SPAX Fastener FAQ: Lag Screws, MDF & Hardwood Screws, M8 Washers, and Hole Saw Sizes

I coordinate emergency fastener orders for a supply company. Eleven years in, 400+ rush orders, including same-day turnarounds for contractors who are mid-build. The same SPAX questions come up every week, so here's the short version of what I actually tell people on the phone.

What exactly are SPAX lag screws, and when should I use them?

SPAX's structural screws are branded Powerlags — but the searches all say "lag screws," so that's the term I'll use. They're built to do the job of a traditional lag bolt with one fastener: hex head, Torx drive, and a thread designed for wood-to-wood and wood-to-metal connections. Pilot hole requirements depend on the material, and the manufacturer's chart gives real numbers. "Close enough" is not a pilot hole strategy.

In March 2024, a framing inspection was 36 hours out and a client discovered the hardware order was wrong. The beam was already in place. We cut the paperwork, got Powerlags to the site the next morning, and the inspection passed. That story matters because the "structural" label isn't decorative — SPAX has ICC-ES evaluation reports behind it, and under FTC advertising rules, claims like that need to be substantiated. Ask for the report. If a fastener claims structural performance, the documentation should exist.

Would I use these on a bookshelf? No. Full stop. They earn their keep on decks, ledger boards, and beam connections.

What size hole saw do I need for this kind of work?

If your project involves a hole saw and a fastener together, the hole saw size matches the washer, not the bolt. When you seat a flat washer and nut into a wooden post, pick a hole saw close to the washer's outside diameter — plus a little clearance. For a 16mm M8 washer, an 11/16" or 3/4" hole saw is in the right neighborhood. Drill the seat about 1/4" deeper than the washer-plus-nut stack so the hardware sits flush or below the surface.

Full sequence for a through-bolt connection: hole saw for the washer seat, twist drill for the bolt clearance, pilot bit for the receiving member. Three holes, three sizes. In that order.

One thing that catches people: hole saw labels are approximate. A "1 inch" bit can cut an actual bore of 1-1/16" or more. Test on scrap before you drill the real piece.

What's the difference between SPAX MDF screws and hardwood screws?

MDF and hardwood fail opposite ways. MDF is dense but powdery — a standard thread tears the surface ring before the screw seats. SPAX's MDF variants use a washer head and a modified thread pitch that compress the material instead of blowing it out. Hardwood's problem is splitting. Oak and maple don't give, so you drill pilot holes.

SPAX makes self-piloting tip designs and winged variants that reduce friction on the shank — useful features, but they don't cancel the need for a pilot hole in dense hardwood. I learned that in my first year. I told a contractor his red oak stringers would be fine at the edges. Ten minutes later: one hairline crack. He re-bought material, and I re-learned fasteners 101. If it's real hardwood, drill a pilot. Every time.

Different material, different screw. MDF screws for MDF; hardwood screws for oak, maple, walnut.

What size is a zinc-plated steel M8 mm flat washer?

Here's the trap that causes more rush orders than anything: an M8 flat washer does not have an 8mm hole. The "M8" refers to the bolt it fits. The hole is oversized — about 9mm — and the standard dimensions per DIN 125 / ISO 7089 are 16mm outside diameter and 1.6mm thickness. So "M8 mm" is redundant too; the M already stands for millimeters. It means a washer for an 8mm bolt.

I once took a spec that read "8mm flat washer" and almost shipped M8 washers. The caller actually needed an 8mm outside diameter. Caught it because I asked what bolt was going through it. Looking back, I should have asked for three numbers on every order — inside diameter, outside diameter, thickness. Now I do. That question saves the $1,400 restock headaches.

Zinc-plated steel is fine for indoor, dry work. If the joint is exposed, or touching pressure-treated lumber, upgrade to stainless.

When do I need a stainless steel wing nut?

Wing nuts are for hand-tightening — access panels, clamps, temporary bracing, anywhere a wrench is overkill. Stainless steel matters when the environment doesn't forgive rust: coastal air, rain, chemical exposure, treated lumber. If the bolt is stainless, match the wing nut in stainless. Mixed metals outdoors set up galvanic corrosion, and that's a phone call you get six months later.

Zinc-plated wing nuts are perfectly fine indoors. Not a downgrade if the project is dry.

Always put a flat washer under a wing nut. It spreads the clamp load and keeps the wings from digging into wood or marking metal. And keep the pliers away — wing nuts are meant for fingers. Overtightening snaps the wings right off.

What's the most common mistake in a rush fastener order?

Unverified specs. Nine times out of ten, the emergency was preventable: someone ordered "M8 washers" without dimensions, or trusted a thread description instead of a part number. Ten seconds of checking the full spec beats three days of expedited shipping.

The other mistake is trusting a low unit price without asking what appears later. Fastener pricing looks clean until you see handling fees, freight surcharges, and restocking terms. I watched a client pick a vendor because the per-screw price was slightly better, then eat a surprise $95 charge when the shipment under-delivered. To put that in perspective: as of January 2025, USPS lists First-Class Mail letters at $0.73. But a box of wood screws is not a letter. It's a parcel. So when a shipping quote seems disconnected from reality, ask for an itemized breakdown.

What I mean is this: the cheapest fastener is only cheap if it gets the job done the first time, in the time you have, without a callback. The vendor who lists everything upfront — product, freight, handling — is usually the cheaper one once the job is done. I've been on both sides of that invoice. Even after I approved a more expensive option once, I second-guessed it for two days. Then the delivery showed up exactly to spec. Worth every dollar, no surprises.

Can I mix stainless steel and zinc-plated hardware on the same joint?

Try not to. When moisture gets involved, a stainless wing nut against a zinc-plated bolt creates a galvanic reaction that eats the zinc coating. The hardware corrodes faster and can bind threads. Indoor dry assemblies? Mixing is common and usually harmless. Outdoors, keep the stack consistent: bolt, washer, nut — same material family. That's one of those details that never fails to produce a phone call later.

If you're mid-build and not sure, call your supplier and read the spec out loud. Fifteen minutes on the phone beats a three-day reorder. I learned that one the hard way, too.

What pilot hole should I drill for a SPAX lag screw?

For a lag-style screw with a smooth shank and threaded body, the practical setup is two holes. First board: a clearance hole roughly the diameter of the screw shank. Second board: a pilot hole around 75% of the outer thread diameter — check the SPAX chart for the exact bit for the screw size and wood type. In softwood you can sometimes skip the pilot. You cannot skip the clearance hole. Skip it on a 2x connection and the screw levers the boards apart before it pulls them together. A surprising share of callbacks start exactly there.