A 4-40 screw’s thread size does not identify its Allen wrench size; the correct tool depends on the screw’s internal hex drive. Among these listings, the DGBRSM and Mellewell sets include a hex wrench with 4-40 set screws, while the other options are explicitly 4mm metric tools—not verified fits for 4-40 screws. Choose based on the screw’s drive dimensions, not its thread designation.
| Product | Best For | Important Detail |
|---|---|---|
| DGBRSM 4-40 Set Screws | Small screw jobs with an included key | 25 screws; listing describes a cup point |
| Mellewell 4-40 Set Screws | Higher-quantity replacement screws | 50 screws and a hex wrench |
| Eklind 4mm T-Handle | Metric socket-head fasteners | 4mm drive; not identified as a 4-40 fit |
| uxcell 4mm Ball-End Keys | Metric fasteners in tight spaces | Three 4mm keys; angled entry |
| szkoo 4mm Ball-End Set | Metric work needing multiple keys | Six pieces; listed hex size is 4mm |
DGBRSM 4-40 Set Screws With Hex Key
This set includes 25 4-40 x 1/8-inch socket set screws and a hex key, so it is a practical option when replacing small fasteners and needing a tool in the same package. The listed screw style is a cup point, with a fully threaded, headless design intended for mechanical applications.
Best for: Buyers who need 4-40 set screws and an included key rather than a standalone Allen wrench. It is selected because the listing directly identifies the screw thread as 4-40 and includes a wrench.
Check before use: The provided information does not specify the wrench’s drive size. Confirm that it matches the screw’s internal hex before applying force. The product description also conflicts on material, referring to carbon steel in the name and stainless steel in the features.
Mellewell 4-40 Set Screws With Wrench
Mellewell’s package contains 50 black alloy-steel set screws and one hex wrench. The screws are listed as 4-40 x 3/16 inches, full thread, with a flat point and an internal hex drive. The listing also states a 2.8mm thread diameter and 40 TPI.
Best for: Buyers seeking a larger supply of 4-40 replacement set screws with a wrench included. The flat-point description distinguishes this option from the cup-point DGBRSM set.
Check before use: The wrench’s size is not provided in the listed details, so do not assume it fits every 4-40 internal hex screw. Confirm the drive fit first to reduce the chance of damaging the socket.
Eklind 4mm Cushion-Grip T-Handle
This Eklind tool is a 4mm hex T-handle with a 9-inch shaft and a 3-inch-wide handle. Its T-shaped grip offers a different handling format from a short L-shaped key, and the listing describes it as an industrial-grade tool made in the United States for internal hex fasteners.
Best for: Users who need a dedicated 4mm metric hex tool and prefer a T-handle. It is included as a nearby size-category option, not as a confirmed wrench for 4-40 screws.
Check before use: The listing specifies metric 4mm sizing, not a 4-40 screw-drive fit. Do not use it on a 4-40 fastener unless the internal hex is confirmed to match.
uxcell 4mm Ball-End Hex Keys
This three-piece set has 4mm ball-end keys measuring 4.33 inches long. The listed construction is heat-treated, drop-forged chrome vanadium alloy steel with a black-plated finish. The ball end allows entry at an angle of up to 25 degrees, which can help reach metric fasteners in confined locations.
Best for: Buyers who need several 4mm keys for angled access to metric socket fasteners. It is not specified as a 4-40 wrench.
Check before use: Ball-end keys are intended for angled access, but the listing does not establish a fit for any particular 4-40 screw. Verify the drive size before turning a fastener.
szkoo 4mm Ball-End Hex Key Set
The szkoo listing describes a six-piece ball-end hex key set and identifies the hex size as 4mm. It states that the keys are made from CR-V steel and describes resistance to rust, corrosion, bending, and wear. The ball end is intended to help reach screws without perfect straight-on alignment.
Best for: Buyers wanting a multi-piece set of 4mm ball-end keys for metric work. It is not a verified 4-40 Allen wrench based on the provided product details.
Check before use: The listing’s stated 4mm size is metric. Compare the tool with the screw’s internal hex rather than relying on the 4-40 thread label.
Buying Guide: How to Choose a Wrench for a 4-40 Screw
Does “4-40” tell you the Allen wrench size?
No. “4-40” describes the screw’s nominal thread size and thread count, not its internal hex-drive dimension. A 4-40 fastener can have different head or drive styles. Check the screw manufacturer’s specifications or measure the socket to identify the correct tool.
Should you choose a metric 4mm key?
Only if the screw’s internal hex is specified for a 4mm tool. These product listings identify the uxcell, szkoo, and Eklind options as 4mm tools, but do not say they fit 4-40 screws. Avoid forcing a key that feels loose or will not seat fully.
What is the difference between the two screw kits?
DGBRSM lists 25 screws measuring 4-40 x 1/8 inch and describes a cup point. Mellewell lists 50 screws measuring 4-40 x 3/16 inch and describes a flat point. Both include a wrench, but neither provided description states its drive size.
When is a ball-end key useful?
A ball-end key can help engage a compatible hex socket from an angle, which is useful when straight access is limited. Confirm that the key is the correct size and seat it securely; angled access does not make an incorrectly sized wrench appropriate.
Frequently Asked Questions
What size Allen wrench fits a 4-40 screw?
The 4-40 thread designation alone does not determine the wrench size. Identify the screw’s internal hex-drive measurement or consult its specifications.
Are 4mm and 4-40 the same size?
No. 4mm is a metric measurement, while 4-40 is an imperial screw-thread designation. They refer to different dimensions.
Do the listed 4-40 screw kits include a wrench?
Yes. Both the DGBRSM and Mellewell listings include a hex wrench, but the supplied details do not state its size.
Can I use a ball-end key on a 4-40 screw?
Only if its drive size matches the screw’s internal hex. The listed ball-end sets are identified as 4mm tools, not confirmed 4-40 fits.
Which listed item is a standalone 4mm wrench?
The Eklind item is a 4mm T-handle. The uxcell and szkoo listings are also 4mm ball-end key options, but their formats and set quantities differ.