I have a 2004 XK8 purchased from my mechanic that had a coil conversion kit installed prior to his possession. I am unable to track down who did the conversion and when.
I have had retention studs breaking on three of the four struts. two in front, one in rear. Has anyone else had a similar problem?

Front passenger side

Front drivers side


Fourth broken stud from rear passenger side

 

I've had two sets of Arnott installed, and never seen this problem.

1. Is it possible they were grossly over-tightened (i.e., air wrench, breaker-bar used on socket)?

2. I assume you contacted Arnott directly - they are very good at responding, even if you don't know the original purchase order? If nothing else, Arnott can advise you on proper way to extract broken stud, and provide replacement studs and the proper torque values.

3. I'm going to rule out something below moving excessively (i.e., control arm) as you stated it is both front and rear, but it's still possible.

4. Try and take a high-res closeup of the broken stud end. That will help identify how it broke ( i.e., snapped due to torque, cracked due to excessive side-loading and/or movement).

 

A while back I had raised the front of my SV8 and it was sitting on jack stands with the hood up. I was working on the engine (not on suspension) and heard a THWACK then something hitting the floor. After a few minutes of head scratching I noticed that one of the studs on the top of the air shock had let go under the extra strain of the suspension hanging on it and banged into the hood on its way out! The nuts must have been horribly over-tightened by a previous "technician" -- most likely with an 3/4 inch impact wrench. (eye roll) I have since replaced the front shocks with new Bilsteins, taking the extra 5 minutes to torque them properly.

My advice to you would be to loosen the remaining nuts and re-torque them properly. They are never in tension while driving unless you're a stunt driver so as long as nothing rattles they should be ok.

Good Luck
Jeff

 

Seconding that position, but regard failure in bending from suspension 'tilt' even less likely, and loading in tension 'not at all'.
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My prime suspect would be drastically over-torqued studs.

Ex:

Suncore 'grey market' (unbranded), not Arnott, but .......I was interrupted whilst installing one of my first two at front. 'Seniour Moment' thing. Left all four studs on that one strut threaded only to nut depth before driving away as-was with it. Eg: With about 1/4" of free play, all four studs?

'Clunked' a skosh on poor pavement ~ 120 mile r/t. No other damage or distortion when I went back in to find the source of the 'clunking' and finished the job of tightening those foiur studs.

As already said - they are ordinarily under 'positive gravity'.