1970s Singer bobbin gear replacement

I was asked if I could fix someone's Singer 514 because it "didn't work".

The patient

A cursory inspection told me that the plastic bobbin drive gears had broken (needle moves and bobbin doesn't when the hand wheel is turned).
This is a very common problem now since the gears last about 40 years before they become brittle enough to break apart. It's probably why these machines don't have much value now, and most people will just throw them away rather than pay to replace them (particularly here in Australia where labour costs a lot). I don't know how long replacements will last, but I imagine you'd get 20 years at least.
I've done this job before, on my daughter's 498K Stylist in 2012. As I remember, the gears for her machine were much better quality than these, but it seems I can't get good ones now.

Where the bobbin gear lived and died

I told her the bad news. Although gears are available (Chinese made, and quite poor quality), the labour cost would exceed the machine's value. She was still keen (she had bought it new in 1973) so I ordered the gear set - they must be replaced as a set because the new ones are cut differently and won't mesh with the unbroken original, and even if they did mesh the unbroken one would break very soon too.
Pre-requisites to doing this job are:
1. You have the replacement gear set in front of you (I ordered them from Amazon).
2. You accept it will take a couple of hours if you haven't done it before, and do the whole job on the same day. You might be surprised how much you can forget in just a day.
3. You know how to time the hook.
4. Have some Singer gear grease on hand.
5. An appropriate Allen key and other tools to remove circlips.
6. Take photos as you go. These pictures are specific to the 514. They're all similar but don't count on them being identical.

Gear replacement

Disconnect the power (duh), remove the machine from its base and remove the bottom plate.

The bobbin mechanical components are then exposed

It's now obvious where the broken gear was.
Our task is to remove both of these shafts and replace the gears, but it's not as easy as you'd think. This is why it takes multiple hours instead of half an hour.
I had to remove the good gear first. Start by removing the circlip at the end nearest the motor (to the right in my pictures), then everything that's holding the shaft components on, including the white plastic gear on the left. You shouldn't need to force the shaft out.

Removal required an Allen key

Here's the shaft halfway out: The eccentrics for the feed dog should just sit there, held in by the rest of that mechanism.

All free and halfway out

The other shaft was tricky. It would not exit easily, and I had to remove the motor mounting in order to get it all the way out (to the right).

Almost there.

It's obvious from this photograph why the motor needs to be removed. The change from the old external motors to these quieter ones necessitated some compromise, and removing and replacing the motor was quite a struggle.
Once the two shafts are out, replace the gears and reassemble. Here are the new ones installed:

New gears installed

That big pointy bit of plastic fouls the movement of the other gear and causes a rough spot on the machine. You might be able to carefully remove a little to eliminate the rough spot, but I left it alone.
This is why I mentioned that the replacement gears are poor quality. It's not hard to make a copy of something this simple, but the people tasked with it still managed to screwed it up.
However, the machine does work acceptably now, where it didn't before, and the stitch quality is as good as it was before the original gear's demise.
After you've got everything back together you will need to correct the machine's timing, because you will definitely have changed to to unacceptable.

Feed timing

Make sure the feed dog is doing what it's supposed to be doing.
When the needle descends to the work, the feed dog will be moving downwards. If I remember correctly the offending plastic protrusion points towards a line etched into the feed timing eccentric (both of which can be seen in the picture above, taken before I'd replaced the latter's grub screw). Align these and you should see correct feed timing. If not, set it manually.

Hook timing

The settings are pretty universal for hook timing. You want it set up so that as the hook passes the needle, the needle has started to ascend and the needle's eye is 3/16" (2mm) below it, the needle position set to the centre. My friend Tammi has much more detail on this on her web site.

This is the setting for every sewing machine