E21: Seat Fitment Guide:

Swapping seats round in a BMW E21 3-series is a nightmare, thanks to the floor-mounts being two different heights. To fit level-based seats from another car requires chopping out the welded floor-mounts and once this is done there is no going back.

1. Check if your floor-mounts have been chopped:

Check the outer floor-mounts on either side, spot-welded in place just inside the door-sill. They should look the one in the left photo below.

If they don't, and they look more like the right photo [with or without the wood], then you have chopped mounts and seats not originally from an E21. You also won't be going back to E21 seats very easily, so to preserve the style and make the ride a lot better, throw in seats from a newer BMW.

2.1 Fitting stock BMW seats from E30, E36 etc. :

This makes sense. E21 seats are sprung and uncomfortable and were dropped in almost every succeeding model. Other BMW seats are easier to come by and cheaper. E30 seats preserve the look almost identically and are far superior in construct. They also fit the inner seat mounts on the E21, the ones against the trans.-tunnel, so only the only outer ones need adapting. I am sure this is the case with seats from other 80s BMWs, possibly the E36 and E46 as I've seen it done, but need confirmation.
Modifying the outer floor-mounts, the ones against the door sills, is impossible as they just sit that much higher than the inner ones, the E21 of course having a crazy odd-height sub-frame design, so they need cutting out. Putting in new ones is pretty straight forward though, you can fabricate them with steel or alloy C-channel, wooden blocks, improvised stilts - these are all popular methods. Tilt-forward seats don't tend to be very strong anyway, so the new mounts don't have to be rock solid as long as they're bolted or welded securely to the floor-pan. Mine used the stock inner floor-mounts and a wooden block on the outer side to get the height - this presented no problem to the UK MOT (roadworthiness-test).

Check out my guide here: http://e21dub.blogspot.co.uk/2013/09/e30-sports-seats-fitted.html

Or check out HSVTurbo's guide HERE [where I got my idea] in THIS THREAD on bimmerforums.

2.2 Fitting rigid Racing bucket-seats:

If you're building a track car then it's obviously best to keep things as stiff and secure as possible. The neatest and most weight-saving method is to cut some steel cross-beams to fit and weld them across the floor-pan, from the trans.-tunnel to the door-sill. This could be done with or without removing the outer floor-mount, but you will probably have to do this to get the seat low enough. Two cross-beams under each seat would probably be best, but it is most often done with just reasonably thick one.

More info. HERE on bimmerforums.

2.3 Fitting non-BMW stock seats (VW, Ford Recaro etc.) :

To get the right height and a central-position with stock seats from non-BMWs there is a lot of modification to the mounts - these seats will always have the bolt-holes in the wrong place. The tidiest way would probably be to weld in rigid cross-beams like in the above guide [2.2], otherwise you will need to fabricate custom mounts. [I used a combination of wood-block and steel cross-beam to fit Ford RS Recaros to mine in THIS POST].

Check out my guide here: http://e21dub.blogspot.co.uk/2013/08/recaro-seat-base-adapter-finally-solved.html

3. Re-fitting stock E21 seats:

I have never seen anyone online do this. If you have level floor-mounts then I see no reason to switch back to E21 seats. All but the Recaros are uncomfortable, sprung nightmares. They give a truly bouncy and uncertain ride - only the most restoration mad would want it. The Recaros are super-rare and overpriced to fit the E21s mounts. E30 Recaros are a fraction of the price, have level mounts and are better anyway. If you are so desperate to restore your E21 dutifully, then I would consider it having cut floor-mounts a deal-breaker and to find one with stock seats, because the car will never be the same again.

There are 3 ways I see to achieve refitting stock seats:

1. Build custom outer mounts using a combination of the methods above, building them back up to the original and rather tall height for the stock seat-frames. [].

2. Locate a pair of original outer floor-mounts that have been cut out of another E21 and refit them to yours. These would first need to have been cut out carefully from the donor car and preserved by someone. Whether they would advertise them for sale is another matter, but asking on the forums can't hurt. Otherwise, you'll need to find someone with a scrap / breaking E21 and ask to neatly cut out its floor-mounts for a few quid. Either way, obtaining a useable pair is going to be tough.
Once you do though, they would just need to be measured up to where yours were cut off and welded into place. How neat a job you do will determine whether they fit or if it's unnoticeable.

3. Fabricate new outer mounts from the ground up out of steel and weld those in. I will endeavour to make a note of the dimensions of my cut one before passing it on and do a quick CAD design to fab-up a simple one.

**Also, I still have the passenger-side (N/S, Euro/US Driver-side) floor-mount [above], so if anyone wants it to try and weld back in message me! [Sadly, I cut the driver-side one out more severely and this is probably going to be the case if yours are chopped as it gives the wood or whatever more to grab on to.]

