These are the two ways to change how a seat fits after you have bought it. A cushion changes your height in the shell. An insert kit moulds the shell to your body. They solve different problems.
Which one do you need
If the seat fits but you sit too low or too high, you want a cushion. If the seat is the right size but you move around in it, or it is a size too big and you want it to hold you properly, you want an insert kit.
Seat cushions
Sold as low, medium and high, and the name is the cushion thickness rather than anything about the seat. It is the cheapest way to fix a driving position that is nearly right.
Check headroom before you go upA high cushion lifts you toward the roof, and in a car with a cage and a helmet that is exactly where you have least room. If you are close, measure before you order. Going the other way, a low cushion is the first thing to try if your helmet is touching.
Most of our cushions are made for a specific seat, currently the Mirco Classic, GT, RS1, RS2, RS7, RTS-2 and RTX-7. Check the compatibility on the product, because a cushion cut for a different shell will not sit right.
Seat insert kits
An insert kit is expanding foam and a bag. You sit in the seat, the foam expands around you, and it sets to the shape of your body. It is what single seater and rally teams use to make one shell fit one driver properly, and it is the answer when a seat is a size too big.
The litre figure is the volume of foam in the kit, not a measurement of you or of the seat. More litres fills a bigger gap. If you are between two, the larger kit gives you room to get it right, and you can trim the result.
You get one attemptThe foam sets once. Read the instructions properly, have someone to help you, and be dressed exactly as you will drive, in your suit, with your helmet and your head restraint on and the harness done up. Any layer you leave off is a gap you will feel every time you get in afterwards.
FIA 3795:1992
Six of our insert kits carry FIA 3795:1992, which is the standard for the foam itself. If you are running in a championship that requires an approved insert, check for it on the product. The rest are for road, track day and club use where no approval is required.
A note on sizing up and padding out
If you are between two seat sizes, our advice on the bucket seat guide is to take the wider one. An insert kit is what makes that advice work. A seat that grips your hips cannot be made bigger, but a seat that is slightly loose can be made to fit you exactly, and it will be more comfortable than either the smaller seat or the loose one.
Not sure which fixes your problem?Tell us the seat, what is wrong with how it fits, and your height. Sitting too low, sliding about and being pinched at the hips all have different answers and it is worth getting the right one. Call 0115 989 3488 or email Sales@GSMPerformance.co.uk.
All figures read from our own product data across the insert kit and cushion categories, checked 2 August 2026.
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