Key takeaways
- Length alone does not identify a strut. One 13.74 in interchange length is catalogued at five forces from 15 to 81 lb.
- F = (G x L x S) / b gives the total force for the lid. Divide by the number of struts; S = 1.1 is a safety factor, not a per-pair allowance.
- Strut force falls about 3.3 percent per 10 C (18 F) below 68 F (20 C), so at 14 F (-10 C) the 1.1 safety factor is already used up.
- Across ten catalogued tool box struts, compressed length runs 57 to 73 percent of extended length.
- Mount rod down in the closed position so the seal stays lubricated.
A tool box lid strut is specified by five numbers, and the one people order by, length, tells you least. One 13.74 in (349 mm) interchange strut is catalogued at 15, 29, 49, 57 and 81 lb. Order on length alone and you have a one in five chance of the right part. Below are the other four numbers, the force calculation worked for a full-size aluminum crossover lid, and the winter correction most sizing guides leave out.
The four measurements, taken center to center
Every length on a gas strut is measured between the centers of the two end fittings, not over the ends. Gastac's tool box guide sets out the procedure:
- Extended length. Pull the strut to full length and measure from the center of one eyelet or ball socket to the center of the other.
- Compressed length. Push it fully home and measure the same two centers again.
- Stroke. Extended length minus compressed length.
- Diameter. Rod over tube in millimeters. An 8 mm rod in an 18 mm cylinder is written 8/18.
Then note the end fittings at each end: eyelet, ball socket or blade, and the size. The 17 in and 14 in tool box interchanges in the Lift Supports Depot catalogue use a 10 mm (3/8 in) ball socket, and a socket must match the ball stud it snaps onto. If the old strut is missing, measure the box: lid mount to body mount with the lid shut is the longest compressed length that will fit, and the same two points with the lid at the angle you want it to stop give the extended length.
Getting the force number
Force decides whether the lid stays up. There are three ways to get it.
Read it off the tube. Gastac notes the force may be printed on the side of the existing strut. It may be in newtons: divide by 4.448 for pounds. The catalogue lists the 81 lb P10207-5 as 360 N.
Weigh it. Gastac suggests compressing the strut against a scale to read the force required.
Calculate it. For a new box, a replacement lid, or a lid that now carries a light or a sign, the answer has to come from the lid itself.
The force formula: F = (G x L x S) / b
Trans-Technik's gas spring guide gives the standard flap formula:
F = (G x L x S) / b
| Symbol | Meaning | On a tool box |
|---|---|---|
| G | Weight of the lid | Weigh it, or use the scale method below |
| L | Distance from the hinge axis to the lid's center of gravity | Half the lid depth for a plain, uniform lid |
| S | Safety factor, usually 1.1 | Use 1.1, then correct for temperature separately |
| b | Distance from the hinge axis to the strut's mounting point on the lid | Measure it with the lid shut |
| F | Total gas spring force required | Divide by the number of struts |
The guide works in newtons and millimeters, but L and b appear as a ratio, so pounds and inches work unchanged.
S is a margin, not a pair factor. The 1.1 is a 10 percent safety factor on the whole calculation. It has nothing to do with whether the lid has one strut or two. Split F across the struts after applying it.
b is treated as a lever arm at right angles. On a tool box the strut usually lies at a shallow angle to the lid when it is shut, and the lever arm it actually gets is b times the sine of that angle: at 15 degrees, about a quarter of b. Trans-Technik notes that the mounting point "affects opening speed and closing behavior through the dead point." If a lid needs a hand lift for the first few inches, that is the geometry, not a weak strut.
The scale shortcut for G x L
You need only the product of G and L. Disconnect the struts, hook a luggage scale under the free edge at the latch line, and lift until the lid is horizontal. The reading R times the distance D from the hinge to the hook is the lid moment:
G x L = R x D
For a uniform lid R is half the lid weight. Latches and T-handles near the front edge show up in the reading automatically, which is why this beats estimating a center of gravity.
Worked example: a 20 in deep aluminum crossover lid
Take a full-size aluminum crossover of the typical envelope described in crossover, side mount, chest and low profile: 72 in (1829 mm) wide, 20 in (508 mm) deep, single lid hinged along one long edge, two struts.
