Broken camshaft

[newtothis]

Whilst riding home the other day the engine dropped onto three cylinders, a look under the cam cover showed why.


Fortunately the ends were just lying on the top of the head. The shims however have disappeared into the bowels of the engine.




Having had a look inside the cylinder with an endoscope there appears to be no damage to the piston! Not even a mark where it must have slammed the valves shut.

Does anyone have a rear inlet cam and/or a pair of rockers? Please!

[scotjell]

I've got a complete rear head if you want it £50 posted

[Morespeedvicar]

No way, I've never seen that before! Hope you find the shims!

[Neosophist]

Hmm.. lack of oil? Maybe camshaft was damaged on reinstallation after valve clearance check at sometime and took time to break?

[Tim400GB]

Id say badly re-installed cam too.
Turned around just in time to watch someone break a cam tightening it down unevenly in the past

You had a look in the sump for the shims yet?

[Neosophist]

Id say badly re-installed cam too.
Turned around just in time to watch someone break a cam tightening it down unevenly in the past

You had a look in the sump for the shims yet?

I've broken one myself before but not on a v4

[newtothis]

There's no sign of any oil feed issue, the bearing surfaces are perfect and everything turns smoothly...just not together!

As a quick damage assessment I had a peer in the cylinder with an endoscope and it looked surprisingly OK.

One shim was lurking under the exhaust rockers, the other remains AWOL

I pulled the head off tonight and am surprised to say the least.


the piston as uncovered, not even wiped clean. No sign of any contact.


not even a clean patch on the valves


after a quick wipe with carb cleaner


So somehow there's not even a witness mark. There are two tiny marks on the valve cut out bosses but they are different on each whereas I'd expect forcibly closing the valves to leave the same marks


The broken ends of the camshaft show that the two parts have turned against one another. The dark areas are the fracture surface and the polished areas show some circular wear marks. The two bits still fit snugly together so there isn't all that much material worn away.



So I somehow appear to have got away with no damage other than the cam itself and a couple of rockers. Considering how bad it could have been I think I need to buy providence a pint !

[newtothis]

I should also have said that prior to pulling the head off I connected an air line into the cylinder using an old tyre valve to see if it would hold pressure. i couldn't hear any leakage through the inlet port so the valves still seal on their seats!

[Tim400GB]

Cant see any signs of creep. Would expect to see some from an engine that has run and subsequently failed.

Dodged a bullet with the valves/pistons there didn't you, the rockers breaking off did you a favour.

[newtothis]

I really can't believe that it got off so lightly, having had race car engines put legs out of bed, drop valve heads, melt pistons etc I'm used to catastrophic failures!

Although the photos don't show the surfaces very well there's no sign of beach marks so it looks like a one-off break, the fairly flat crack line doesn't look like a torsion failure. As you said earlier it would be most likely to break if / when incorerctly tightened. If that implies that it was running broken but just didn't slip then I wonder how long it had been like it?