BMW E30 Bilstein B12 Kits: Check 45 mm Or 51 mm Struts Before Ordering
BMW E30, part number by part number
An E30 model name will not get you to the right Bilstein B12 kit. Two things separate parts that otherwise look identical in the listing: the front strut housing diameter, and the permitted axle ratings.
Cameron sees the same order go wrong the same way. Somebody reads "320i", finds a 320i kit and orders it, and the car turns out to be on the other housing. Garreth points out the bit that makes an E30 worse than most: these cars are forty years old and a lot of them have been apart before, so what the factory fitted and what is on the car now are two separate questions.
Seven B12 listings cover the E30 on our shelves. Here they are side by side, with the figures that actually decide between them.
- 45 mm and 51 mm are front strut housing diameters, not lowering figures
- All seven listings give roughly 30 to 40mm at the front and 30mm at the rear
- Front axle maxima run from 805 kg to 865 kg across the seven
- The rear is 905 kg on five of them and 945 kg on two
- The Convertible is a separate listing on both housing sizes
- Two kits can share a housing diameter and still not be interchangeable
- A previous engine or suspension conversion has to be checked against the car as it is now
What 45 mm and 51 mm actually mean
Both figures refer to the front strut housing diameter in the Bilstein application listing. They describe the tube the damper insert lives in, and they have nothing to do with how far the car comes down.
Establish which housing the car is on before you choose the kit. If there is any doubt at all, the installer can measure it, and measuring takes a couple of minutes against the alternative.
All seven E30 listings, side by side
Grouped by housing diameter. Open the product page for the full application dates and fitting conditions, and match BOTH axle ratings against the vehicle identification plate rather than just the front.
| Part number | Listed application | Front axle max | Rear axle max | Approximate lowering |
|---|---|---|---|---|
| 51 mm front strut housings | ||||
| 46-000101 | 320i, 323i, 325i and 325e 2.7 | 825 kg | 905 kg | 30 to 40mm front, 30mm rear |
| 46-000118 | 316i, 318i and 318is | 805 kg | 905 kg | 30 to 40mm front, 30mm rear |
| 46-000132 | 320i and 325i Convertible | 865 kg | 945 kg | 30 to 40mm front, 30mm rear |
| 46-000163 | 320i, 323i, 325i, 324d and 324td | 865 kg | 905 kg | 30 to 40mm front, 30mm rear |
| 45 mm front strut housings | ||||
| 46-180797 | 320i and 323i | 825 kg | 905 kg | 30 to 40mm front, 30mm rear |
| 46-180803 | 316i, 318i and 318is | 805 kg | 905 kg | 30 to 40mm front, 30mm rear |
| 46-180834 | 320i | 865 kg | 945 kg | 30 to 40mm front, 30mm rear |
Read across, not down. The lowering column is identical on all seven, which is precisely why it is no help in choosing between them.
The four 51 mm kits
Four listings on the larger housing, covering the saloons and the Convertible. The engine application is what separates them, with the axle limits as the second check.
The three 45 mm kits
Three on the smaller housing. Note that there is no separate Convertible entry here, so a Convertible on 45 mm housings is a phone call and not a click.
Why the axle limits matter as much as the housing
Take 46-000101 and 46-000163. Both are 51 mm front housings. Both give the same lowering. Their front axle maxima are 825 kg and 865 kg, and their engine applications are different, so a matching strut diameter does not make those two listings swappable.
The same pattern runs through the 45 mm group. 46-180797 and 46-180834 are both 45 mm kits and both cover a 320i, and they are 825 kg and 865 kg at the front with 905 kg and 945 kg at the rear. Use the engine application and both axle limits together, every time.
Forty year old cars have history
An E30 that has had an engine conversion, a strut swap or a previous lowering kit is no longer described by what it left the factory as. The housings may have been changed. The front end may be carrying more weight than the original application allows for.
None of that is a problem so long as it is known. It is only a problem when the kit is chosen from the V5 and the car turns out to be something else.
Measure the housing
If nobody can say for certain whether it is 45 mm or 51 mm, have it measured. Two minutes with a caliper settles it and nothing else does.
Read the plate
The vehicle identification plate carries the permitted axle loads. Compare the front and the rear separately against the listing.
Declare the conversions
Tell us about the engine swap or the previous suspension work up front. It changes which listing applies far more often than people expect.
What to send us
- Registration or VIN
- Body style, saloon, Touring, Coupe or Convertible
- Engine, and whether it is the original one
- Front strut housing diameter if it is known, or a note that it needs measuring
- Front and rear axle ratings off the identification plate
- Any suspension conversion the car has had, however long ago
With those six we can confirm the listed application and the current supplier lead time before you order, and before the car is booked in.
Common questions
How do I find out whether my E30 is on 45 mm or 51 mm struts?
Measure the outside diameter of the front strut housing. If the car is not in the air, your installer can do it in a couple of minutes. It is the one figure you cannot get off the registration document.
Do the 45 mm kits lower the car less than the 51 mm kits?
No. All seven listings give roughly 30 to 40mm at the front and 30mm at the rear. The millimetre figure in the title is the strut housing diameter, not the drop.
My car is a 320i. Why are there three different kits for it?
Because the housing diameter and the axle ratings vary across the application. There is a 320i entry on the 51 mm housing at 825 kg front, another at 865 kg, and a 45 mm entry at each. The badge narrows it down and does not finish the job.
Can I use a saloon kit on a Convertible?
Not on the listed applications. The Convertible has its own entry with higher axle maxima at 865 kg front and 945 kg rear, which is there because the car weighs more.
Is the B12 kit height adjustable once it is on?
No. B12 Pro-Kit is a fixed lowering package of Bilstein dampers and matched Eibach springs. If you want to set the height yourself, that is a threaded kit and a different conversation.
Will it need an alignment afterwards?
Yes. Dropping the car changes the geometry, so book the alignment as part of the job and follow the installation and approval documents supplied with the kit.
Technical reference: the Bilstein manufacturer application programme, cross checked against the current UK product listings. Follow the installation and approval documents for the exact kit supplied. The rest of the range is on the Bilstein B12 category page, and the differences between the series are covered in our B12, B14 and B16 guide.
Send the number with the registration and we will confirm the housing, both axle limits and the listed application, so the box that turns up is the right one.
Registration or VIN, body style, engine and anything you know about previous suspension work. We will check the listed application and the current supplier lead time with you, which is a far better use of ten minutes than sending a kit back.