Lid weight. Lid sheet runs 0.063 to 0.125 in thick on aluminum boxes, and aluminum is about 0.098 lb per cubic inch (both from aluminum vs steel vs polymer truck boxes). A 72 by 20 in top with a 2 in front lip is 1,584 sq in of sheet:
| Lid sheet | Volume | Sheet weight |
|---|---|---|
| 0.063 in | 99.8 cu in | 9.8 lb |
| 0.090 in | 142.6 cu in | 14.0 lb |
| 0.125 in | 198.0 cu in | 19.4 lb |
Add the hinge leaf, latches, handles and seal, and take G = 15 lb (67 N) all in. Weigh yours: this input moves most between boxes.
Center of gravity. Uniform lid, so L = 10 in (254 mm).
Mounting point. Assume the struts' lid ends sit b = 5 in (127 mm) from the hinge axis.
Total force. F = (15 x 10 x 1.1) / 5 = 33 lb (147 N).
Per strut. 33 / 2 = 16.5 lb (73 N).
That is the room-temperature answer. For a truck parked outside through a northern winter it is the wrong one.
Why struts go weak in winter, and how much
A strut's gas pressure follows temperature. Bansbach states that "the extension force of the gas spring changes by approx. 3.3% per 10°C (base + 20°C)." That is ordinary gas behavior: pressure is proportional to absolute temperature, and going from 68 F to -4 F (293 K to 253 K) cuts it by 13.6 percent, close to Bansbach's 13.2.
Trans-Technik gives the operating range as -30 to +80 C (-22 to 176 F). That is the range the strut works in. It is not a range over which force stays constant.
| Air temperature | Force change | 16.5 lb strut delivers | 20 lb strut delivers |
|---|---|---|---|
| 104 F (40 C) | +6.6% | 17.6 lb | 21.3 lb |
| 68 F (20 C) | 0 | 16.5 lb | 20.0 lb |
| 32 F (0 C) | -6.6% | 15.4 lb | 18.7 lb |
| 14 F (-10 C) | -9.9% | 14.9 lb | 18.0 lb |
| -4 F (-20 C) | -13.2% | 14.3 lb | 17.4 lb |
| -22 F (-30 C) | -16.5% | 13.8 lb | 16.7 lb |
Read the 14 F row against the worked example. Without the safety factor the lid needs 15 lb per strut (15 x 10 / 5 / 2). A strut sized at exactly 16.5 lb gives 14.9 lb at 14 F (-10 C): the whole 1.1 factor is gone and the lid sits on the edge of falling. The 1.1 covers friction and tolerance at room temperature. It was never a cold-weather allowance.
That settles the usual argument. Gastac's FAQ says extreme cold "can slightly alter gas pressure." A 13 to 17 percent loss is slight for most purposes and decisive on a lid sized with a 10 percent margin. If the lid holds once the truck has sat in a warm shop for an hour, the struts are cold, not dead. If it falls in the shop too, they have lost charge and need replacing, as a pair.
Sizing for the cold
To keep the full 1.1 margin down to a design temperature T in C, divide the room-temperature requirement by the fraction left at T:
F cold = F / (1 - 0.033 x (20 - T) / 10)
For the worked example at -22 F (-30 C): 16.5 / 0.835 = 19.8 lb per strut. Round up to a 20 lb pair.
The cost is closing effort. At 68 F, two 20 lb struts on b = 5 in produce 200 lb-in against the lid's 150 lb-in. The 50 lb-in excess, spread over the 20 in to the free edge, is about 2.5 lb of push to close, rising to about 3.2 lb at 104 F. Neither is heavy, but the same excess on a short b, or a strut two sizes up, gives a lid that springs open when the latch releases. Keep both struts the same force, because unequal struts twist a 72 in lid and load one latch harder than the other.
