At 11:22 AM 8/3/04 -0600, Andrew Martens wrote:
>"Must one loosen the distributor clamp plate bolts AND the pinch bolt to adjust the timing?"

Only if the plate is bent or installed wrong. Slots in the clamp plate are to allow for some deformation of the plate and still fit without binding on the two base bolts.

>"Why the elongated holes in the clamp plate?"

The slots may have nothing to do with adjusting timing (depending on your point of view). If you had easy open access to the side of the engine (which you do not in the MGA, MGB or post 1960 Midget) you might loosen the bolts on the base clamp to rotate it a small amount for timing. This would be limited to the extent of the clearance in the slots. The MGA and early MGB distributors have a knurled thumb knob on the vacuum advance unit that allows for fine adjustment of timing without loosening any bolts, assuming it's in the ballpark to begin with. Consult the shop manual for instructions. The primary purpose of the slots is to allow for some misalignment of the plate to the engine block without binding on the screws, even if the plate might be badly bent.

You can test fit the plate on the distributor body first, while the dizzy is on the work bench. The plate should be flat, and the sides of the slotted opening at the pinch bolt should be roughly parallel and have a generous gap. If the ears touch when the bolt is tightened, the plate is badly bent and needs straightening. Picture at right shows a badly bent clamp plate and broken housing. Click for larger image.

Note" The picture shows the plate assemble wrong side up. The bolting ears need to lay flat against the engine mounting surface, while the center part of the plate is raised slightly around the distributor base.

To straighten the plate, clamp it in a vice and bend the ears away from each other to be roughly parallel with a sizable gap. The center hole should also be close to round and the plate should rotate freely on the dizzy body. Light tightening of the pinch bolt should pull the plate tight on the dizzy body such that you cannot turn it by hand. A range of only about one turn of the nut on the clamp bolt should transition from free rotation of the clamp plate to tight enough that you can not turn the plate by hand. If you have trouble getting the plate configured to function properly, buy a new one. And in the future, be careful not to over tighten the clamp bolt. Over tightening the clamp bolt can badly damage the die cast aluminum housing.

Proper procedure is to tighten the pinch bolt first (not too tight), which will center the bore of the clamp plate on the distributor body. Then center the slots roughly on the screw locations, and tighten the two base bolts. Next time you loosen the pinch bolt the distributor should turn with light hand force only without touching the base bolts. But if the plate is bent out of shape because someone had over tightened the clamp bolt, all bets are off.