DYNAMO CHECKS AND SWAPS
(from the Bristol Austin 7 Club Magazine)
The first tip is to have a good browse through the two excellent sets of articles that appeared in the A7 Association magazines. The first, by Harry Hales, is titled “A closer look at Dynamos, Cut-outs and Switch Panels” and is to be found in 1979 C & D. The second, under the title “Electrickery” is by Monty Northam and appears in 1985 A, B, C 7 & D. All models are covered and there are plenty of illustrations. If you haven’t got the magazines there are plenty of people who would willingly get you copies of the articles.
The second tip is to make sure you can easily remove the dynamo for closer inspection, or swap it for your spare. Having a good spare is a good idea, especially if you are intending to maximise its load with “brighter” lights etc.
The aluminium support which carries the dynamo, has a long stud (just possibly a long bolt) as one of its four fastenings to the crankcase/front cover. On the later dynamos the bearing housing at the cam gear end has a conical section which allows the dynamo to be withdrawn without fouling this stud. Disconnect the wires to the distributor, undo two long bolts and a short one holding the housing and that’s it, you’re free to lift out the dynamo. The earlier type is cylindrical rather than conical and has a groove in the housing, for the stud. This traps the dynamo and requires the aluminium housing to be detached. The dodge is to file out the housing in line with the bottom of the groove so that it clears the stud.
When fitting a dynamo, make sure the two long bolts have slotted heads so that you can put them in with a screwdriver. Put the top bolt in first to take the weight and locate the gasket. Nip up the long bolts, fully tighten the top one and then, using a hefty screwdriver, finally tighten the bottom bolts. Use this method even when access is good (e.g. engine out of car) to minimise the hassle when removing with engine in place. If you have pulled the bolts up tight with a tube spanner you may not have this piece of kit when the chips are down!
To avoid messing up the timing when checking out the dynamo or doing a swap, remove the distributor cover and note/sketch the exact position of the rotor arm. Don’t turn the engine during the swap and when engaging with the cam gear on reassembly make sure the rotor arm lines up as per your earlier sketch.
If you are rebuilding a dynamo for a coil ignition engine, its worth remembering that the later brush gear housing on the C35M type can be swapped for the housing on the C35A. The latter takes tagged brushes and they are much easier to fit than the ones with the horseshoe shaped slots. If you do have these earlier ones, and are struggling to fit them, you may find it helps to back off the long bolts sufficient to ease the housing off its 7” long locating pin and rotate it to get better access. If either of the locating pins aren’t there it is most advisable to get them reinstated. If the housings move relative to each other, or even oscillate, the timing will be upset (variable!) and you may not even know why!!
A useful tool for changing a standard ½” i/d x 5/8” o/d Oilite bush can be made from a 6” length of 5/8” diameter bar. Take one end down to a couple of thou’ under ½” diameter then take a couple of thou’ off (say) a further 2” length. If you have an engineer’s vice, you will be able to wind the old bush out and the new one in using this mandrel. Don’t forget to give the new bush a long soak in oil before fitting.
If, however, you have procured a steel backed split bush, first check if it will slide/rotate on the armature journal. The chances are that you will need to use an expanding reamer to achieve the correct running fit.
Thanks to the BA7C for this article which first appeared in the excellent magazine