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Nearly...
Mr Whippy - 18/8/14 at 06:55 AM

I took the rocker cover off to do a quick cylinder head swap and got a nasty shock... thought I'd have to do a whole engine swap instead which is a pest if you need the car the next day to go to work! but managed to recover the broken bit and there's no other damage so very lucky indeed.

Can you spot the issue?




[Edited on 18/8/14 by Mr Whippy]

mookaloid - 18/8/14 at 07:20 AM

Is that a broken cylinder head bolt sticking up between the exhaust valves there?

Mr Whippy - 18/8/14 at 07:31 AM

yip not sure why it went and fortunately the gasket didn't blow which is amazing, never seen this happen before myself

There was a really small bit of the threaded section sticking out the block when I took the head off and it just spun out no bother so very luckly the block is fine

loggyboy - 18/8/14 at 07:37 AM

8 valves are missing?

Mr Whippy - 18/8/14 at 07:38 AM

quote:
Originally posted by loggyboy
8 valves are missing?


I wish

jollygreengiant - 18/8/14 at 09:49 AM

quote:
Originally posted by Mr Whippy
quote:
Originally posted by loggyboy
8 valves are missing?


I wish


It's alright, you've got the regulation 16 valves, just not all in the same place, 8 on the engine and 8 on the work bench.

Grimsdale - 18/8/14 at 09:57 AM

have you got a photo of the fracture surfaces? i bet it's a fatigue failure due to undertightening / loss of bolt tension.

liam.mccaffrey - 18/8/14 at 10:46 AM

Might be being a numpty but how would under tightening cause fatigue failure?


quote:
Originally posted by Grimsdale
have you got a photo of the fracture surfaces? i bet it's a fatigue failure due to undertightening / loss of bolt tension.

wilkingj - 18/8/14 at 10:50 AM

quote:
Originally posted by Mr Whippy
quote:
Originally posted by loggyboy
8 valves are missing?


I wish


I got 16 valves in my old car... 8 in the engine, and 8 in the radio!


[Edited on 18/8/2014 by wilkingj]

Mr Whippy - 18/8/14 at 11:04 AM

quote:
Originally posted by Grimsdale
have you got a photo of the fracture surfaces? i bet it's a fatigue failure due to undertightening / loss of bolt tension.


I'll take that when I get home, looks a very sharp jagged crack. The bolts have a 14 on the top so may be grade 14?

[Edited on 18/8/14 by Mr Whippy]

coyoteboy - 18/8/14 at 12:47 PM

quote:
Originally posted by liam.mccaffrey
Might be being a numpty but how would under tightening cause fatigue failure?

Combination of factors including, but not limited to:

1) Unloading of clamping force means shear loads are carried by the bolt, not the clamping surfaces.
2) A loose bolt can be subjected to bigger stresses as the materials around can deform more meaning loading isn't axial
3) Stresses about zero seem to be harder on parts than stresses about a raised mean, it's to do with fracture propagation.

[Edited on 18/8/14 by coyoteboy]

Grimsdale - 18/8/14 at 04:07 PM

quote:
Originally posted by liam.mccaffrey
Might be being a numpty but how would under tightening cause fatigue failure?


quote:
Originally posted by Grimsdale
have you got a photo of the fracture surfaces? i bet it's a fatigue failure due to undertightening / loss of bolt tension.


In a correctly tightened ideal situation, the bolt clamping force is higher than the force experienced by the joint as a whole, so the bolt never sees a cyclic load, and a fatigue fracture cannot occur.

In an under tightened situation, the load on the joint can be greater than the clamping load exerted by the bolt, so the bolt experiences cyclic loading which, if above the fatigue limit, will allow fatigue failure.

Mr Whippy - 19/8/14 at 06:03 AM

Hi

Here's a picture of the broken bolt, quite a sharp break and it almost fits back together perfectly. Not sure why it broke though to me it looks like it's been twisting at the time so maybe over torqued?

Cheers

Grimsdale - 19/8/14 at 08:42 AM

Yeah, that's fatigue.

The fracture surface is split into two main regions - one planar region and one angular distorted region.
On the planar region you can see circumferential rings, particularly near the outer edge. These are known as beachmarks and are generally indicative of a fatigue crack progressing from the outer surface inwards. As the crack progresses, the effective cross sectional area of the bolt reduces to a point where it cannot withstand the tensile load, and the remainder of the bolt fails by tensile overload.

Undertightening is not the sole cause of fatigue failure of bolts, but it is common. Other factors such as inadequate stiffness of the joint as a whole, loss of tensile loading (head warping, gasket failure etc).

Mr Whippy - 19/8/14 at 11:15 AM

that's interesting thanks

The Black Flash - 19/8/14 at 10:36 PM

Seconded - I find this stuff fascinating.

haz87 - 20/8/14 at 08:17 AM

Very informative