Tech Q&A: Spring 2003

By Kelvin Dodd

MG Caliper Rebuilding

The Lucas Service Bulletin calls for replacing the bolts when “halving” calipers in order to replace the O-ring. The bolts, however, have never been available. Rebuilding calipers without replacing the inner O-ring doesn’t seem safe. Am I stuck replacing my calipers just because they need O-rings?

—Ben Severson

mg caliper

The workshop manual describes how to replace the dust seal and fluid seal without splitting the caliper. This is the approved method because the caliper half-bolts are designed to be only used once (no torque specifications are available). If the calipers appear to be in otherwise good condition and the O-rings are suspect, some owners have reportedly successfully done the “halving” job by keeping the caliper mating surface clean and using Loctite on the threads when reinstalling the bolts. However, this procedure isn’t recommended by the factory, the caliper’s manufacturer, or Moss Motors.


What’s involved in doing an overdrive swap in a ’70 B? I’m swapping the engine out at the same time. Is the car already wired for the overdrive? Also, can I reuse my existing clutch?

—Dave Reparian

mgb od swap

I’m assuming your car was not originally fitted with an OD transmission. Luckily, retrofitting one is very straightforward. From 1968-76 the wiper switch also operated the OD. The switch pushes forward and back to connect power from the white ignition feed wire to the yellow OD feed wire. The yellow feed wire appears in the bundle of wires that connect to the rear harness on the right-hand side of the engine firewall where the fender meets the firewall. You will need to either purchase an OD transmission sub-harness or fabricate one. The wiring is very simple, with a yellow wire going to the Third/Fourth gear isolation switch on the shifter extension, then to the OD solenoid.

You can reuse your existing clutch if it is in good condition. However, I suggest at the very least replacing the disc and throwout bearing—these are reasonably priced and tend to wear the most.

Also plan on installing new transmission mounts because they often tend to be in poor condition. Remember to clean out the threads on the transmission (5/16 coarse) while the unit is out. Nothing is worse than installing the engine and transmission only to find that the mount-location threads are damaged.

Luckily, from 1968-80 the driveshaft can be re-used with the overdrive, making the conversion that much simpler.

Weber vs. SU

I bought an MGB that has a Weber downdraft carb on it. Would it be better to switch back to SUs?

—Andrew Johnson

weber vs su

This is a controversial and popular topic. My views are based on listening to and working with both sides of the discussion. The Weber downdraft carburetor conversion is a simple and complete way to replace the defective or worn original fuel system. It works pretty well and is easy for inexperienced mechanics to tune. Twin SU carburetors are more in keeping with the heritage of MG and the enjoyment of a British sports car. The constant-depression design of the SU carbs also better matches the requirements of the MG dual-intake port head, giving better efficiency and power. My suggestion is that you enjoy your MG and drive it as much as possible. When you are ready for an interesting project, converting to twin SU carbs will give you more of the sports car experience.

Crack-Addicted MGB?

I have a little crack that’s adjacent to the door-mounted mirror and the quarter-vent window. It keeps growing. Is this a common problem? I’ve heard that pulling the door shut by the quarter-vent is responsible—or is this just a genetic MGB weakness?

—Aaron Templeton

If you take a close look at where the crack starts, you’ll see that the door skin was folded over to give a fixing lip for the rubber glass seal. This folding process left a sharp corner with no reinforcing. Any stress on the quarter-vent causes this area to move and finally crack at the weak point. Incorrect alignment of the vent window to windshield, pulling on the vent window, or sitting on the door top all will increase the stress at this point and contribute to the crack worsening. So you are correct, the problem is a weakness found on all MGB roadsters.

6V or 12V Coils?

My ’77 Spitfire has a Crane XR-700 ignition system with a 12V coil. It runs beautifully but takes a while to start. According to the manual, I should be running a 6V coil and ballast resistor with the stock electronic ignition system. Crane’s instructions say that system is compatible with the stock coil, so does it matter whether I go with a stock 6V or a hotter 12V?

—Jim Jenkins

6v vs 12v 2

6v vs 12v

Your car was originally fitted with a 6V coil that was fed through a ballast resistor or resistance wire during regular running. During starting, the resistor is bypassed to achieve a hotter spark and fast start. Installing a 12V coil on this car without bypassing the resistance circuit dramatically reduces the output of the coil. For best performance, either keep the original 6V coil or use a high-performance coil designed for use on a ballasted system.

If you choose to install a high-performance coil, check the workshop electrical diagrams to determine if your car has a resistance wire or ballast resistor. If the high-performance coil includes a ballast resistor, this is used in place of the factory resistance to achieve correct output.

Thermostat Housing Corrosion

I’m having problems with corrosion on the aluminum thermostat housing where it contacts the radiator hose. I’ve noticed this problem on other cars but particularly on my Austin-Healey BT7. The housing only lasts about a year.

—James Robinson


Electrolysis is likely attacking the aluminum housing. If you’re using water only in your cooling system, consider going to the recommended dilution of anti-freeze, which contains corrosion inhibitors. Also, use distilled water instead of tap water to limit the amount of minerals in the cooling system.

Next time you buy a replacement housing, protect the aluminum by either having it powder-coated inside and out or sprayed with a thermal barrier coating such as those popularly applied to exhaust manifolds and headers.

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