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Westclox Mainspring Chart

Mainspring sizes for Big Ben, Baby Ben and other old Westclox alarm clocks.

I provide this chart to assist those who repair old Westclox clocks. ClockHistory.com is not a parts supplier - please do not ask us for parts. The fine clock parts suppliers listed on my links page will be happy to send you a catalog if you would like to order mainsprings.

If you know of anyone that sells old Westclox parts, and I will put the information on this web page.

America (ca. 1927)
Time spring Same as Big Ben Loud alarm
America (ca. 1927) Alarm spring Same as Ironclad
Bunkie and Sleep-Meter Intermittent Time spring Same as Big Ben Loud Alarm
Bunkie and Sleep-Meter Intermittent Alarm spring 5/16" x .013" x 48"
Baby Ben (mov't. 1, 1912 to mid-1914) Time spring 6.2 mm (just under 1/4") x .0102" x 29" hole end

New replacement: 1/4" x .010" x 26" loop end, convert to hole end. (Note: 29" long would be better, but I haven't found one available. The 30 inch long spring will run the clock for about 30 hours.)

Baby Ben (mov't 2 - 4, mid-1914 to 1960) Time spring 5/16" x .0087" x 30" hole end (measured in type 2 & 3 movements)

5/16" x .009" x 30" hole end (SPR-3414 factory material)

New replacement: 5/16" x .009" x 36" loop end, cut off 6" and make hole in end. (CMH-80.2 or 80.3)

Baby Ben (all two key wind) Alarm spring 7/32" x .0082" x 27" hole end (measured in type 2 movement)

7/32" x .0087" x 27" hole end (SPR-3246 factory material)

New replacement: 7/32" x .008" x 26" hole end (CMH-56.1)

Baby Ben single key wind (mov't. 5)   9/32" x .0085" x 41" hole end
Big Ben Loud Alarm (mov't. 1 - 2, 1908 to 1956) Time spring 5/16" x .015" x 40" hole end (CMH-80.13)
Big Ben Loud Alarm (mov't. 1 - 2, 1908 to 1956) Alarm spring 1/2" x .0138" x 40" loop end (style 1 & 2 cases)

1/2" x .0138" x 32 1/2" loop end (style 4, 5 and 6 cases)

Big Ben Chime Alarm (mov't 3, 1931 - 1956) Time spring Same as Baby Ben mov't 2 - 4

5/16" x .0088" x 30" (SPR-3418 factory material)

Big Ben Chime Alarm (mov't 3, 1931 - 1956) Alarm spring 9/32" x .0105" x 20" loop end (measurement of factory original spring) (CML-70.6 is close)
Clock of Tomorrow   5/16" x .010" x 72" (CMH-80.10)
Ironclad Time spring Same as Big Ben Loud Alarm
Ironclad Alarm spring 3/16" x .0155" x 25 3/4" hole end (measured in clock)

New replacement: 3/16" x .012" x 31" loop end (CML-48.4), cut off loop and make a hole

Model 66 (Bingo, Raven, Waralarm, etc.) Time spring 5/16" x .0125" x 24" loop end
Model 66 Alarm spring Type one: 1/4" x .008" x 18 1/2" loop end

Type two: 1/4" x .0085" x 14" open loop end. SPR-3254, model 66AH. This type of spring may be installed without dismantling the movement.

Sleep-Meter (also fits Black Bird, Blue Bird, Ironclad, Jack o'Lantern) ca. 1924 Alarm Spring 7/32" x .01" x 40" loop end (measured in Black Bird dated 1924 on movement.)
Travalarm   5/32" x .0093" x 35" (measured in 12-65 example). Can use CML40.3 (5/32" x .009" x 38 1/4") from S. LaRose and shorten it by 3".
Two-inch movement (time only)   7/32" x .012" x 29.5" loop end (measured in clock)

New replacement: 3/16" x .012" x 31" loop end (R & M and Timesavers)

Compute the correct length of the time spring for the Big Ben Loud Alarm:

Length = Pi(B squared - A squared)/2t

where Pi = 3.1416 B = barrel radius A = arbor radius t = mainspring thickness
In this case: B = 18 mm A = 4.65 mm t = .015"

Doing the computation, Length = 49.2". If a spring this long were used, the clock would run longer on a winding than with the standard 40" spring.

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