The 4AD is the most common Singer in the U.S. here is a variety of information on the 4AD also known at the SM1500 Roadster

For starters grab this PDF it is a copy of a Motor Trader article on maintenance of the 4AD, when I purchased my 4AD back in the 1974 it was the only manual I had and I got my car on the road with this article. (you will need Adobe Acrobat Reader for this)

• 4AD and SM1500 pictures
• 4AD Rear Axles
• 4A Steering Box Overhaul
• Serial Numbers Where to find them and what they are.
• Singer 4AD DashBoard Instrument Panel Layout
• Setting Valve/Camshaft timing The happy (or smiley) face method.
• SINGER 4AD mechanical details This is the exploded component drawings.
• SINGER Post war dating
• SM Roadster Series 4AD Maintenance and lubrication schedule.
• Solex 30 FAIO The Singer 4AD Carburator.
• Solex 30 FAIO Parts List
• The Armstrong Lever Action Shock
• 4AD Sales Brochure, this is also in the Period Advertisements area, but it is 4AD specific
• 4AD Wiring diagram… Warning, this file is big ! 255K I had to do that to keep the quality. At 9600 it will take 3 minutes 37 seconds, 14.4 it will take approximately 2 minutes 55 seconds to download. At 28.8 it will take 1 minute 31 seconds, 33.6 it will take 1 minute 18 seconds. This assumes a clean line to my server, since there will be many hops and bottlenecks, its likely to take longer (I am leaving the modem information just to show how old this site is when we had to worry about those things)
• Warning, The identification plate on the firewall. This file is HUGE, 107MB’s, yes 107 MB’s, I left it that big as a TIF file in case you want to make a new one, you can use this original for your pattern. It will need some cleaning up since my scanner left some spots on it but it will get you everything you need. And if you make a new one make sure you tell all of us.