Understanding the Issue
In a detached lever escapement, drop lock (the initial locking depth when an escape wheel tooth lands on a pallet jewel) is too shallow on both the entry and exit pallets. Yet the drops (the free motion of the escape wheel after unlocking) are equal and correct, implying the basic geometry (pallet span and escapement depth) is set correctly. However, the slide – the additional travel of the pallet jewel along the escape tooth after lock, as the lever moves to the banking – is excessive on the entry side, while the exit side’s slide is normal. Fork horn and guard pin clearances are correct, so the safety action isn’t the culprit. This combination (shallow locks both sides, too much slide on entry) points to a mis-adjustment in the pallet jewel positions rather than an issue with banking pins.
Cause: Pallet Jewel Position Imbalance
In a Swiss lever escapement, the locking depth on each pallet (entry and exit) is governed primarily by the pallet stone positions in their slots. If both pallets have insufficient lock, it usually means the stones are set too far into the pallet fork (not sticking out enough into the escape wheel’s path). This shallow engagement can make the escapement “unsafe” (prone to unlocking too easily). At the same time, an imbalance in stone positions can cause one side to have more slide than the other. In this case, the entry pallet likely has too much slide because the opposite (exit) pallet stone is a tad too far inside the pallet (shallow). When one pallet jewel is set incorrectly, it affects the other side’s lock and slide due to the lever’s geometry.
- Drop lock (1st lock): Ideally, when an escape tooth locks on a pallet’s locking face, it should penetrate about 1/5 to 1/6 of the pallet jewel’s face width (high beat movements are less). In our case it’s less on both sides, indicating inadequate locking depth.
- Slide (run-to-banking): After the tooth locks, the lever continues to move to the banking pin, and the tooth slides along the pallet’s impulse face (this is often called “slide” ending at total lock). Normally, the total lock (drop lock + slide) should be about 1/3 of the jewel’s face. Excessive slide on the entry side means the lever is traveling too far beyond the initial lock on that side. This often happens when the opposite pallet stone isn’t providing enough lock, forcing the other pallet to compensate with extra travel.
Corrective Adjustment: Move the Opposite Pallet Stone Outward
To restore proper escapement function, adjust the pallet jewel positions rather than the banking pins. In particular, focus on the exit pallet stone, since the entry side is the one with excessive slide. The standard remedy for too much slide on one pallet is to move the opposite pallet stone slightly deeper into the escape wheel’s path. In this case, that means shifting the exit pallet stone outward (toward the escape wheel) by a very small amount. This adjustment increases the exit stone’s locking depth and, due to the lever geometry, simultaneously increases the entry pallet’s lock as well. Its effect, the entry pallet will engage the escape tooth a bit sooner and with more Drop lock (1st lock), which reduces the amount of slide needed on the entry side. See the image below for the net effect. Basically, what is happening here is that total lock does not change because total lock is determined by the banking. What does change is that when the drop lock on the entry side increases, some of the slide is taken away from the total sum of the drop lock + slide distance.
Why this works
Moving one pallet jewel alters the angle at which the next escape wheel tooth is caught by the other jewel. By moving one stone out it will increase the drop lock on the other. By moving the exit stone further out, the escape wheel tooth on the entry side will lock a bit earlier and deeper.
Here’s how moving one pallet stone out affects the slide on the other pallet stone:
• If you “Pull out the entry stone Only” (move the receiving pallet stone outward):
◦ The slide on the receiving (R) stone “remains the same”.
◦ The slide on the exit (L) side “decreases”.
This is the opposite of what we want to do.
• If you “Pull out exit stone (L) Only” (move the discharging pallet stone outward):
◦ The slide on the discharging (L) stone “remains the same”. Remember it was set correctly.
◦ The slide on the receiving (R) side “decreases”. Which is what we want.
This means that:
• Moving the receiving (entry) pallet stone outward will decrease the slide on the discharging (exit) pallet stone.
• Moving the discharging (exit) pallet stone outward will decrease the slide on the receiving (entry) pallet stone.
Remember the potential total look has not changed because the banking pin does not move. By increasing the lock on the entry side, that increase in the drop lock just takes away from what slide was present on the entry side. Again look at the image.
This fixes the shallow drop locks (1st lock) on both sides since the exit stone itself now locks deeper, and the entry stone benefits from the exit stone’s new position and corrects the entry slide excess. Basically, the exit stone’s outward adjustment compensates for its previously insufficient lock, bringing the escapement back into balance. The result should be roughly equal lock depths on entry and exit (around 1/5 of the jewel face at initial lock), with a modest, equal slide to the banking on each side (total lock ~1/3 of jewel.
Why Not Adjust the Banking Pins?
Because the drops are already equal and correct, the banking pins are likely properly positioned for drop. Adjusting banking to get more lock would also alter the drop symmetry (inside drop & outside drop) and isn’t the best solution here.
In fact, banking pins primarily limit the lever’s swing and set the drop, but lock is determined by pallet stone protrusion. If both locks are too shallow, it indicates the stones themselves need repositioning rather than simply widening the banks.
By correcting the pallet jewel depth, you retain the correct drop on each side while achieving proper lock. Once the pallet stones are set for correct lock and slide, then you can fine-tune the banking pins if necessary to eliminate any excess drop without disturbing the lock (in this scenario, however, in this case the drop was reported “fine and equal,” so banking adjustment is probably not needed).
Implementation and Caution
Adjustment of pallet stones is delicate. Even a .005mm change is significant in escapement geometry. The exit pallet jewel should be moved very slightly outward. This is typically done by heating the pallet fork (to soften the shellac) and nudging the jewel with an appropriate tool.
After adjustment, re-check the escapement thoroughly: each pallet should now show a healthy lock (around 20% of impulse face) when a tooth lands, and a slight additional slide as the lever moves to the banking (bringing total lock to ~30% of the impulse face). The entry-side slide should no longer be excessive once the exit stone is corrected. Both entry and exit locks will be deeper (safer), fixing the original issue of insufficiency.
Finally, ensure the escapement runs smoothly: the watch should unlock and impulse correctly on each beat without flutter or tripping. By focusing on the pallet stone adjustment – in this case, moving the exit pallet stone outward to increase lock – you target the root cause of the shallow locks and unequal slide, thereby restoring proper escapement function without compromising the equal drop or the safety action.
