Drill Bars Explained: Snatch Block, Steam Boat and the Types Wescon Makes
By the Wescon Engineering Team · Last updated October 8, 2026A drill bar holds a drill bushing directly over a hole so a drill or reamer runs true instead of following the old hole. Aircraft mechanics call it a snatch block, a pull bar or a steam boat. It is the tool for realigning a hole that came out crooked or does not line up with the frame underneath.
It is easy to run into alignment issues when drilling long or large aerostructure skins and panels. Picture lining up a row of holes along the edge of a wing panel. Even careful work can leave a few holes misaligned, which causes two problems:
- The hole is angled, so the rivet cannot sit flush with the surface.
- The hole does not line up with the matching hole in the frame underneath it.
Photo: Wescon Industries Inc., CC BY 4.0How an Adjustable Drill Bar Works
When a hole comes out angled, or does not line up with the one beneath it, fixing it is called pulling the hole. Drilling freehand to correct it is difficult even with a steady grip and good drill gun posture, because the bit follows the path of the existing hole instead of cutting a new true one.
A drill bar holds a bushing directly over the true location, so you can ream the hole oversize or clean it up and bring it back to its intended center.
The bar has two parts: a slotted segment that clamps to the structure, and a bushing holder segment that carries the bushing.
Using an adjustable drill bar
- Mark the true location of the hole, and have the drill gun ready with the correct bit installed.
- Align the slotted segment over the neighbouring holes.
- Fasten the bar to the aerostructure with temporary fasteners or cleco pins.
- Insert the correct drill bushing into the bushing holder segment and secure it.
- Check that the bushing sits directly over the true location mark, then drill.
Remove the temporary fasteners when the drilling is done. The hole is realigned and ready for the next step.
How to Choose Your Adjustable Drill Bar
Step 1: Choose your bushing type
Decide which bushing type you will use, or check what your workplace already standardizes on. The two main options are Slip/Fixed (SF) renewable bushings, sold in the aircraft trade as slip fit bushings, and threaded bushings.
Step 2: Choose your housing type
Look at where the clecos will sit and where the misaligned hole is, then pick the housing that reaches it.
Single ended. Holds one bushing. The basic snatch block, for a flat work surface where the hole lines up straight.
Double ended. Holds two bushings, the larger size on one end and the smaller on the other, so one bar covers two jobs and there is one less tool in the kit.
Up-down swivel. The bushing holder segment moves up and down, keeping the bushing square to an angled surface while the slotted segment clamps to the surface beside it. Use it when the hole sits at a different angle or height than the clamp points.
Left-right swivel. The bushing holder segment pivots side to side, centering the bushing over a hole offset from the line of the slotted segment. Use it when both the clamping holes and the misaligned hole sit off to one side.
Drill Bar Selection Tables by Bushing Size
Check which bushing type you use, then read the matching table. Table 1 if you use an SF bushing, Table 2 if you use a threaded bushing.
Table 1: SF bushing drill bars
A drill bar takes a 1/2 inch long SF bushing. Read the ANSI code of your bushing, in the format SF-XX-X, and find the matching bar below.
| SF bushing specs | Single ended | Double ended | Up-down swivel | Left-right swivel |
|---|---|---|---|---|
| SF-32-8 (1/2" OD x 1/2" long) | WES5800-1 | WES5801* | WES5802-1 | WES5803-1 |
| SF-48-8 (3/4" OD x 1/2" long) | WES5800-2 | WES5801* | WES5802-2 | WES5803-2 |
* One end of this drill bar holds the smaller bushing size, the other end the larger size.
Any standard SF drill bushing to ANSI B94.33 matching those dimensions fits these bars, not only ours.
Bore range is set by the standard, not by us: ANSI B94.33 allows 1/16 to 11/32 inch in SF-32-8, and 5/16 to 9/16 inch in SF-48-8.
Table 2: Threaded bushing drill bars
Check the thread size of your bushing and find the matching bar below.
| Threaded bushing specs | Single ended | Double ended | Up-down swivel | Left-right swivel |
|---|---|---|---|---|
| 3/8"-16 (WES5501) | WES5900-1 | WES5901* | WES5902-1 | WES5903-1 |
| 1/2"-13 (WES5502) | WES5900-2 | WES5901* | WES5902-2 | WES5903-2 |
* One end of this drill bar holds the smaller thread size, the other end the larger size.
Any standard 3/8-16 or 1/2-13 threaded drill bushing screws in, not only ours.
What to Look For
Material and weight matter, especially if you are holding the bar up for any length of time. Wescon bars are anodized aluminum, about half a pound. A bar that takes SF bushings will accept bushings from any maker.
Wescon adjustable drill bars are made in USA and ship in about four days. Pick the housing that reaches your hole, in an SF or threaded version, from the drill bar catalog | Contact our engineering team
Read next: the same bushings in a drill guide
The SF bushings that go in a WES5800 drill bar also fit Wescon's egg cup bushing holders and dog bone drill guides, so one bushing set covers both tools. Read how to choose an egg cup bushing holder.
