Sheared off rod bolts...bike

[Truckedup]

Not my bike, but a friend runing in the same land speed racing class had a rod punch through the crankase at 130 plus mph...Like mine, a vintage Triumph 650 running the best parts inside..The blow up gave no warning, just boom...
The connecting rods are H beam forged steel with a repuatation for absolute reliability...The rod bolts are ARP 3/8 inch threading into the rod...No signs of a spun bearing or seizure...A 3/8 bolt for a 20 cubic inch cylinder at 7500 rpm seems more than adequate..
He recently had the engine disassembled and put it back together using the same rods and bolts He uses a bolt stretch guage rather thn a torque wrench.
He is beating himself up because he should have replaced the rod bolts....He is experienced with these engines and currently holds the class speed record..The rods and bolts had about 75 runs down a 1-1/2 mile standing start track
Yes many variables here, but in general...Do you think reusing rod bolts caused this failure?

[BillK]

Considering where they broke I would think the rod was "stretching" for lack of a better word and stressing the bolt at the parting line. Is there anything left of the bearing that you can look at ?

On the other that is a lot of runs on that length track. Probably the same as 500 1/4 mile passes ? You said he is experienced with the engines, how have they lasted in the past ?

On another note, man that is a long rod ! I had a 650 Bonneville but never had it apart to look at the internals.

[Truckedup]

To the best of my knowlege, there have been no failures of the type of connecting rod even in vintage road racing...The rod is 6.5 inches long, the stroke is 3.23... These are the rods being used most of us racing the old turds..

[digger]

It's hard to tell but If it's a fatigue (reuse) issue there would be striations present on the fractured face. Maybe some better pics would help

[BillK]

He uses a bolt stretch guage

Considering this failure and the fact that he has a stretch gauge it would probably be a good idea to start recording the length of each bolt before assembly. That way when they disassemble it to freshen it up they can compare the bolts to their original length. I think ARP has something on their website as far as what is acceptable stretch after running. That way you might have an idea if something is starting to go wrong.

Then again for the price of 4 bolts in an engine it might just make sense to replace them every time.

[Truckedup]

Best photo of the sheared bolts....He will take a better photo...

[mag2555]

In any racing motor you do not want to torque up the rod bolt more then 4 times without replacing them and checking the Bore and of course using the stretch method.

[Truckedup]

In any racing motor you do not want to torque up the rod bolt more then 4 times without replacing them and checking the Bore and of course using the stretch method.

My engine ,very similar to the blown up one of my friend, uses R&R aluminum rods...It has about 35 runs down the track and maybe 20 pulls on the dyno for tuning....I used only a torque wrench to check the big ends and bearing fit... and for assembly... The engine has been torn down for inspection twice.....the bike is retired for the foreseeble future..
With these old Triumphs, the crank is almost always the first to go....But he has a custom billet crank that probably has a longer lifespan ..

[Zmechanic]

Did he keep track of the unstretched length? That's usually an easy first line of defense of catching fatigue. If you notice the unstretched length changing, something is yielding. It's not a foolproof method of course, just one more tool in the toolbox.

What rod bolt material? 3/8" ARP2000 rod bolts will hold on a 305 sprint car that spends a lot of time 6800-7200rpm for at least a season. If they are in good shape, I'd be surprised if they weren't strong enough to do the job. We had a set on their 3rd retorque (by me) and sold the motor and car after a full season. AFAIK it's still going strong.

[Truckedup]

Sorry I still don't understand the fascination with the heavier H beam con rods. Bolt fatigue or what ever for high revs maybe a lighter rod would be nice.
And for highly stressed bolts in a performance application where it is cheaper to replace bolts than the whole engine, I would not reuse them, and the replacements should be checked for flaws. Even top known name brand doesn't always mean perfect.
https://www.onallcylinders.com/2012/08/ ... vs-i-beam/

These things have limited choice of parts...On my scale, the MAP H beam steel rod weighs 415 grams.The R&R aluminum rod weigh about the same...The stock aluminum rods weigh about 400 grams.. The stock rods were adequate in their day but running 50 year old aluminum rods with unknow cycles is very risky..
Car and modern bike tuners wonder why we fuss with old tempermental engines....There's some sort of wierd satisfaction of getting 1.7HP per cubic out of 1930's technology with the engine on the limit of reliability ...

[Racer71]

I used to build a lot of the cb750 Honda motors for TQ midgets and we turned them much higher than that, I had a few failures. We’re they the arp 2000 bolts or 625+? I had several sets of the 2000’s break after the second time being torqued/stretched. I never after those three motors used arp again, went to Carrillo bolts exclusively unless customer wanted to run the arp. I wasn’t the only builder that had this issue with them in a two year span.

[Truckedup]

I used to build a lot of the cb750 Honda motors for TQ midgets and we turned them much higher than that, I had a few failures. We’re they the arp 2000 bolts or 625+? I had several sets of the 2000’s break after the second time being torqued/stretched. I never after those three motors used arp again, went to Carrillo bolts exclusively unless customer wanted to run the arp. I wasn’t the only builder that had this issue with them in a two year span.

They are ARP2000....I assume the rod bolts are general purpose and not special design...So maybe he should find other bolts and have the new rod resized?

[Racer71]

We went to the sps Carr bolts that Carrillo sold in their Honda rods what’s the length on those bolts? I have piles of rod bolts from the Honda motors. I would use the Carr bolts three times just as a precaution. But know guys who used the Carr bolts for 10+ years, but stuffs to expensive to chance even though dropping that much on rod bolts all the time was a bit exspensive.

[Geoff2]

That piston has a pretty hefty dome on it, more weight. That, plus too much rpm, has probably caused the rods to elongate/flex at the parting line...... & flexed the bolts one too many times.

[MadBill]

A few up-close shots of the fracture surfaces could end a lot of speculation...