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Fitting New Seats

How to upgrade your seats

by Julian Edgar

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At a glance...

  • Changing the seats in your car
  • Selecting new seats
  • Seat belt implications
  • Air bags and pretensioners
  • Size and features
  • Making brackets
  • Hardware
  • Results
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This article was first published in 2009.

A seat that is comfortable and supportive is integral to enjoying driving. So what if your car has flat, hard seats that allow you to flop around on every corner? The answer is to fit new seats – either brand new, or from another car. In this story we’ll look at the latter – sourcing secondhand seats and then installing them.

However, first a warning. Unless the seats are from another version of your car, installing new seats is usually a lot more work than it first appears: what looks like it might take a few hours often in fact takes a few days. Also, when changing seats, there are safety, legal and insurance implications. (And before beginning, also check out the breakout box at the very end of this article!)

Picking New Seats

Selecting the right new seats is critical to success – and there’s actually a lot to get right in the selection process.

  • Seat-Belts

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What do seat-belts have to do with seat selection? In some cases, a lot! In many cars, the seat-belt buckle is attached to the seat. This means that as the seat is adjusted forwards or backwards, the buckle moves with the seat – always placing it in the correct position.

However, it also means that seat-belt loads are being borne by the seat. So in a crash, the seat-belt buckle is pulling on the seat, rather than directly on the car’s bodywork. Both the seat and the seat‘s fastening to the bodywork have to be strong enough to withstand these forces.

Other (rarer) seat-belt systems place both lower anchorages on the seat – that is, the seat-belt attaches to the seat at the buckle and also on the other side.

In any situation where the car’s original seat-belts attach to a seat (either at the buckle or on both sides), the replacement seat must use the same seat-belt design. Don’t ever be tempted to consider installing non-seat-belt seats and then attaching the seat-belts to them with custom lugs or bolts. If the seat didn’t originally carry seat-belt loads, it won’t be strong enough to withstand the required crash forces.

Another point to consider in relation to seat-belts of any kind is that the new seat must position the occupant so that the seat-belt still functions correctly. For example, if the upper seat-belt anchorage is low (and not adjustable), a tall and wide seat may prevent the sash part of the belt from sitting properly over the shoulder.

Finally, does the car use electronic seat-belt sensors to detect when the seat-belts are done up? If the seat-belt buckle is attached to the seat, the old buckle will need to be swapped to the new seat to retain this function.

  • Air Bags and Pretensioners

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If the original factory seat contained air bags, it’s very unlikely that you’ll be able to easily source a better quality seat that is electronically and mechanically compatible with the car’s airbag system. (The exception is if you take a better seat from a sports version of the same car.)

If you want to improve the quality of an airbag equipped seat, the best approach is to have the seat re-upholstered with better padding and/or material. But again that can be problematic – the airbag must still function as it should, for example deploying correctly through the new seat material.

If the car uses only front airbags (or the side airbags aren’t in the seats), do the factory seats have sensors to detect when a person is sitting in them (or the weight of that person)? If so, these sensors will need to be swapped into the new seats.

If the seat-belt buckle is attached to the seat, and the buckle uses an active pre-tensioner (ie a device that tensions the seat-belt in a crash), can these buckles be swapped across to the new seats? A pre-tensioner may have an extensive assembly under the buckle itself – and there may not be room for this in the new seat.

  • Size and Features

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Seats vary a great deal in base width, backrest height and width, height off the floor, and length of the base. So the starting point in working out what will and won’t fit is to do some basic measurements of the existing seats.

Will a wider seat fit? (And by ‘fit’, I mean not just fit in the car, but also allow the occupant to still reach the seat adjustment mechanisms with the door shut?) Will a longer seat base bite too much into rear legroom? Will a higher head restraint block rear vision – or make rear seat occupants feel claustrophobic? If the seat sits higher off the floor, will you have headroom or visibility problems?

Does the seat need to flip forward (eg in a two door car)? Do you want lumbar support? Height adjustment? Electric seat controls? Leather or cloth? What colours will suit the rest of the car’s interior? Do you want seats with rear pockets?

Visit a wrecking yard and there is a bewildering array of options!

  • Runner Design

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When selecting seats, the design of the runners will make a major difference to how hard it is to put the seats in your car.

Look at the original seats and measure the distance between the seat runners, and also the runner length. In addition, look at how the runners are bolted to the floor – for example, do they use flat brackets or are the brackets curved? Check carefully – are both runners the same length, same height and attached to the floor in the same way?

The closer that the donor seat runner design matches the original, the easier it will be to fit them.

Sourcing Seats

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Seats can be bought wherever secondhand parts are sold. However, there are some downsides to buying privately or sight unseen.

The major reason that you’re putting new seats in your car is to improve comfort and support – so you need to sit in the seats you’re thinking of buying! That ‘sit before you buy’ is best accomplished if the seats are still in the donor car – that way you get a feel for how they are when mounted with the original height and inclination.

If you have access to a large wrecker where you’re free to wander around the yard, you can try out lots of seats, measure lots of seats, and inspect the runner and seat-belt arrangement of lots of seats!

