Movement Stories: Bulova 7AP - Assembling a “Basic” Movement
In my ongoing series exploring vintage watch movements, my focus has been on movements with interesting design quirks or features, but I thought it might be helpful to go over a very “vanilla” watch movement, with a typical design and no complications whatsoever. Accordingly, the subject of today’s post is the Bulova 7AP, a very simple but quality movement manufactured in the late 1930s. When I received it it had clearly been serviced previously (by someone a bit careless with their screwdrivers), but not for many years. It was dirty and hopelessly gummed up, but although it wasn’t running at all everything looked to be intact. So, following disassembly and a thorough cleaning, it was time to put things back together. Hopefully I didn’t hurt its feelings by calling it vanilla…
Assembling the Wheel Train
If there’s one thing about this movement that is a bit unusual, it’s the lozenge shape, as it was designed to power similarly shaped tank-style watches. It still has a very simple layout, with the train wheels arranged in a nice gentle arc from the mainspring barrel. The sequence of photos below show the bare man plate, the barrel and train wheels in place before the bridges are installed, the installed bridges, and finally everything after installation of the parts that allow the watch to be wound: the crown wheel, ratchet wheel, and click:
One thing to note here is that the 4th wheel, which carries the sub-second hand, sits above the 3rd wheel, with its pinion facing down. You will also see similar movements where the 4th wheel sits below the 3rd wheel with the pinion facing up. I’m honestly not sure what the design trade-offs are here, although I suppose the pinion-facing-down arrangement, as in the 7AP, may ease manufacturing of the 4th wheel by making installation of the brass gear onto the steel pinion a bit simpler.
Another thing you may not have noticed is the little mark visible in the last two photos on the right side of the barrel bridge, below and slightly to the left of the bridge screw. This is a special symbol that Bulova put on their movements manufactured from the mid 1920s through the mid 1940s indicating the year of manufacture. This mark tells me that this particular 7AP was manufactured in 1939.
About those jewels…
Just an aside about jewling in these “basic” watches. You can think of 15 jewels as something of a baseline for quality watch movements. These 15 jewels cover the escapement (9 jewels) plus top-and-bottom jeweling of the 3rd, 4th, and escape wheels (6 jewels). This is sufficient to ensure good timekeeping performance. However, for reasons that may be lost to history, a 17 jewel count became something of a gold standard for mechanical watch movements. So, where to place those extra 2 jewels? Since they aren’t really that critical to the performance of the watch, designers had some choices here. What you will see most frequently is jewling of the center wheel pivots, capping of the escape wheel or other train wheel pivots, or some combination. The 7AP takes the combination approach: the upper pivot of the center wheel is jeweled, and the lower pivot of the escape wheel is capped, bringing the jewel count to 17. Personally, I prefer it when both pivots of the center wheel are jeweled, as opposed to capping escape wheel pivots. That’s because, while neither of these things have much impact on timekeeping performance, jeweling the center wheel pivots is good for movement longevity. However, I suspect jeweling the large-diameter center wheel pivot hole in the main plate was probably the most expensive option for adding an additional jewel, so use of cap jewels to bring up the jewel count is commonplace.
On to the Keyless and Motion Works
With the train of wheels in place, it’s time to flip the watch over and reassemble the keyless works and motion works. First, the winding pinion and sliding clutch are installed, followed by the stem and setting lever, with grease applied to the interfaces between all of these parts. Next, the yoke, canon pinion, minute wheel, and motion works intermediate wheel are installed. At this point the center and 3rd wheel pivots on the dial side need to be lubricated, since they will be covered up. Next the yoke spring is installed. This is under a good bit of tension, so the setting lever spring/top plate is installed immediately thereafter, before anything goes flying. Note that the interfaces between the setting lever & yoke, setting lever & setting lever spring, and yoke & yoke spring are all lubricated with a bit of grease. This assembly sequence is illustrated in the photos below:
The Escapement
With that done, it’s time to flip the movement back over and finish things off by assembling and installing the escapement. First, the pallet fork, which has been soaking in a bench solvent for cleaning, is installed. After giving the watch a bit of a wind to check that the pallet fork is operating correctly, the pallet jewels are lubricated with a tiny bit of Moebius 9415:
My practice for non-shock-protected movements, such as this one, is to remove the balance and disassemble the balance cock before cleaning to ensure everything gets thoroughly cleaned. So, now, the balance cock needs to be reassembled:
With that out of the way, I lubricate the upper balance jewel (I usually “cheat” and do this with an automatic oiler after assembly, as keeping a drop of oil in place on the cap jewel while screwing everything together can be a real hassle), reinstall the balance complete on the cock, and lubricate & reinstall the lower cap jewel. Finally, the balance assembly is reinstalled, and…….
Voila! Back from the dead and still going strong after 86 years. All it took was a thorough service.
UPDATE 04/06/25 - The watch whose movement was the subject of this post is now for sale in the shop:
