At 10:40 PM 10/3/04 -0600, Nicholas Von Staden wrote multiple questions:
>"....1958 MGA 1500. I want to know if its worth it to put MGB disc brakes on the front."

Take one question at a time. You have to get the answer right for this one before even considering the rest.

Without getting into the long philosophical discussion about changing any part of the car from original spec, I only need to say that the front drum brakes work okay for normal driving activities. Having driven hundreds of thousands of miles in the MGA, I still have original type drum brakes on mine, and I don't feel the urge to spend the money to change them.

If you have a sufficiently heavy foot, the drum brakes are capable of strong force and indeed even locking up the wheels at any time for a single stop from speed. I have done literally thousands of short laps during autocross competition with no brake problems. The most significant advantage of disk brakes is reduced fade with repeated use and heating such as you might encounter during road racing or while hustling down a mountain side through switchbacks. One advantage of drum brakes is slightly less unsprung weight, which may give a little less harshness on road bumps and a little better tire grip in a turn with a bump.

So first ask yourself why you might want to convert it to disk brakes. If it is only as an urge for the wow factor, I don't think I'd do it even if it was free. The conversion is not economically justified unless you expect to win enough prize money in professional competition to pay for it. But if you're about to spend a lot of money to go vintage racing, then the conversion is probably a good idea. That is, if it doesn't throw your car into a different non-competitive racing class.

>".... with that adapter that MOSS makes ....a little expensive for the adapter"

Moss part no: 180-522, a pair required. Wow! That is expensive. I had glossed over it without much attention when Moss first announced the new part, because I didn't have any use for it myself. I suppose this adds some incentive to keep it original. That adapter is only needed to mount MGB calipers on MGA knuckles.

>"but calipers and discs are cheap enough."

I don't know where you got that idea. The only way to switch to the (currently) less expensive MGB rotor is to change the hub and bearings along with it. And no guarantee that the MGB parts will still be cheaper the next time you need them. While the MGB caliper is not terribly expensive, the rebuild kits for either A or B calipers are still way cheaper than changing calipers. MGA calipers purchased as cores and rebuilt should still be cheaper than MGB calipers and the special mounting adapter brackets.

But there are other alternatives to installing disk brakes. One way is to install all MGB parts from the kingpins out. That only requires a couple of thick spacer washers at the top trunnion. This might be economically viable if you have an MGB parts car handy, or may otherwise get the used MGB front end parts cheap. The other way is of course to install all original MGA disk brake parts. You can use the same knuckle (bearing spindle) and all parts inward, but it requires a different bearing hub, rotor, caliper with mounting bracket, and a change of flex hose (including banjo fitting and bracket with special bolts) and steel hydraulic line. In either case you would likely need to rebuild the calipers.

Apparently MGA calipers are not currently available new, and there may be a shortage of rebuildable MGA calipers on the market right now.

Addendum, June 2016:
MGA calipers are now available new at reasonable price:
www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29232
https://mossmotors.com/180-511-brake-caliper-right-front-new
https://mossmotors.com/180-521-brake-caliper-left-front-new

But there are still plenty of MGA parts cars around, so no problem picking up used calipers from a bone yard, or through eBay, or a swap meet, etc. Calipers are nearly always rebuildable. The nice feature about calipers is that the sealing surface is on the side of the piston, so if it gets all scarfed up you just install a new piston and seal and it's like new again with no machining required. The only problem there is the current cost of the piston for the MGA caliper at $XX.XX list price vs $xx.xx for the MGB part (times four required). But the solution to that is simple.

Addendum, June 2016:
Pistons for MGA calipers are also now available at much reduced price:
(same link as above).

There is a nifty trick to allow installing the MGB piston in the MGA caliper. Without any long explanation of the function of the center guide pin and associated parts, just suffice it to say that the factory decided (discovered) it wasn't needed by the time they started building the MGB. So just pull out the center guide pin from the MGA caliper bore, and the cheaper MGB piston works in the MGA caliper.

1500 hose mount high and outboard

1600 hose mount low and inboard

Where the flex hose connects to the steel line there is a welded bracket on the MGA frame (back of the coil spring mount area). For the 1500 this has a large hole. For the 1600 this has a smaller hole and a steel clip called Locking Plate. The locking plate has a hex hole to grip the hose end fitting and prevent rotation of the hose (strain relief). You may need to modify the frame bracket accordingly. You may need to grind the bracket narrower to match the locking plate, and tack weld a narrow washer inside to reduce the hole size. Alternative is to cut off the bracket and weld on a new one to suit the application.

ET Taylor in North Carolina, USA has another solution to this. He wrote: "Problem I encountered was the need to add a small bracket to move the lines out a bit".
While this likely works, the 1600 hose was originally coming out the top of the frame bracket.

If you use MGB calipers on the MGA, you need to procure a flex hose to suit. The MGB has the pipe mount bracket on the body inner fender, so the MGB hose is different length. I am not sure at this time if the MGA banjo fitting fits on the MGB caliper. Be sure the hose has sufficient length to allow for full steering motion at full drop of the suspension. I have not personally done this, so perhaps someone who has may help fill in the blank here.

There is one more detail to consider. The disk brake MGA has a taller cover on the master cylinder to increase volume of the reservoir. Disc brakes will work as well with the flat cover, but you need to be diligent about maintaining proper fluid level in the reservoir. As brake pads wear and the caliper pistons advance outward, substantial fluid volume is displaced, and the 1500 reservoir could run low on fluid. It is a good move to install the taller 1600 type cover on the master cylinder for use with disc brakes.

Addendum July 21, 2024:

Bob Shafto has provided this drawing showing original locations of the front brake hose mounting tabs welded to the frame of MGA 1500 with drum brakes and MGA 1600 with disc brakes. This is followed by a drawing for an adapter bracket that can be bolted onto either frame to provide the other hose mounting location. This bracket is useful if you are converting a 1500 from drum brakes to front disc brakes. It works as well for converting a 1600 chassis with front disc brakes to drum brakes. You might think no one would want to do the latter, but it has been done occasionally. This bracket will provide the proper hose mounting location for either case.

For example, below is a picture series showing this bracket being used for installing the 1600 type disc brake hose (and pipe) when converting the 1500 car to disc brakes.

Here is a link to the original PDF document: MGA_FrontDrum2DiscBrakeConversion.pdf