Fitting Aftermarket Seat Frames: Bolt Point Tolerances, Side Mounts And Sliders Explained
Seat frames, side mounts and sliders, in the order they bolt up
A customer rings to say the frame has arrived, the seat is on the bench and one of the four bolt holes sits about 6mm out from where it should be. Cameron has taken that call more times than either of us can count, and he opens with the same line every time: that is normal, and here is why. Garreth takes the other half of the call, which is the order of parts that stops it happening twice.
This page is the whole conversation written down. It covers the plus or minus 10mm tolerance on frames, side mounts and seats, how to install from what each supplier publishes, and the one part that takes most of the pain away. Read it top to bottom and the frame, the seat and the slider should arrive as parts that bolt together.
- OMP, Sparco and various other brands work to a plus or minus 10mm tolerance on the bolt points of their sub frames, side mounts and seats
- Seat sub frames are base mount, so a side mount seat needs side mounts, or adaption bars over the frame
- A seat slider removes the need to modify anything, because its base holes match the frame and its top holes match the side mount
- A standard seat belt clip can be used with a motorsport seat, with a few ways to deal with the bracket
What plus or minus 10mm means on your bolt points
OMP, Sparco and various other brands build their sub frames, side mounts and seats to a tolerance of plus or minus 10mm on each bolt point. Put two parts together that both sit at the edge of that range and the holes can land up to 20mm apart, which is why a frame can look perfect in the photo and still need persuading on the day. That is the reason a measured fit beats a catalogue photo.
In practice it means that one or more bolt points may need extending for a clean fit. Cameron says that is normal, and for someone who fits seats every week it adds about five minutes. The OMP Renault Clio Mk3 frame and the BMW E46 frame are the usual examples: two bolt points need extending by 5mm in about 75 per cent of installations.
The side mount seats have the same tolerance on the holes in the flank of the shell. The distance from the rear bolt point to the front one is 290mm, with the exception of some Racetech seats. Any levelling of the seat is done with the mounting equipment or the side mounts.
Base mount tolerance, drawn out
A base mount seat meets the frame or slider on a drilled rectangle. Sparco and OMP both use 345mm across by 271mm front to back, which is why a Sparco seat usually drops onto an OMP frame and the other way round. Each of those four holes carries the same plus or minus 10mm, so the pattern you measure on the bench can differ from the one drawn in a catalogue.
One base mount pattern, with the tolerance at every bolt point
The dashed rings only show the size of the tolerance, and the pattern sizes for every brand are in our seat bolt patterns guide.
Frames are base mount, so what about a side mount seat?
Seat sub frames are made for base mount seats: a flat plate drilled to match the floor of the seat, or the top of a slider. A side mount seat has threaded holes down each flank and nothing useful underneath, so it needs a pair of side mounts, or adaption bars sitting over the frame, to meet it. If you are not sure which type your seat is, our base mount or side mount guide settles it in a couple of minutes, and this one covers a seat that did not come from us.
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Base mount seat
Lands on the frame or on a slider. Holes are matched to the frame, within the tolerance.
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Side mount seat
Needs side mounts, or adaption bars over the frame, before it meets a base mount frame.
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Seat slider
Base holes match the frame and top holes match the side mount, so nothing needs modifying.
Seats and frames from different brands are not all drilled the same way. Mixing Recaro with Cobra, or OMP with Corbeau, for example, can mean extending the holes for a successful fit. Tell us the seat, the frame and the car and we will say which combinations work before you spend anything.
Three common installs
1
Base mounted recliner
Usually the simplest. The recliner has several base holes that match a sliding runner kit and a vehicle specific frame: one seat, one slider kit, one frame.
2
Base mounted bucket
Often as easy as a recliner, but few fixed back buckets have base holes and almost none with a fibreglass shell. Ask us before ordering the seat and frame together.
3
Side mounted bucket
The most involved. A seat, a set of side mounts, a universal slider and a flat base frame, usually from different brands.
Race seats are designed to sit on side mounts bolted to weld in bars, fitted to meet the regulations. Most road and track cars want the original carpet kept and no changes to the car, which is where the multi brand route comes from. It is also where the tolerances stack up.
A typical build is a Bimarco Futura bucket seat, a set of steel side mounts, a Sparco sliding runner kit and an OMP Renault Clio Mk2 base frame. Cameron likes that one because every part is easy to get. Garreth likes it because the result is solid and the seat is not welded to anything.
When the holes do not line up
Here Cameron and Garreth do not say quite the same thing, so here are both. Cameron has fitted enough of these to say that opening a bolt point a few millimetres is part of fitting aftermarket seats. If the holes are inside the 10mm the tolerance allows, a skilled fitter will extend them to suit, check that everything sits square and torque the bolts to the maker figure.
