Customer Question: What Size Bolts Do Bucket Seats and Brackets Require?

At GSM Performance, we often get asked by customers, “What size bolts do bucket seats and brackets require?” It’s a great question – after all, if you’re upgrading to aftermarket bucket seats, you’ll need the right hardware to bolt everything together safely. The simple answer is that almost all aftermarket fixed-back bucket seats, reclining sports seats, and their related mounting equipment use M8 bolts (8mm metric). The one exception is Corbeau’s side-mounted fixed-back bucket seats (their racing bucket models), which use M10 bolts for mounting those seats to side brackets. (Note: This exception does not include Corbeau’s reclining sport seats – those still use standard M8 bolts like every other seat.)

So why does this matter? Using the correct bolt size ensures your seat is securely mounted – critical for both safety and comfort. In this blog, we’ll break down which seats and brands use what bolt size, and go over the different connections in a typical seat install: between the seat and side mounts or base, between sliders and brackets, and how everything bolts to your vehicle’s floor. Let’s dive in!

Standard Bolt Size for Aftermarket Seats: M8 (The Norm)

For the vast majority of aftermarket seats and brackets, the bolt size you’ll be working with is M8. This means the bolts have an 8 mm diameter (with a typical thread pitch of 1.25 for M8). Whether it’s a fixed bucket seat or a reclining sport seat, almost all brands design their mounting holes to fit M8 bolts.

Popular seat brands like Sparco, OMP, Recaro, Bride, Cobra, and even Corbeau’s reclining seats, all use M8 bolts for their mounting points. This applies across the board to most components of the seat installation:

Seat to Mounting Bracket: If you have a bottom-mounted (base-mount) seat – common for reclining seats – the four bolts that attach the seat base to either a slider or a fixed bracket are M8. Likewise, if you have a side-mounted bucket seat (fixed-back), most seats use four M8 bolts (two on each side) to attach to the side brackets.
Seat Sliders: Sliders (adjustable rails) typically have holes that line up with seat or bracket mounting holes, and these are drilled for M8 hardware as well. The bolts fastening a slider to the seat or to a base frame are usually M8.
Side Mounts to Base Frame: For side-mount setups without sliders (or with sliders in between), the side mount brackets attach to your car-specific base frame or bracket using M8 bolts in almost all cases.

Why M8? M8 bolts have become something of an industry standard for seat mounting. They offer a good balance of strength and size, and most seat manufacturers have standardized around them so that their seats and mounting accessories are compatible with each other. If you purchase a set of universal side mounts or sliders from a brand like Sparco or OMP, they often come with M8 bolt kits (or you can purchase an M8 hardware kit easily) because that’s the expected size.

In short, if you’re fitting any typical aftermarket bucket or sport seat, M8 bolts are what you’ll use 99% of the time for connecting seats, sliders, and brackets together. Now, let’s talk about that other 1% – the special case.

M8 Bolt Packs – Standard or Standout Options

To make seat installation even easier, we offer a range of M8 bolt packs to suit your setup and style. Whether you’re after a no-fuss black bolt kit for a clean and OEM-like install or you want to add some flair with custom titanium bolts, we’ve got you covered.

Endurance Motorsport bucket seat M8 bolt pack
£4.14 Incl. VAT£3.45 Excl. VAT Add to basket
Endurance Motorsport Bucket Seat Titanium M8 Nut Pack
From £9.60 Incl. VATFrom £8.00 Excl. VAT Select options This product has multiple variants. The options may be chosen on the product page
Endurance Motorsport Bucket Seat Titanium Bolt Pack
From £23.98 Incl. VATFrom £19.98 Excl. VAT Select options This product has multiple variants. The options may be chosen on the product page

The Exception – Corbeau Side-Mounted Seats Use M10 Bolts

If you’re using Corbeau fixed-back bucket seats with side mounts, you’ve discovered the one major exception to the M8 rule. Corbeau’s side-mounted racing seats (their fixed-back bucket seat range) require M10 bolts for mounting. This means the threaded holes in the side of those Corbeau seats are sized for 10 mm diameter bolts, and their matching side mount brackets are drilled for M10 hardware as well.

