Stainless is brittle and I would not use stainless bolts in a safety role, standard bolts are stamped 8.8 end higher spec allen/cap screws could be used.

I have used plated domed cap screws in the top plates in 10 mm,over engineerd for that bit extra as Mark say's.

You,ll be ok Gaz , as Mark says, those 8 bolts holding the roll hoop to the chassis will be as strong if not stronger that the welds around the suspension boxes ,
When I was fitting my rear back stays there was a discussion about how many bolts should be used to secure them to the chassis....some said 1, some said 2.....I,ve used 4 on each side so in effect I have 16 8.8 strength bolts holding my roll bar/back stays to the chassis....should be enough :eek: :D

cheers
andy

Bolt strengths.
 

I read somewhere that all vehicle construction bolts should be a min 8.8 sierra use 10.9 holding the front brake callipers on as std. Ford would not spend money on 10.9 bolts if 8.8 would be ok.
The majority of stainless bolts sold will be A2 unless specified as A4. Screwfix do list A4 as an option although i suspect that most IVA inspectors would not accept A4 in stressed areas.

Stainless A2-70 = 700N/mm2 Tensile Strength 450 N/mm2 Yield Strength
Stainless A4-80 = 800N/mm2 Tensile Strength 600 N/mm2 Yield Strength
Normal Steel 8.8 = 800N/mm2 Tensile Strength 640 N/mm2 Yield Strength



Hope that Helps.

Try here for 10.9 high tensile fixings ;)

Shop4Fasteners

Have a good poke arround in their store, they have a mountain of fixings available :cool:

Not the cheapest on the net but by far the biggest range of fixings ;)

So I'm still a bit unclear about the length's I should use, I know I need a bit of thread visible after it's screwed in but I'd to go and buy ones that are too long or too short. Can anyone tell me what length's they used?

3mm plate x2 + height of the nut ~8mm + 1mm washer = 15mm so a 20mm bolt would be fine. I think 16mm would be too short.

/Cool, same again for the stays at the back?

The Ford upgrade I would think is because of the potential shear forces involved in a stationary object trying to stop a rotating mass and as stated Ford must have got it right.

Over engineered won't hurt........

Hmm been digging on those two sites and can't see the 10.9 or 12.9 nuts only 8.8 nuts? Will it be ok to use 8.8 nuts with 12.9 bolts?

Bolt grade is more important,could be wrong but nut does not need marking for IVA and as stated before you dont need a roll bar.