I'm going to bolt my seats through the floor and the two box sections that run the length of the cockpit floor either side of my Avon.
Do the bolts have to be high tensile (i.e. grade 8.8 or above) or can I use ordinary roofing bolts which have a nice rounded head that won't dig into the underside of the seat squabs?
The seats are made by Intatrim and hence have a fibreglass base. I will also put penny washers under both the heads and nyloc nuts to spread the load.

Andy.

I would suggest bolts should always high tensile really.

Rich.

Mine are 10. something in strength and are domed heads. I went to my local go kart store and got them

I'd go for socket button head screws. If going stainless they aren't as strong as plated (which are 10.9 usually), but A2 should be OK for a seat (IMO).

Not expensive for the piece of mind.

re: 8.8 or wehatever - in this case, does it matter? Surely the fibreglass will tear before the bolts snap? (ofcourse depends o nthe bolt diameters - I used 8 mm so this is what I refer to).

Stainless button head and coach bolts since the s. b. h. were not available in the same length, same seats as yours.

Drill the holes in the floor and use a piece of 20 mm square tube from B&Q as spacer, fit the bolts, then paint the ends of the bolts so they mark the seat bases when seats in place. Drill the seats. Clear away a bit of the foam and use penny washers on the seats.

I failed my sva for not having some high tensile blts in different places so my advice would be to use high tensile and nyloc nuts (or double bolt with loctite) everywhere you can

I use HT whenever possible but can't personally see the benefit of using them when attaching the seat to the floor. Your mass is retained by the straps not the seat. In the event of a smash, the seat will only be subject to deceleratory force due to it's own mass. The seat (not to mention the rest of the car) would be toast long before M8.8 bolts sheared!! I used standard 8.8s (I used B+Q's best domed-with-a-cross-on-them roofing bolts) and no complaints @ SVA... They did complain I'd attached them to an ali floor without any repair washers though!!! Fair enough!!!

Yep. Totally agree with Ben B it is the belts that retain the driver in an accident not the seat .

However if the seat is not firm up against the rear firewall and the chassis it will move about on track which can be but annoying and dangerous. 8.8 should be fine with something to spread the load eg Large washers / spreadplates.

- Jim