Common tool box strut sizes
Catalogued tool box lid struts from the sources below. Several are aftermarket interchanges rather than the original part. Forces are per strut.
| Part | Extended | Compressed | Stroke | Force | Rod/tube, fittings | Fitment noted by source |
|---|---|---|---|---|---|---|
| Gastac GT8181000110 | 10 in | 7.10 in | 2.90 in | 10 to 110 lb range | 8/18 | Universal tool box |
| Strong Arm 7083 | 11 in | 7 in | 4 in | 155 lb | 10 mm ball socket | Interchanges 1015926 |
| P10210-1 | 11.9 in | 7.75 in | 4.15 in | not listed | not listed | Replaces Snap-on 1-3069 |
| Gastac GT8181200110 | 12 in | 8.03 in | 3.97 in | 10 to 110 lb range | 8/18 | Universal tool box |
| Weather Guard 7251-2PK | 12.5 in | 8.3 in | 4.2 in | not listed | not listed | Saddle Box Model 116, set of 2 |
| P10207-6 / -2 / -4 / -1 / -5 | 13.74 in | not listed | not listed | 15 / 29 / 49 / 57 / 81 lb | 8/18 (listed for -5) | Replace Snap-on 1-2669A, 1-2069, 1-2369, 1-1969, 1-2469 |
| Gastac GT8181489110 | 14 in | 8.90 in | 5.10 in | 10 to 110 lb range | 8/18 | Universal tool box |
| PM3858-1 | 17 in | not listed | not listed | not listed | 10 mm ball socket both ends | Interchanges 9772PK |
| P10206 | 19.76 in | 11.22 in | 8.54 in | 20 lb | not listed | Replaces Snap-on 1-2769 |
| Gastac GT8182000150 | 20 in | 12.13 in | 7.87 in | 50 to 150 lb range | 8/18 | Universal tool box |
The 13.74 in family is a force ladder. Five parts share one length and run from 15 to 81 lb. They replace Snap-on numbers, the tool storage the catalogue files alongside truck boxes, which is why a length-only search returns them. At 13.74 in, the force on the tube is the part number that matters.
Compressed length is 57 to 73 percent of extended. That holds across the ten struts with both lengths published, and the longer the strut, the lower the ratio (60.7 percent for the 20 in Gastac, 73.1 percent for its 7 in). If the lid-shut distance between your mounts is 8.5 in, the longest strut that can close fully is roughly 11.5 to 14 in extended. Confirm against the specific part.
8/18 is the default. Every size in Gastac's tool box range is 8/18 mm, as is the P10207-5.
Strut length and how far the lid opens
Extended length sets where the lid stops. A longer strut on the same mounts, or the same strut mounted closer to the hinge, carries the lid further past vertical. Gastac's fitting method starts from the angle: open the lid where you want it, fix the cylinder end to the lid, then swing the rod end to the body mount.
On a crossover hinged on the tailgate side, that angle is a clearance question. A 20 in deep lid held at 110 degrees has its edge 6.8 in toward the cab from the hinge line; a longer strut holding it at 115 degrees pushes that to 8.5 in. Before fitting anything longer than the original, run the swing check in how to measure a truck bed for a tool box against your rear glass and any light bar.
Mounting rod down, and strut life
Gastac says the strut "should always be mounted with the 'rod down' in the fully closed position to ensure proper seal lubrication." Trans-Technik adds that piston rod downward "ensures lubrication of seals and full damping effect at the end of the stroke." The closed position counts because that is where the strut spends almost all its life. On a lid, that means cylinder on the lid and rod on the box body.
- Protect the rod. Trans-Technik warns against scratches, wear and dirt on the piston rod, because the rod slides through the seal. If you are refinishing a tool box, take the struts off first rather than masking them.
- Replace both at once. Gastac recommends replacing tool box struts as a pair for balanced support.
- Expect a finite life. Gastac gives 3 to 5 years or more, depending on lid weight and conditions. Bansbach designs for 50,000 to 100,000 double strokes on average. A service truck opened forty times a day, 250 days a year, reaches the lower figure in five years, so the two estimates agree.
A strut is not a security part. Lid geometry and latches decide how a box resists a pry bar, covered in jobsite box security.
When this method stops applying
The formula and the temperature correction cover a single rigid lid on a horizontal hinge.
- Gull wing lids. Each wing has its own G, L and b. Calculate each separately.
- Lids carrying gear. A light or sign on the lid adds to G and usually moves L outward. Redo the scale reading with it fitted.
- Below -22 F (-30 C) or above 176 F (80 C). That is outside the standard range. Bansbach offers special seal sets for down to -55 C (-67 F); do not assume an aftermarket strut has them.
For the clearance numbers that decide whether a box fits in the first place, the truck bed tool box fit finder checks your measurements against a box's envelope.