Why did Corbeau do this? In recent years, Corbeau updated some of their FIA-approved racing seats to use larger M10 fixings, likely to meet stricter safety standards or to add extra strength. These models (for example, the Corbeau Pro-Series or Revenge seats) have bolt holes that are bigger than the norm. So, if you have a Corbeau FIA bucket seat, you’ll be using four M10 bolts (instead of M8) to attach the seat to the side mounts, and you’ll also use M10 bolts to attach those side mount brackets to the sliders or base frame.

It’s important to note that this M10 bolt size ONLY applies to Corbeau’s side-mount, fixed-back bucket seats and their side brackets. Corbeau’s reclining sports seats (and older non-FIA models) still use standard M8 bolts for mounting like everyone else. So, don’t worry – if you bought a Corbeau sport recliner for your street car, you’re not hunting for special bolts; it will use M8s.

What if I mix and match components? If you are using Corbeau side mounts with a Corbeau seat, stick with M10 bolts as specified. But if you try to fit a Corbeau seat on non-Corbeau brackets or vice versa, be aware of the bolt size difference. For example, Corbeau side mounts have larger holes for M10 bolts; if you were to use them with a seat that has M8 threads (or on a slider drilled for M8), you’d need to use reducing washers or different hardware to make it work safely – not an ideal scenario. Generally, it’s best to pair Corbeau seats with their intended Corbeau side mounts so everything matches up. If you have any doubts, ask us – we can guide you to the right combination or hardware.

Outside of Corbeau’s specific case, no other major seat brand uses M10 bolts for their seat mounts as of now. So this really is a unique exception. Always double-check the instructions that come with your seat or mounts – if it’s Corbeau, it will usually highlight that M10 bolts are needed (since that’s unusual). If you’re unsure, feel free to reach out to our team for clarification.

Side-Mounted vs. Bottom-Mounted Seats (Bolt Interfaces Explained)

It might help to clarify how seats are mounted because terms like “side mount” or “base mount” determine where the bolts go:

Fixed-Back Bucket Seats (Side-Mounted): These seats (common in motorsport) have mounting holes on their sides. You use side mount brackets that bolt to the seat’s sides (usually two bolts per side). Then those side brackets bolt down to either an adapter frame or to sliders (if you want the seat to slide). In a side-mount setup, the critical bolt connections are:

Seat to Side Mount: Four bolts (M8 for most seats, M10 if it’s a Corbeau racing seat).
Side Mount to Slider/Base: Usually another set of four bolts (two per side bracket) which are M8 in most cases. (Again, with Corbeau side mounts, these would be M10 as well, since the bracket holes are larger.)

Reclining Sports Seats (Bottom-Mounted): Recliners and many aftermarket “sport seats” use bottom mounting. The seat has four threaded holes on the bottom. These seats often come with universal sliders, or you attach them to a base bracket or sliders using bolts that go up through the bracket into the seat base. The bolt connections here are:

Seat base to Slider or Bracket: Four bolts (M8 for essentially all brands, including Corbeau recliners). The bolts typically thread into the seat’s bottom.
If sliders are used, Slider to Seat and Slider to Bracket: four bolts for each interface, all M8. Many reclining seats come with sliders that align to standard hole spacing and include the required M8 bolts.

In summary, side-mounted bucket seats use bolts on the sides, and bottom-mounted (reclining) seats use bolts underneath – but in both cases, M8 is the standard size for those bolts (with the one side-mount M10 exception we covered). No matter the mounting style, if you have a non-Corbeau seat or bracket, you’ll be grabbing M8 hardware to put it together.

