M42-4.5 x 400mm Hex Head Cap Screws: Specs and Use

By Mutual Screw & Supply | Published: 2026-11-03

Category: Guías prácticas

Learn how to specify and install M42-4.5 x 400mm Class 10.9 hex head cap screws. Check thread pitch, grip length, and plain-finish handling before you torque.

Quick answer

To use an M42-4.5 x 400mm hex head cap screw correctly, first confirm the 4.5mm thread pitch matches your tapped hole or nut, then verify that the 400mm length and partial thread give you the grip you need. Install with a proper wrench or socket and treat the plain finish as a maintenance item, not a corrosion barrier.

Why the 4.5mm pitch and 400mm length need separate checks

M42 fasteners are not interchangeable by diameter alone. The 4.5mm pitch is a specific coarse thread for this size, so a nut or tapped hole cut for a different pitch will not seat correctly. Check the pitch with a thread gauge or a known mating nut before installation.

The 400mm length is measured from under the head to the tip. Because the screw is partially threaded, only the threaded portion grips; the unthreaded shank adds length but no holding power. Confirm that your clamped parts are thick enough to leave the unthreaded shank inside the joint and enough thread engaged in the nut or tapped hole.

What Class 10.9 and plain finish mean for your application

Class 10.9 steel gives this screw high tensile strength, but it also means the material is heat treated and can be sensitive to hydrogen embrittlement if plated or exposed to certain environments. The plain finish avoids plating but offers no corrosion protection, so indoor or dry, controlled conditions are the safer fit.

For outdoor, washdown, or chemical exposure, plan on a protective coating, regular inspection, or a different fastener material. Do not assume the plain finish will resist rust simply because the screw is large.

Step-by-step installation for M42-4.5 x 400mm cap screws

Verify thread pitch and diameter: Use a thread pitch gauge or a known M42-4.5 nut to confirm the screw matches your tapped hole. Common mistake: assuming all M42 threads are the same pitch. If the nut spins on by hand more than a turn or two, stop and recheck the pitch.
Measure grip length against your joint: Calculate the total thickness of the parts being clamped. The unthreaded shank should sit within that thickness, and the threaded portion should engage the full nut or a depth at least equal to the screw diameter. Common mistake: ordering by overall length without checking thread length. A partially threaded screw can bottom out in a blind hole if the unthreaded shank is longer than the clamped stack.
Prepare the mating threads and surfaces: Clean tapped holes and nut threads, remove burrs, and check that washers or clamped surfaces are flat. Common mistake: using damaged or painted threads that increase friction and reduce clamp load. Chase threads with a tap if a new nut will not thread on by hand.
Lubricate for consistent torque: Apply a suitable anti-seize or thread lubricant to the threads and under-head contact area unless your specification prohibits it. Common mistake: torquing dry and then reusing the same torque value after lubricating. Lubrication changes the torque-tension relationship; use the torque value recommended for your lubricated condition.
Tighten with the correct hex socket: Use a six-point socket that fully engages the hex head. Tighten in a controlled pattern to the torque specified for M42 Class 10.9 in your application. Common mistake: using an adjustable wrench or worn socket that rounds the head. If you cannot find a published torque for your joint, consult an engineer rather than guessing.
Inspect and protect the plain finish: After installation, check for exposed bare steel and apply a corrosion-inhibiting coating if the environment is damp or outdoor. Recheck torque after the first thermal cycle or load cycle. Common mistake: leaving plain-finish screws unprotected in wet areas. Schedule periodic inspections for rust or loosening whenever the joint is safety-related.

Product facts

M42-4.5 X 400mm Hex Head Cap Screws | Class 10.9 Steel Plain | Partially Threaded (1 / Pkg.): M42-4.5 x 400mm hex head cap screw, Class 10.9 steel, plain finish, partially threaded, sold as 1 per pack. This single-piece pack suits one-off repairs or prototypes where you need to confirm fit before ordering larger quantities.
M42-4.5 X 400mm Hex Head Cap Screws | Class 10.9 Steel Plain | Partially Threaded (1 / Pkg.)