Interestingly, I also found that the wrecker prices were cheaper than private secondhand prices – an unusual occurrence.

Legalities?

Changing seats – especially if that requires altering seat belt mounts – requires legal approval. That process will vary from location to location but in here in Queensland, what is called a ‘blue plate’ needs to be affixed to show that approval has been given for the modification. The inspection and provision of the blue plate can be performed by an accredited workshop. The next step is to inform your insurance company of the modification.

Installation

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The original seats in this Honda Insight are OK, but on longer trips they lack support. There’s also no height or lumbar adjustment. The seat-belts mount to the seats on both sides.

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One of the new pair of seats. The seats are from a Holden (Opel) Vectra. Of all the hundreds of seats I inspected in a wrecking yard, only Astra and Vectra seats had two seat-belt mounting points. The seats feature leather facings, height adjustment on the driver’s seat, lumbar support and electric seat heaters. They cost $250 for the pair.

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The Honda’s seats are held in with fine-thread 8mm front bolts...

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...and 10mm rear bolts. Because these bolts handle seat-belt loads, they are very important. Note how the rear bolts are thicker – in a frontal crash they undergo tension, whereas the front mounts are being compressed.

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The Honda and Vectra seats side by side. I am using a level to ensure that the base of each seat is about the same distance off the ground. A greater distance in the new seat would mean that headroom would suffer (however, note that headroom also depends on how much the seat compresses with body weight).

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The underside of the two seats – the original Honda seat is on the right. The green lines show that the Honda’s seat rails are (1) each of the same length, and (2) are longer than either of the Vectra seat rails (red). Also note how the Vectra’s seat rails are different lengths.

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The Honda seat rails are further apart (green) than the Vectra’s seat rails (red).

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Finally, the front of the Honda seat rails (right) have a riveted and welded lug for floor mounting, while the Vectra seat (left) doesn’t appear to have any mount at all! (An advantage of having watched the removal of the Vectra seats from the donor car is that I know the Vectra rails slide into a cavity in the body of the Vectra – that’s why there appears to be no mounting facility.)

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The first step was to remove the seat belt buckles from the Vectra seats. The donor car had been in a minor accident resulting in damage to the front-left corner of the car. The seat-belt pretensioner on the left-hand seat had operated, pulling on the seat-belt. You can see how this assembly is now much shorter than the driver’s side unit, and how a tell-tale flag has extended.

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Here is one of the Vectra’s seat-belt mounts. Note how dual rivets (arrowed) are used to hold the mount to the seat assembly.

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The Honda’s seat-belt buckle mount needed only this locating tag ground off before it could be bolted to the Vectra’s seat-belt mount...

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...using the original Honda bolt. Note the arrowed shoulder on the bolt, that allows the buckle to swivel even when the bolt is done up tightly.

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The wiring for the Honda’s seat-belt sensor was transferred and secured with cable ties.

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Both the seat-belt and seat hold-down bolts in the cars were Loctited into place. This practice should be followed when making the swap!

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The first mounting step was to drill holes in each of the Vectra front seat rails. Mild steel angle (40 x 40 x 6mm) was then used with high tensile bolts, nuts and hardened steel washers to join the two front rails, providing a very strong mounting point. (High tensile hardware must be used.)

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The angle could then be rested on wooden packing while...

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...the seat was levelled and its height adjusted.

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Lugs were then attached to the angle to provide the front mounting points that bolt to the original Honda floor mounts. The lugs were made from 40 x 8mm mild steel bar, with being nickel-bronze brazed to the cross-member and also bolted in place (the top bolt also secures the seat). Note how the greater width between the Vectra sat rails is catered for (green arrows show seat mounting holes.)

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The lugs were cold bent in a vice, using a big hammer. A piece of bent wire was used as a guide.

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A similar design approach was taken for the rear seat mounts. With the seat installed, the outer seat-belt mount could be Loctited and screwed into place. (The seat-belt buckle was installed before the seat was placed in the car.) The fact that the Honda and Vectra used the same seat-belt mount bolt diameters and threads made this part of the exercise very easy.

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The new seats in the car.

Conclusion

Unless the new seats use identical mountings, swapping seats is no simple and quick task. However, if the new seats can be picked up cheaply, it remains a very cost-effective improvement.

However....

After fitting the new leather seats to my Honda, was I happy? No, I wasn’t – and the reason I wasn’t is interesting.

In short, the new seats transmitted too much vibration, a movement that made me feel sick.

The Vectra seats use firm foam rubber while the original Honda seats use a combination of steel springs and foam. The Vectra seats – no doubt absolutely fine in the softer riding, more refined Vectra - were not soft enough for the 3-cylinder, much more hard-riding Honda.

So, despite the large amount of work involved in installing the seats, I removed them.

Two points come from this: (1) install the driver’s seat first and then do plenty of driving to make sure that you’re happy with it, and (2) not all seat padding/springing/suspension designs suit all cars.

However, based on our previous experience of seat swaps, the problem I found in the Honda occurs only rarely.

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