V12 Head Stud Failure

Talk about the E-Type Series 3
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Whitact
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#1 V12 Head Stud Failure

Post by Whitact » Thu Sep 28, 2017 12:30 pm

Had a single head stud on the OTS fail while on the autobahn in Germany. A very clear fatigue failure just above the level of the washer. What is surprising is just how far across the stud the crack had propagated before the final failure. Without the acorn nut the coolant was being lost at a surprisingly rapid rate!
The head gasket seems intact with no water in the oil or loss of compression.
Perhaps not surprisingly the stud could not be removed with an extractor so the head will now be coming off.
Has anyone else experienced this? Are there a bunch of poor quality repro studs out there? This engine was rebuilt with some new studs about four years ago. The dilemma now is whether to replace all 52 or just the one that failed.
Cheers,
Adrian Turner
S3 OTS & FHC
S1 FHC
XK140 FHC

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christopher storey
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#2 Re: V12 Head Stud Failure

Post by christopher storey » Thu Sep 28, 2017 1:23 pm

How very strange. Do you know what type of failure it is ? I would presume with a head stud that the most likely failure would be a tension failure, which characteristically produces ( I believe ) a helical shaped break . ? Overtorquing ? Or could it be a fatigue fracture from temperature cycling - which to a layman like me sounds very unlikely. If it is a fatigue fracture you will see "beaching" which is like ripples in the sand, where the fracture progresses a minute amount further with each fatigue cycle . I think you need to identify the origin of the failure before deciding whether to replace them all

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Whitact
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#3 Re: V12 Head Stud Failure

Post by Whitact » Fri Sep 29, 2017 7:15 am

Hi Chris
Its a very clear tension fatigue failure. The initial defect is clearly visible as a small arc and then the bands of progressive failure run on from there in expanding arcs across nearly two thirds of the cross section. The area of the final yield failure is surprisingly small, less than a third of the total cross sectional area of the stud. The initial defect is within the length of the threaded section which makes me think that this was a manufacturing defect rather than anything else.
I'm hoping to get the heads off over the next few days to get a better look at the rest of the studs.
Cheers
Adrian Turner
S3 OTS & FHC
S1 FHC
XK140 FHC

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christopher storey
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#4 Re: V12 Head Stud Failure

Post by christopher storey » Fri Sep 29, 2017 8:12 am

Yes, that sounds like a fatigue fracture . I would think as a result that it is likely to be confined to that one stud rather than endemic

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PeterCrespin
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#5 Re: V12 Head Stud Failure

Post by PeterCrespin » Tue Oct 03, 2017 7:58 am

Perhaps check for an acorn/stud mismatch? Short early acorn on fractionally long or incompletely inserted stud or thin washer could lead to bottoming-out and a torsional failure mode?
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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