Camshaft failure - Stag

[Rod1883]

Someone I know from rally's in France has suffered a catastrophic failure of one of the OH camshafts on his Stag. Just wondered what those with far more knowledge than me think the cause of this might have been

 

 

 

Edited September 2, 2023 by Rod1883

[RobH]

Enlarged and enhanced break area - looks like a brittle fracture. Maybe just a bad casting ?   Roger will probably know. 

 

[RogerH]

Hi Rod,

not a very good  picture - Low Res and poor light.  However there are some clues.

At the 3-o-clock position there is a 1/2 round shadow - almost certainly the start of a slow moving crack.

Moving slightly left from this shadow there is a vertical line at an angle of 1-o-clock to 7-o-clock  This area from  the 1/2 round to the line was a more rapid growth.

The area to  the left of the line is a quite rapid failure.

You may notice some smudging in this area - probably where the two broken halves rubbed together.

Question - What cause  the initial crack

Without a quality pic and a great deal of expertise (that I do not have) then the answer is a mystery.

Roger

Edited September 2, 2023 by RogerH

[RogerH]

Hi Rod/Rob,

could well be a type of brittle fracture https://inspectioneering.com/tag/brittle+fracture

The vertical line to the right hand side may well have an associated crack on the outer radius of the cam behind the 1/2 circle shadow.

The 1/2 circle could be the stress raiser for brittle failure to become reality.

But what caused the stress raiser !!!

 

Roger 

[Rod1883]

Thanks for the comments Roger, Rob.

The car owner confirmed it was the original camshaft exhibiting little obvious wear when the engine was rebuilt 4 years ago by a specialist in Poole. All a mystery at the moment - the engine builder has the car and will be stripping the head/engine to check everything - if it were me I would want to find something that clearly was the root cause before rebuilding. Comments on the Stag Forum indicate this to be a very very unusual failure, if not unheard of. As a Stag owner I hope it's not an indication that original components are getting to an age where this type of failure might become more common?

[RogerH]

Hi Rod,

If what I think is a crack (3-o-clock position) then one needs to find out why it was cracked. The actual failure is of little interest.

Could something have happened to the cam during the rebuild 4 years  ago!  

If the cam was dropped onto a hard surface that may produce a weakness in that area - but without evidence it will remain a mystery.

 

Roger

Edited September 3, 2023 by RogerH

[Rod1883]

Thanks again Roger

I will pass the comments and observations on to the owner.

Rod

[Ian Vincent]

If there are no obvious causes, I’d be inclined to put it in the ‘s**t happens’ category and replace it with a known good quality item. If it’s the original cam and assuming it’s not having to deal with extra strong valve springs, there won’t have been any additional resistance to work against.

I know, not a very helpful comment. 

Rgds Ian

[RogerH]

That sounds about right  

 

Roger

[Waldi]

Worth checking if all valves/tappets can move freely. And are the timing chain and tensioner ok?

Waldi