The rules for reinforcing plates are covered in NCOP. There is also some additional information in VSB5B (the National Code of Practice Standards Bulleting 5B Construction and Installation of Additional Seats by Individuals). The NCOP guidance on reinforcing plates requires:
• the plates to be made of mild steel,
• the surface area of the plates must not be less than 3,750mm2 and have a thickness of 3-4.5mm (1/8-3/16”). VSB5B allows not less than 75mm x 50mm x 3mm (3750mm2), but requires the plates to be no thicker than 4mm,
• the plates must be contoured to fit the vehicle body,
• the plates must have a minimum 5mm corner radius with the plate edges closest to the body chamfered.
• the bolts used must be 7/16” UNF SAE Grade 8.
The engineers requirement for 100x150 give you 15,000mm2... that is waaaaay larger than the 3,750mm2 required by either NCOP or the VSB. In reality, it is not the dimensions that matter, more the surface area (more area = less pressure on the pillar - less likely to pull through the pillar). To get that surface area in the B-pillar, you will need a long, thin plate with a nut in the middle. I used these for the FB, but only to a 3,750mm2 surface area.
Cheers,
Harv
