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. ***
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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.