Starter Motor Problem with 12/4

[Neil McL]

I'm hoping for some advice with regard to a starter motor problem on my Austin 12/4 (1926, short stroke). The starter used to work fine, then sounded slightly different on a few trips, and then stopped working. I can hear the motor spinning, but it does not engage to turn the engine. I have charged the battery, so I assume that there's no shortage of power.

I have removed the motor and the whole spiral-drive assembly just rotates freely around the motor spindle. I have not inspected a starter motor before, but I think that I understand the typical principle of operation for engagement. There is quite a large gap between the end of the spiral-drive assembly and the housing which seems a bit odd. Is that due to wear of the housing and the end of the spiral-drive assembly? Any suggestions on the nature of the problem and how to dismantle the motor / fix it ?

Thanks,
Neil

[Neil McL]

The system keeps deleting my photos without warning ! Here's the flywheel teeth:

[Media-Moderator]

Have spoken to Tony Smallbone about this? He may be able to tell you if the gap you describe and show in the pictures is correct.

[Martin27]

Hello Neil
Looking at the photo of the drive (the one showing the large gap) you can just see the head of a hex head bolt which attaches the end of a coil spring to the sleeve with the spiral. My guess is that this spring is broken. If you find that this is the case you will be pleased to hear these are usually available from VAR stores. The purpose of the spring is to provide a "cush drive" ie it softens the impact as the drive pinion hits the flywheel and drive is taken up. Be aware that these springs are available as either right or left hand. I think yours will be right handed. Have a word with Steve Newton. Make sure you get the correct one!

Best regards
Martin

Edit
LEFT handed on a clockwise motor. The spring coils then tend to tighten under load.

[Martin27]

Hello again Neil
To dismantle. Look at the back of the motor you will see two screw heads. These are the through bolts. Unscrew these and the drive end casting will pull away. Since the drive spring is broken (we assume) the spiral sleeve and pinion will pull off the armature shaft and you will easily see the problem. On reassembly, lightly grease the armature shaft before replacing the sleeve. The pinion and spiral is supposed to run dry and clean.

Hoping I'm not trying to teach grandmother how to suck eggs!
Martin

Edit
There are two devices designed to prevent the pinion running up to the flywheel under gravity. The pinion hub is machined away on one side so as to make it heavier on one side than the other. Gravity then holds the pinion with the heavier side at the bottom. Also there is a small spring and a plunger in the pinion arranged so that the plunger engages with a depression in the spiral sleeve which should prevent accidental movement. Make sure you don't lose these, although if you don't separate the pinion from the sleeve you won't see them.

[Neil McL]

Thanks for all the advice. I have dismantled the motor now and the coil spring was indeed broken which I have discussed with Tony Smallbone and ordered a replacement from VAR Spares c/o Steve Newton.

The sleeve which sits underneath the coil spring is fractured too, but it looks okay to reuse for now and I will hopefully be able to turn a replacement later this year. The middle bearing is a rather sloppy fit so I'll see if I can find a replacement for that. Tony also mentioned the need to shim the motor shaft to control its axial alignment and that the shimming washers have often been "lost" over the years.

Martin's final comment provides a very useful insight since I had wondered about the purpose of both the non-symmetrical machining of the pinion and the plunger - a very "neat" design.

Thanks,
Neil