My 401A machine, "Trouble", did not have a tension unit when I bought it. I've gathered up spare parts and this video shows how to install and "zero out" the needle thread tension unit. Includes rust removal and cleaning and oiling of the tension unit when needed. A good tutorial to increase your knowledge of SInger tension units, their maintenance, installation, and adjusting. Singer models 401A, 403A, 404.
ALL ABOUT TENSION PLAYLIST:    • ALL ABOUT TENSION on Vintage Singer S...  
The BOBBIN TENSION METER I use in the video can be found here:
https://singer-featherweight.com/coll... and at other online sellers.
KRUD KUTTER CLEANER/DEGREASER: https://www.rustoleum.com/product-cat...
MAGNITIZER: https://www.homedepot.com/p/Husky-Pre...

MUSIC: SUMMER NIGHTS by the126ers
00:00 INTRO
00:50 TENSION PARTS
02:52 RUST REMOVER
05:29 TAKEUP SPRING
06:00 TENSION PLAYLIST INFO
06:45 NEW SPRING?
10:32 WASHING PARTS
14:25 PARTS OILING
17:30 OIL HOLES FOR PARTS
18:20 INSTL TENSION STUD
21:00 INSTL RELEASING PIN
24:30 INSTL THREAD GUIDES
27:50 INSTL DISCS-SPRING-GUARD
30:33 INSTL INDICATOR
30:50 INSTL BEEHIVE SPRING
31:34 INSTL STOP WASHER
32:05 INSTL TENSION FLANGE
34:18 ZERO OUT
38:08 OPTIONAL METER TEST
41:00 INSTL NUMERATED DIAL
42:50 INSTL KNURLED KNOB
44:16 INSTL CAP & SCREW
46:00 THREAD PULL TEST
47:00 REMV ENTIRE TENSION UNIT
49:55 SET TENSION ON SPRING
50:35 INSTL ENTIRE UNIT