Mounting to the Vehicle Floor – Use Your OEM Bolts

One area people sometimes overlook is how the seat bracket or frame attaches to the car’s floor. When you remove your factory seats, you’ll typically unbolt them from the floor pan using the original seat bolts. These are often larger, hardened bolts (for example, many OEM seat bolts are M10 or M12 with specific thread pitches) that are engineered for your car’s anchor points.

For installing your aftermarket seat setup, you will usually reuse those original car seat bolts to secure the new seat base or mounting frame to the vehicle’s floor. The aftermarket seat bracket (often called a subframe or vehicle-specific base) is designed to line up with the factory mounting holes in your floor, so you can bolt it down using the same bolts that held your stock seat. This is important because:

The original bolts are the correct size and thread to match your car’s welded nuts or threaded inserts in the floor.
They are high-strength bolts meant to withstand crash forces – always use grade 8.8 or higher bolts for seat mounting if you ever replace them, but generally the OEM ones are best.

Do not attempt to use random M8 or M10 bolts to bolt the frame to the floor – those may not fit your car’s threaded holes and would likely be too small in diameter anyway. Your car’s floor mounting points are specific, and the safest practice is to use the manufacturer’s hardware (or hardware supplied with a vehicle-specific bracket kit, if any). In short, the interface between the seat bracket and your car uses the car’s original bolts, not the generic seat bolts that come with your aftermarket parts.

A quick checklist for floor mounting:

Save the bolts you removed from your factory seats – you’ll need them to install the new brackets.
If your vehicle uses studs or a different method, follow the bracket manufacturer’s instructions (some kits provide new nuts if the car has studs, etc.).
Always tighten these floor bolts securely (refer to your vehicle’s manual for specs if needed – they must be good and tight since they hold the seat in during impacts).

By using the OEM floor bolts, you ensure the aftermarket seat is anchored as solidly as the original seat was.

Summary: Bolt Sizes for Seats, Mounts, and Brackets

To make things easy, here’s a quick summary chart of the typical bolt sizes used for various seat mounting components and brands:

Seat / Mounting Component	Bolt Size Required
Fixed-Back Bucket Seat (Side Mount) – Most brands (Sparco, OMP, Cobra, Recaro, etc.)	M8 bolts (standard)
Fixed-Back Bucket Seat (Side Mount) – Corbeau only (racing/FIA models)	M10 bolts (exception)
Reclining Sport Seat (Bottom Mount) – All brands (including Corbeau recliners)	M8 bolts (standard)
Side Mount Brackets – Universal or non-Corbeau	M8 bolts for attachment
Side Mount Brackets – Corbeau-specific side mounts	M10 bolts for attachment
Seat Sliders (if used, between seat and base frame)	M8 bolts for all connections
Base Frame / Seat Subframe to Car Floor	Car’s original seat bolts (OEM hardware)

As you can see, M8 bolts are the go-to for nearly every part of an aftermarket seat install, except when dealing with Corbeau’s unique side-mount setup which calls for M10. And regardless of the seat brand, the final connection to your vehicle’s floor uses the stock bolts that came with your car.

We hope this clears up the confusion! The good news is you usually won’t need an assortment of odd bolts – a set of quality M8 bolts will do the job for most seat installs, and if you’re working with a Corbeau race seat, just make sure to get the correct M10 hardware for it. Always use high-tensile steel bolts (typically supplied or available from us) for safety. If you’re ever unsure about what hardware you need for your specific seat or car, just ask us at GSM Performance – we’re happy to help ensure you have the right parts for a secure installation. Happy motoring with your new seat setup!

Final Thoughts – Getting the Basics Right

When fitting aftermarket bucket seats or recliners, the bolt sizes you need are simple to remember. Most seats and mounting gear use M8 bolts, which are the standard across brands like Sparco, OMP, and Cobra. The only exception is Corbeau side-mounted bucket seats, which use M10 bolts for their side mounts.

For securing the subframe to your car, just reuse your factory bolts – they’re made to fit your vehicle safely. Getting the right bolts means an easier installation and a safer setup. If you’re ever unsure, our team at GSM Performance is always happy to help.

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