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I've always considered Japanese aerial bomb fuzes to be almost art forms (go figure).

Type D-2(a) & B-3(a) Aerial Bomb Tail Fuzes - Navy
Here are the Type D-2(a) and the Type B-3(a) bomb tail fuzes used by the Japanese Navy.
D-2(a) on the left is a mechanical time fuze for air burst used on the T-99 naval incendiary.
Made of brass with an internal steel mechanism.
This type fuze was introduced prior to 1940. There are three variations of this type (A, B & C).
It is marked with the Imperial Japanese naval anchor.

B-3(a) on the right is a double action impact type used for heavy (500 kg) air-dropped ordnance.


D-2 Cutaway Diagram

This shows the B-3(a) fuze an activated state. The prop unscrews while falling thereby freeing the arms which  fly forward on impact which drives the firing pin (a big one inside) forward. The firing pin is also driven forward if the arms are pulled to the rear.  There are holes for other safety or shear wires to restrict the firing pin motion. The big cotter pin (see the previous photo) is obviously the main safety, removed before flight.
 
Type A-6(b) & B-1(a) Aerial Bomb Fuzes - Army


At first glance, these appear to be nose impact fuzes of the same design, just different sizes. They both have a firing pin with a threaded end. Once the prop unscrews the firing pin (held up by a creep spring) is free to move down and initiate the detonation. Pretty straight forward.

The small one, A-6(b), is from the Type 3 1/2kg Cluster Munition and is a nose fuze. It unscrews as air flows past and once fully unthreaded it remains captured at the end of the rod, which makes sense as you would want this to activate even if it hit soft earth.


The larger one (B-1a) at first glance seems to be the same, except of one big difference... the pitch of the air-screw is reversed. This one would tighten down as the air flows through it if mounted on the nose, so it is a tail fuze. On this one, the vane completely unscrews and departs. The remaining piston/firing pin is free to move forward on impact. It was used on 50-100Kg HE types and typically was in used conjunction with a nose fuze.
Diagram of B-1(a) Fuze with Detonator
Army and Air Force 1953 training manual:
TM 9-1985-4


Markings

 

D-2(a) with the Kanji symbol "Ko", probably signifying "Aerial" or "Air Service".
Same as found on the Type 2 Sub-Munition.
B-3(a) Markings. My guess is this is an arsenal marking preceding the serial number. No I.D. yet.


Here are the remains of the label on the time fuze
and the picture a complete one:
In vertical columns, right to left:
to-kei hatsu-jo ma-i-te a-ru
"Clock Spring is Wound Up"

Thank you Yanaka & Teri!

So it would appear that this fuze has an internal spring-driven clock mechanism which is activated, (rather than being powered), by the spinning wind vane.



 

Both have the Imperial Naval Anchor and the Kana symbol "To" in a circle.
This is the incendiary tail fuze.

05.02.06
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