E21 316: E30 Sports Seats - fitted.

MJNewbs from eBay got back in touch and we finally negotiated a low enough price for the seats. The fronts are in great nick, only a slight fray to the driver's back-rest, but the rear seat back-rest has a few tear that I'll address in a later post.

Thankfully, the E30 seat bases fit onto the inner stock mount, next to the trans. tunnel, on the E21 so they are already sited neatly in the centre and only require the outer mount to be lowered. Here's how I finally got them to an equal height.

The outer seat-mount was cut away about 1.5 inches height from the lowest point of the floor-pan and the top half completely removed from the sill. This leaves a large flat T-piece section onto which a length of '2by4' style wood-beam was placed that sits at the same height as the inner seat-mount, give or take 2mm.

To strengthen the edge of the wood where the seat sits a piece of 2mm-thick angle-steel was screwed in place, clamping the wood to the remaining edge of the cut seat-mount. The M10 seat bolts run through the wood and are bolted to the floor-pan, so the wood only acts as a riser, not an anchor and the angle-steel helps to distribute any downward force on the wood, or twisting motion as the seat is adjusted.

I chose to bolt my seats directly through the floor-pan for strength and this leaves a gap under the base of the wood at the inside corners, which I stacked out with a nut and large washers. This gives the added benefit of the seats being bolted first to the wood, then through the floor-pan with the same bolt. The downside is the sealed floor-pan nuts will need removing should the seat need to come out in future. The alternative method, used by HSVTurbo in his build here, is to bolt the seats to a thicker metal bracket, then bolt the bracket through the wood and floor-pan. This way the seat can be removed from inside the car leaving the wood fixed in place, but I just don't feel this setup will be strong enough, even with the thicker metal bracket.

I used M10 nyloc-nuts with a dab of stud-lock inside and large-diameter [40mm] 'bumper' washers. A good blob of silicone-sealant was put between the washer and body, then round the outside of the seated nut. Finally, I coated them in a thick layer of Finnegan's WAXOYL Underbody Sealant, for protection obviously, but also to blend the shiny nuts and washers into the car's underside.

I still have the rear-seats, including the folding arm-rest, along with the door-cards/handles and rear trim-panels, so we'll have to see how much will fit after the MOT.

E21 316: Stretched Toyos first try + more spending.

Tried out the 8Js on the front with the new 195 Toyo T1Rs using 20mm spacers and there's a lot more room at the arch-lip now. The tyre doesn't catch at all turned hard-left, thought it would going over a bump. Hard-right the tyre is still catching the front valance slightly, but about 1cm cut away should be enough now and a little off the fender where it protrudes from the bumper - none of the bumper itself though. I had hoped to get away with smaller spacers on the front, but even with the 20mm spacers from the rear, the inside rim of the wheel is still 5mm inboard of the 15x7Js and there is not a lot of headroom.

I just haven't found time to try out the 9Js on the rear today as the valve is bust in one and on the other, about 6 inches of the bead has just been refusing to seat. My leg just couldn't take any more foot-pumping so I got hold of an air-compressor from a friend and took the tyre to nearly 90psi this morning, but it still wasn't having any of it. Eventually, after two goes deflating the tyre almost completely and trying to brush in soap between the tyre and rim, then taking it back up to about 65psi, the bead finally popped. The maximum pressure rating on the tyre is 50psi, though it says use no more than 40 to seat the bead, so it just goes to show that when the seal is tight enough it can take over twice that and still not seat. Bead-blaster machines can jet up to 200psi in one go and that doesn't even shred the tyre to bits so I'd be keen to know at what pressure a good new tyre would rupture at. Even so, always take the utmost care when inflating tyres past their rated pressures at home!

Apart from 4 new valve-cores (£2.99) and a valve removal-tool (£4.99) from Halfords, I've had to do yet more spending on parts as the project can't move on without them and I want to be getting somewhere near finished next weekend when we have the bank-holiday off. First up are adjustable camber-plates for the front, a must for style and it will likely give me a bit more room to play with before starting to cut. They're not approved for sale in the UK, so lets hope they make it here from Poland before the weekend - fingers crossed eh! Looking forward to these, though quite pricey @ £105 delivered, but whatever, they're the only ones of their kind. It's also time to stop digging my heels in and buy another set of spacers to go on the front, swapping the ones over from the rear is becoming a chore. I'd hoped to get some alloy ones to save hub weight, but again for easy access, the right centre-bore without more spigot-rings and to get the bolt-on kind I want, yep you guessed it, more steel ones. Another £59, oh well, roll on next weekend.