|The MGA With An Attitude
DISTRIBUTOR DRIVE GEAR ALIGNMENT - IG-103
There will be a long list of people who have a problem with misalignment of the distributor drive gear, whether they did it themselves of inherited it from the DPO. A misaligned gear can cause great confusion in setting up orientation of the distributor body and the position of spark plug wires. When the gear is properly aligned, most anyone can install the plug wires correctly on the first pass. When the gear is misaligned, even an experienced mechanic will have problems figuring out where to put the plug wires and how to orient the distributor body. That might mean you get a larger bill for the time required to service your car. In the long run it will likely be easier (and cheaper) to put the gear right rather than to fuss with it each time the car is serviced.
There are a few reasons why so many of these gears are misaligned. Part of it stems from a difference of dialect between British English and other languages (including American English). Part of it stems from the way the shop manual is organized. Part of it comes from people who don't bother to read the shop manual. But even when they do read it, it can be confusing or misleading if you do not pay very close attention. The drive dog on the bottom end of the distributor shaft may also be disoriented.
In the section discussing engine assembly and installation of the cam drive sprockets and timing chain there is a fairly clear picture of the sprockets aligned using punch marks on the face of the sprockets. The book mentions positioning the crankshaft at TDC. What is not mentioned (or is commonly overlooked) is that this orientation of the sprockets (and camshaft) has the engine with #1 cylinder at the end of the exhaust stroke, and #4 cylinder on the compression stroke.
A few pages farther on the book details installation of the distributor drive gear. The very first sentence says, "Turn the engine until No. 1 piston is at T.D.C. on its compression stroke". For the unsuspecting, this requires one complete rotation of the crankshaft from the position it was in when installing the timing chain, so the camshaft rotates one half turn. If you do not do this, but continue to follow the instructions, the dizzy drive gear is likely to be installed 1/2 turn out of position.
The book also does not mention the intended (original) orientation of the distributor body or the final orientation of the rotor. This might be intentional, so the instructions may lead you to proper connection of the spark wires and proper final spark timing even if some things are assembled improperly. But getting it right and being consistent from one car to the next is important for ease of servicing. So for the record, the intended orientation of the distributor body has the vacuum unit on the left side and pointing nearly straight up, and the rotor should be pointing roughly in the direction of #1 spark plug (with #1 cylinder at TDC on compression). Then the high tension wires are installed accordingly, starting with #1 wire nearest #1 plug, and commencing in firing order 1-3-4-2 anticlockwise around the distributor cap. For the side entry distributor cap this may require a closer look. The #1 terminal inside the cap is connected to the bottom wire on the right (assuming the drive gear is installed correctly).
If your rotor is not pointing in the right direction, you should realign the drive gear. To do this start by removing the distributor and the clamp plate, and then remove the base casting from the engine block. Remove one large flat head screw, and give the casting a good firm twist and pull to break it loose for removal. Use a long 5/16" fine threaded bolt or stud for a handle for the gear. A tappet cover stud works well for this. Screw the stud well into the drive gear so it cannot possibly come loose in handling, and pull the gear out. Do NOT drop the gear into the engine, or you may find yourself removing the oil pan to retrieve it.
With the engine at TDC on compression stroke for #1 cylinder, hold the gear with the slot positioned horizontally BELOW the centerline, and insert the gear into the engine. Notation in the book "large offset uppermost" means the larger part of the split hub should be on top (slot slightly below center). Insert the gear. When the helical gear teeth engage the mating gear on the camshaft, the gear will rotate slightly anti-clockwise until fully seated, at which time the slot should be pointing to the 1:30 o'clock position, approximately in the direction of #1 spark plug. There are 9 teeth on this gear, so if you miss by one tooth it will be 40 degrees out of position (nearly straight up or nearly horizontal), so it's not difficult to get it right. Unscrew your handle, being careful not to dislodge the gear. Then reinstall the base casting back into the engine block with the flat head screw, and reinstall the distributor.
Having done this, if you find the rotor pointing away from #1 spark plug, the drive dog on the distributor shaft would be installed backward. To correct this, punch out the retaining pin, rotate the drive dog 1/2 turn, and reinstall the pin. When finished the rotor should be pointing toward #1 spark plug, still with #1 piston at TDC on the compression stroke (keep repeating that). And having fixed all of this, you may subsequently need to correct the position of the spark wires in the distributor cap. Once you get all of this right, future servicing will be much easier (being much less confusing), and various distributors and distributor caps should be easily interchangeable.
John Twist has a nice video on this process.