Garreth would not make re-drilling the first answer. Cutting a slot or a new hole changes the edge distance and how the load spreads, so his order is to check the part numbers and the car first, then ask the manufacturer for guidance. Anything beyond a tolerance sized extension, or any change to the structure of a frame, is best assessed by a competent fabricator before metal is cut.
Where we landSmall extensions within the tolerance are routine for a skilled fitter. Anything bigger, or anything structural, gets assessed first. If you are not sure which side of that line you are on, drop us an email or click to open a chat with the part numbers and a photo of the holes.
The slider route: nothing to modify
This is the easy way out of the tolerance problem. A universal seat slider has base holes that match the frame and top holes that match the side mount. The seat goes on its side mounts, the side mounts go on the slider and the slider goes on the frame, with no drilling in between.
For a base mount seat, fit the seat to the sliders first and the sliders to the frame second, so any fine adjustment is made on the base and not on the seat. That works from a simulator rig to a car floor mount. Garreth would put a slider in nearly every mixed brand build for that reason, while Cameron points out that a one driver car can run fixed and sit a little lower. Our complete guide to universal seat sliding runners shows which kits suit which seats.
Seat belts: the standard clip still works
You can use a standard seat belt clip with a motorsport seat. If the original buckle is in the way, an Endurance Motorsport seat belt bracket can sometimes be fitted between the slider and the seat. If no harnesses are being used at all, the base of the seat can be modified to take the clip. Call us before cutting anything, so we can say whether that suits your seat. Belt receptacle brackets are generally not supplied with frame kits, so mention it when you order.
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The parts we reach for most often when a customer asks about a slider, a bracket or a set of side mounts. The prices on the cards are live.
A typical side mount install, step by step
- Fit the side mounts to the seatUse the bolts supplied or the maker specified size, and keep both sides level to each other.
- Fit the sliders to the side mountsThe holes can be within the 10mm tolerance, so extending one may be needed here.
- Fit the slider handleIt sits between the two rails and needs some pressure to hold in place. The rear of the handle may need adjusting to sit on the slider teeth.
- Offer the seat unit up to the frameSet it where you want to sit and mark where the bolt points land.
- Extend or drill the frame as neededRemove the seat first. See the section above for what is routine and what to get assessed.
- Bolt the frame into the carThen sit in it, check both rails lock and check nothing catches a wire or belt.
With parts from different brands, expect the seat unit to need a small adjustment before it fits the frame.
Common questions
Is a 10mm tolerance on seat frame bolt holes normal?
Yes. OMP, Sparco and various other brands work to plus or minus 10mm on the bolt points of their sub frames, side mounts and seats, so it is normal for one or more points to need extending.
Will a seat frame fit a side mount seat?
Not on its own. Frames are base mount, so a side mount seat needs side mounts, or adaption bars over the frame, or a slider whose top holes match the side mount.
Do I need a seat slider?
It is not essential, but a slider is the simplest way to avoid modifying anything, because its base holes match the frame and its top holes match the side mount.
Can I use the normal seat belt with a motorsport seat?
Yes, a standard seat belt clip can be used. An Endurance Motorsport seat belt bracket can sometimes be fitted between the slider and the seat, or the base can be modified if no harnesses are used. Ask us first.
Is it fine to re-drill a seat frame?
A small extension within the tolerance is routine for a skilled fitter. Anything larger, or anything structural, should be checked with the maker or assessed by a competent fabricator first.
Why do my seat and frame bolt patterns not match?
The order may be missing a slider, side mounts or adaption bars, or parts from different brands have been combined. Check every part number before moving a hole.
Can I fit my own seat frame?
Plenty of customers do, with a spanner, a drill and a patient afternoon. If you would rather talk it through first, ask us and we will go through what is involved.
Where to go next
If you are still deciding what the seat bolts to, start with the base mount and side mount guide, then the bolt patterns by brand. If the seat did not come from us, this guide works out what it bolts to, and the universal sliding runner guide covers the part that saves the drilling.
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The short versionTell us the car and the seat, and we will name the parts between them
Cameron checks the frame, the mounts and the slider against each other before anything ships, and Garreth will tell you straight if a combination is not worth the effort. Come and have a look in the showroom, or ring us on 0115 9893488.
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Can I fit a side mount seat to a standard seat frame?
Not directly. Seat frames are base mount, so a side mount seat needs side mounts or adaption bars over the frame. A seat slider with base holes that match the frame and top holes that match the side mount avoids modifying anything. Bolt points on OMP, Sparco and other brand frames, side mounts and seats carry a tolerance of plus or minus 10mm, so one or more may need extending.