Can BMW 320i front brake rotors be used as a direct swap?

[Gordy]

The parts interchange is down. Does anyone know if the BMW 320i front brake rotors are able to be used as a direct swap with the larger 75 Manta calipers (and line adaptions) on a 1970 1.9 GT?

[FPManta59]

As an aside, there is an Alfa Romeo caliper that is the same as the '75 Manta caliper. The difference is that it is aluminum and a bunch lighter! If there is interest, I can have the parts guys at Greenfield Imported research it again.

Ed

[Killer Texas GT]

The "Piston" diameter for the 1.9 Kadett, GT, Manta and Ascona (including the 75"big brake) is 48mm. The "Piston" diameter for the 1.1 Kadett and GT is 44 mm.
The 1.1 calipers are also smaller in physical size and mounting bolt pattern compared to the 1.9s. The "75 big brake" is larger in physical size than the typical brake caliper from a 1.9, but has the same bolt pattern mounting.
As for a BMW 320-I rotor working in conjunction with the 75 big brake I believe the diameter of the rotor is a few mm larger in diameter, and is also vented which is wider and will not fit within the standard 75 Big brake, there is a spacer for the 75 Big Brake which will accept the vented rotors.
Then there is a French OPEL floating caliper which has the same bolt pattern for mounting as the 1.9 calipers, and has a 42mm "Piston" but has 1/3 more friction contact area that the 75 Big brake.

[blancojp]

Originally Posted by Gordy The parts interchange is down. Does anyone know if the BMW 320i front brake rotors are able to be used as a direct swap with the larger 75 Manta calipers (and line adaptions) on a 1970 1.9 GT?

Here is the full scoop:

1) The rotor on a GT is 238mm in diameter. Solid rotor, 10mm thick
2) The '75 Manta rotors are 246mm in diameter. Solid rotor, 10mm thick.
3) The Alfa rotors are 261mm in diameter. Solid rotor, 11mm thick
4) The BMW 320I front rotor is 255mm in diameter. Vented rotor, 22mm thick.

The manta caliper mounting points are offset outwards 4mm to accomodate the larger diameter rotor.

The BMW rotor must be cut down in diameter to fit with the Manta calipers. Also the rotor will now be used as a hat over the hub due to the 75mm height, versus the 55mm Opel rotor height. This is not a bolt on scenario since you will need to cut down the rotor 9mm in diameter and install the E321-009 kit on the caliper to widen it. the caliper kit is almost imposible to find and does not directly fit the caliper. BMW calipers can be installed with minor modification but the weight is twice of an Opel caliper.

Alfa rotors can be installed with a solid rotor Volvo caliper. This setup doubles the pad area, increases the number of pistons to 4 and it is mostly a bolt on scenario. The bad part about this is that you now have a larger solid rotor to warp.

I have every year Opel caliper in stock from 1965 to 1975. From what I have seen, threre are three piston sizes, 38mm, 45mm and 48mm marked on the calipers. The 38mm is very rare and I have not seen it used in any year other than '65.

The part that nails you when upgrading the brakes is the center bore. Most rotors on the market today have a 66-68mm center bore and the Opel hub is 73mm so you end up reworking the rotor. I have always reworked the hub so a rotor replacement can be done easily, without having to machine parts all of the time.

I will be posting my new "Poor man vented rotor system" in the next few weeks. You increase the rotor diameter 22mm, vented rotors and same Opel calipers all at a very low cost.

[Gordy]

I went to the Tire Rack and just ordered the proper rotors for the 1975 brake set up so there should be no issues. They've got a good selection of Koni and KYB shocks as well. Ordered the calipers from Rock Auto at $45 each.

[kwilford]

These posts were related to upgrading the brakes, not the sizes and interchangeability between various Opel OEM calipers (which was the topic of the original thread), so they were split off to their own thread.

And FWIW, the BMW 320i calipers are a direct fitment as an alternative to the 1975 "Big Brake" Opel calipers, but you still need to use the Opel (or early Lotus Esprit IIRC) rotors. These are available off the shelf, and I recently saw drilled and radiused ones on eBay for $118 a pair

[opeletti75]

i am in need for 75 calipers can you help me on those alfa calipers

[FPManta59]

I'll talk with them tomorrow and get back with an answer ASAP.

Ed

[blancojp]

Originally Posted by kwilford These posts were related to upgrading the brakes, not the sizes and interchangeability between various Opel OEM calipers (which was the topic f the original thread), so they were split off to their own thread. And FWIW, the BMW 320i calipers are a direct fitment as an alternative to the 1975 "Big Brake" Opel calipers, but you still need to use the Opel (or early Lotus Esprit IIRC) rotors. These are available off the shelf, and I recently saw drilled and radiused ones on eBay for $118 a pair

You are talking about the BMW solid rotor caliper and I was refering to the vented rotor caliper. In any event, minor mods need to be performed since you will have to cut down the backing plate so the caliper will fit. Some of the BMW calipers I've seen use a smaller bolt to attach to the spindle mounts; which you will have to enlarge for the Opel bolt to fit. To me that is alot of work and money just to increase the size of the rotor from 238 to 246mm, very little benefit. The second part is that most wheels will not fit with this arrangement, or at least the ones I've tried.

Though the backing plate will require cutting, the true bolt on scenario is the Honda rotors and Volvo calipers. This would be a true upgrade since the rotors are 262mm, four pistons instead of two and the pads have twice the area.

But if you want to retain you 13" wheels and have a vented rotor, I posted a GEO metro vented rotor upgrade which can be done for less than $120 complete.

[kwilford]

Originally Posted by blancojp In any event, minor mods need to be performed since you will have to cut down the backing plate so the caliper will fit.

True, if you are converting a pre-'75 Opel to the '75 OEM Big Brake package, but not true if you are simply looking for new calipers for your 1975 Opel. But as you say, converting to virtually any bigger brake system requires that the backing plates be modified.

Originally Posted by blancojp Some of the BMW calipers I've seen use a smaller bolt to attach to the spindle mounts; which you will have to enlarge for the Opel bolt to fit. To me that is a lot of work and money just to increase the size of the rotor from 238 to 246mm, very little benefit. The second part is that most wheels will not fit with this arrangement, or at least the ones I've tried.

I haven't run across the hole diameter difference, although I have only bought two sets of BMW calipers, and both fit perfectly. I was also able to buy one set of rebuilt Opel OEM calipers at a very reasonable price.

For many folks, the conversion to the 1975 OEM "Big Brake" is desirable, as it maintains the OEM nature of the brakes, and while the rotor is only 3.36% bigger, by the law of squares relating to rotor swept area, that is at least a 11.3% increase in braking capability. And the 1975 pads are probably 30% larger than the earlier pads. For quite a minor bit of work to convert, the increased braking capability of the 1975 brakes are worth the trouble. Sure, vented rotors are nice when it comes to brake fade under repeated hard use, but for everyday driving, the decreased effort supplied by the 1975 brakes are a definite improvement over the older design.

Finally, while it is true that the 1975 "big Brakes" require at least the larger 1975 13x5 1/2J wheels, virtually any larger wheel (14 inch and larger) will clear these brakes.

I am not at all saying that the Geo or Honda conversion isn't better, just that the 1975 brakes are quite easy to convert to, and offer substantially better braking than the OEM brakes.

IMHO

[blancojp]

The points are well taken but when you look at the 400 or so brake upgrades I have done to Opel and MG's, you start to see things differently. Now I look at the benefit versus the $$$$ employed and the availability of the components.