The Seiko Calibre 4006…
Another comparison post, this time the 17 and 27 jewel versions of the 4006A calibre found in most of Seiko’s Bell-Matic watches. While the majority of this information can be found by digging in the archives of various forums, I thought I’d highlight the differences here with a few pictures.
The production run for the 4006A calibre is thought to have been between 1967 and 1977 and it was manufactured with three different jewel counts, 17, 21 and 27 jewels. When the watches were assembled, the calibre inside was clearly marked on the watch dial…
(Click pictures to enlarge)
Opening the caseback reveals the first difference between the calibres, the winding rotor is marked with the jewel count, which should match the dial markings if everything is in order.
Removing the rotor and winding bridge reveals the first technical difference, additional jewels for the third and fourth wheel pivots on the 27 jewel model.
(Click picture to enlarge)
Also notice that this particular 27 jewel movement has an extra long sounding spring. The longer spring isn’t fitted to all 27 jewel movements, I’ve only seen it on the really early models. (It doesn’t seem to make the alarm any louder).
Removing the ratchet wheels for both the mainspring and the alarm spring barrels reveals two more of the additional jewels, for the barrel arbors. This side by side comparison of the barrel and wheel train bridges clearly shows all four extra jewels.
The rest of the jewels can be found on the other side of the calibre under the dial and calendar mechanism. The remaining six jewels are all underneath the date ring, embedded into the calendar plate…
While these jewels do reduce friction for the date ring, six jewels for this task is overkill, which makes me think that Seiko may have been planning additional upgrades for this calibre. Here’s a side by side shot of the calendar plates showing the extra jewels.
(Click picture to enlarge)
When I compared the 6106 calibres in a previous post, I found similar jewels under the date ring on the earliest version, the 6106A. These jewels were then used in revisions B & C of the calibre, adding Diafix caps for the third and escape wheel pivots. As the 6106A and 4006A calibres were both introduced in 1967, I wonder if Seiko had similar plans for the 4006A…. a 4006B perhaps?
If a 4006B was planned, it never made it into production, Seiko opting instead to discontinue the 27 jewel version of the calibre in 1974 and finish the production run with just the 17 jewel version.
So, is it worth paying a premium for a 27 jewel Bell-Matic? I think it is. Though technically ‘functioning jewels’, the six jewels under the date ring don’t bring much to the party, but the additional jewels for the mainspring barrel arbor and the third and fourth wheel pivots means that the majority of the going train is now jewelled. This will reduce wear, increasing the accuracy and longevity of the movement. The 27 jewel models were made earlier in the production run, and there are fewer of them, which also increases their appeal and collectability.
I was hoping to include a 21 Jewel movement in this comparison but as they only had a production run of a year or so (1967-68), they are rarely seen. I’ve read that the 21 jewel model has a mixture of the upgrades here, with two extra jewels each for the calendar plate and the barrel and wheel train bridge. I’ll post again when (if?) I get hold of one.
** I’ve since acquired a 21 jewel model, read the post here **