frudemoo,

There are two basic and a couple of sub classes of Singer cams. The flat cam and the top hat cam.

Here is a pic of each on my 401:
http://i119.photobucket.com/albums/o...0pics/Cams.jpg

Some Singers require you to raise the needle out of the work, and move the stitch width lever all the way to the widest setting as you install and remove the cam. My 338 is like that and I think yours is too.
If you machine does not have any built in stitches it will need the #1 cam (if it uses flat cams) or the #O cam if it uses top hat cams in the machine to do ZZ stitches.
If there is no cam in the machine it's very easy to destroy needles. The cam keeps the needle bar from moving too far to either side.

The cams can be had from eBay and other on line sources.

Oh,and here's a site with lots of pics and descriptions: { http://www.singersewinginfo.co.uk/pattern_cam_list/ }

Joe

I have a 503a and it has the "top hat" cams. they're super. with mine you move the width to straight stitch and remove/install a new cam. the stitches are just a little fun extra. i always have a cam in, but if one isn't in, you can only do a straight stitch.

the link in your last post didn't work for me, but i think you mean that your cams are under that little trap door i was referring to.