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[Figureman52]

Planning to change both centre boxes and back box tomorrow. are there any special tips I should know about when doing it? I have heard that the front bolts can easily shear off.

[Andy B]

... I have heard that the front bolts can easily shear off.

For reasons known only to someone at GM the holes in the downpipes are threaded instead of clearance, so the bolts rust solid into them. You can try a proper penetrating oil ie Plusgas a few days before you do the job, but you'll either be drilling out the bolts & re-tapping the holes M8 (tapping size 6.8mm or 17/64") or drilling clearance holes in both. 

[albitz]

Wot Andy said. Its a bit of a pig of a job with the frontpipes/cats still on the car.The higher you can (safely) raise the car the better.

[tigers_gonads]

With all the bolts, give them a good coat of plusgas the day before 

Also hang your new system on its hangers and push together.
Refit the bolts BUT leave them a little loose until you had got everything in the right place without bits touching THEN nip all the bits and pieces up 

The amount of times I have changed one piece on my exhaust and nipped it all up only to find something has moved a little and it starts banging and rattleing 

[tidla]

if you have access to oxy acet heat then your sorted.. or get a garage to do that part.(heat nuts, bang on a socket that fits and remove)
save your self a load of hassle.

[Figureman52]

Thanks for the info, I'll give it a go and see what happens.

[Figureman52]

Job done. Gave the bolts a good spray with penetrating oil and got 'em all out. Thanks again for your advice.