replaced because its not worth the risk.

[BSA_54A10]

Every material has a limiting factor called it's "Fatigue Life"
The fatigue life is the number of reversed stress cycles the material can handle before it rips itself apart internally.
Aluminium is not particularly long.
There are bucket full of things that will increase or decrease the number of cycles, like the magnitude of the stress cycles the temperature etc etc etc.
Then there is consideration of the method of forming , casting will have a lot lot shorter life than a forging.
Good thing about aluminium rods is that start to stretch long before they fail catasrophically which is why you measure them when they come out and as soon as they start to get longer you toss them out.
Without getting too technical, all metals have lots of atoms in the wrong place, or an empty hole where an atom should be.
We exploit this in semi conductors to make transistors or in silicon to make computer chips.
In metals when they get stressed the mistakes can move inside the lattice till they bump into each other and when enough bump into each other, an internal  crack forms .
So on every stress cycles all of these mistakes move, just a little bit till eventually one will get big enough to split the rod into several places.

Now back in the days when you could pop down to the local scrap yard & pick up another engine for 1/2 days wages , no body cared much.
However now days a 1/2 days wages just might cover the cost of a gasket set

FWIW most aluminium alloys will start to fail in around 10,000,000 cycles, when subjected to the largest sub critical stress for that particular alloy . Now if you want to work out what 10,000,000 revolution works out at in miles be my guest.
When Ducatti were fitting the Campliago magnesium alloy wheels we worked out the fatigue life of the wheels were around 80,000 km, provided they were just running on nice smooth roads at 110 kph ( NSW max speed ) . These wheels were failing at distances from around 60,000 km.
Originally the assumption was the idiot rider did some thing stupid causing the crash and the wheels failed during the crash but eventually we managed to convince the Commonwealth government that it was the wheels so they did some tests & Ducatti replaced all of the magnesium wheels for identical looking aluminium wheels which were good for around 200,000 km .
Funny enough we have design & material standards for Truck wheels,  car wheels, push bike wheels, wheel chair wheels, even wheel barrow wheels , but none for motorcycle wheels & AFAIK there still is none.

So your answer is YES

[Triton Thrasher]

FWIW most aluminium alloys will start to fail in around 10,000,000 cycles, when subjected to the largest sub critical stress for that particular alloy

You’re bound to know the answer to these simple queries:

At what rpm are A10 rods subjected to “the largest sub critical stress for that alloy?”

At 75% of that rpm, how many cycles does it take to cause failure?

[BSA_54A10]

One would hope that the largest sub critical stress would occur well above the maximum rpm .
Rods are supposedly only stressed in compression & tension so it will be the tensile force applied by the weight of the piston & rod and the piston speed.
having not done the math for the past 40 years I have no desire to do the maths right now.
But if the rods were being used to the limit of their tensile strength then you would expect it to let go in 10,000,000 revolutions
Roughly 1500 minutes @ 7000 rpm or about 24 hours which is why race engines get their rods replaced regularly
We used to do the testing with a sample of around 1/2" which is held tight at one end and the other is connected between a pair of flywheels to make sure there is no twist moment .
Then you just turn it on & wait for it to go bang
The stress vs cycles curve is hyperbolic.
The maximum "safe" working stress is generally taken as the point of inflection and is called the critical stress.
For aluminium it was around 2/3 of the yield stress which is quite low because aluminium is FCC so it slips quite easily along a lot of slip planes

[Rex]

Given a long enough/stressful enough life they will inevitably go bang.
Outside factors like oil pumps, oil supply, Taiwanese big end bolts etc aside, will they fail in the usual use 60+ year old bikes receive?
Unlikely.

[JulianS]

Never had a conrod fail but a few years back did have one of the radial flywheel bolts sheer which punch the whole bottom out of the crankcase.

[Black Sheep]

A7 and A10 (big bearing) rods do seem pretty tough - certainly compared to Norton 99 ones, though oddly the same Norton rods in an 88 seem to last well. The longer stroke of the 99 does not suit them.
So much depends on how long at what rpm they have endured.
Even if your rods have used 90% of their fatigue life, if you never exceed 4,000 rpm they should last for years of normal usage.
With these machines which are way beyond their design life, you really don't know what they have been subjected to. Are they 50% or 99.9% life expired? Just how many miles a year are you going to put on them? Is it worth the expense?

[t20racerman]

Wow - as always some incredibly well informed comments and advice here. As a two-stroke racer (TZ350 and '67 T20) I do know how short engine life is when thrashed, and with so many TZs out there, one tends to have a damn good idea what fails & when. Unfortunately, with a well used 60 year old A10, the data isn't really so available, although I was fascinated to read the stories of Goldies and Norton steel end-caps. Seems racing experience points to the A10 rods as a weak point. Fortunately with an SRM bottom end I can't see oil starvation causing me any problems - a known cause of failure.
I guess that, as I originally thought, the alloy rods are a minor weak point on a road bike, although having them polished at SRM back in '84 when my crank was done was probably a useful move to longevity.

Thanks folks - some fascinating feedback here.

[BSA_54A10]

We come full circle to the fact that the motorcycles have long outlived the criteria used in their design.
Remember it was 20,000 miles between sludge trap removals
A rebore to every 2 or 3 cleans and there were 3 oversized pistons
so very roughly the engine would be considered good for around 180,000 miles by which time BSA expected you to go & buy a new one .
Rods are supposed to be checked every time the bottom end is done .
Now this is fine if you live in the UK or most of the EU as 200 miles is an annual holiday and 5 miles to work & back is considered excessive.

Down here the case is different 40,000 miles would be considered a reasonable annual useage.
Back in the 50's the NRMA considered 20,000-30,000 miles to be average annual car use which eventually caused new car warranties to "blow out " to 24,000 miles or 12 months whichever came first .

It is also why it is near impossible to find good cranks down here unless the bike has been laid up for 20 years and then the cranks are generally excessively rusty.

[muskrat]

G'day fellas.
Well I must be the odd one out.
Maybe the shorter A7 rods are stronger.
I raced my 57 A7SS for 9 years (6 meetings X 6 races & practice per year) and only used 2 pair of rods (old used originals). Snapped cranks and lifted barrels a few times but never had a rod failure. That's running 14:1 on methanol and revving to 7500rpm (8K on occasions). 
Cheers