Socket size for rear engine cover mount?

[fab5freddy]

Any one know the size of the deep socket required to tighten up the rear engine cover mounts at the back of the engine over the turbo?

My after market heat shield has developed a rattle and i need to nip them down   

[rich83]

12mm?

[fab5freddy]

It's bigger than that, i've been through all the deep sockets in my advanced halfords set and can't match it.

I went to halfords this morning and tried a 24mm, to no avail. That was a regular fit though, thinking i need a 24mm triple square if any one can confirm?

[rich83]

21?  Sure there's a 2 and a 1 in there 

[Beej]

Yeah, it's definitely either a 21mm or 24mm, but a multi hex rather than a 6 point

[fab5freddy]

Just for future reference, it's a 24mm deep 12 point socket