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SPAX screws buying guide: finish, thread, then bit

Updated: 22 October 2026

You are about to order a SPAX box, and the screen mixes yellow packs, one stainless pack, and star ratings that do not say whether the screw can live outside. Finish comes first, then thread, then the bit you own, then length. Prices sit on the [SPAX screws comparison](/hub/screws-and-accessories/spax-screws). Peter F., DIY & tools editor, wrote this from those amazon.co.uk listings and SPAX GB guidance, not from a lab pull-out test, on 21 October 2026. A qualifying purchase can earn a commission at no extra cost to you.

You are about to order a SPAX box, and the screen mixes yellow packs, one stainless pack, and star ratings that do not say whether the screw can live outside.

Our top picks

SPAX SPAX A2 4 x 40 mm

Best overall

SPAX SPAX A2 4 x 40 mm
9.0/10about £20.02
View offer
SPAX SPAX YELLOX 4 x 50 mm

Alternative

SPAX SPAX YELLOX 4 x 50 mm
8.2/10about £18.95
View offer
SPAX SPAX YELLOX 4 x 30 mm

Best value

SPAX SPAX YELLOX 4 x 30 mm
8.6/10about £4.27
View offer
On this page
  1. 1.Finish before diameter
  2. 2.Full thread or partial thread
  3. 3.The bit you already own
  4. 4.Length, pack, and the fields that disagree

Finish before diameter

The environment chooses the metal. YELLOX is a yellow coating on steel for interior work and for construction that stays protected. It is not stainless. A dry carcass or a loft board over a dry joist can start on a yellow box. A loft open to driving rain cannot. WIROX is SPAX's sheltered coating, and the lexicon says it does not replace stainless. None of these five titles is WIROX. When rain will sit on the head, the box in this set is the A2 4 x 40 mm, partial thread, about £20.02. SPAX states low resistance to salt water and acids. A4 and the decking heads are on SPAX stainless or yellow zinc.

Full thread or partial thread

The 4 x 30 mm and 4 x 50 mm boxes are full thread. The A2 4 x 40 mm box is partial thread. Full thread suits a bracket into a stud, and it can jack a gap if the thread also bites the top piece. Partial thread leaves a plain shank so the top piece can slide. In this set that shank is only on the stainless screw. Boards need SPAX screws for chipboard before you treat full thread as the tighter joint. Hardwood or an edge still wants a pilot.
SPAX SPAX A2 4 x 40 mm

Testsieger

SPAX SPAX A2 4 x 40 mm
9.0/10about £20.02
View offer

The bit you already own

Cross recess Z2 is Pozidriv. The bit is PZ2. A Phillips bit cams out, which is the warning in Toolstation's screw guide. T-STAR plus is the star recess on the A2 title, which prints T20. UK reviews report T15, a cross-head delivery, and no bit in the box. Look at the recess that arrived. The about £21.49 kit is the only box here whose bullets include bits, and its piece count disagrees with itself.

Length, pack, and the fields that disagree

Screwfix's guide says the screw should enter the bottom piece by at least half that piece's thickness, without bursting the back. Use 4 x 30 mm when that reach is enough, and 4 x 50 mm when it is not, at about £18.95 against about £4.27. Buy 200 of one known length, or the 245-piece case at about £20.80 for 3.5 x 16 mm through 5 x 60 mm. Measure one screw on arrival if the card prints powder coated, 8 mm, #0-80, 1 stuk, gray, or three piece counts.

Common questions

What should you decide before you look at the star rating?
Decide where the screw will live, then the thread, then the bit you own. A 4.8 from 4,208 ratings is the stainless 4 x 40 mm box, and that sample does not make it the indoor carcass screw. The 4.7 from 1,209 on the about £4.27 box is a short YELLOX screw whose card disagrees with the title. Stars do not choose the coating.
Is YELLOX the same as stainless?
No. YELLOX is a yellow coating on steel for interior and protected work. Stainless in this set is the A2 4 x 40 mm box, stated for direct weather, with low resistance to salt and acids on the maker's page. WIROX is a third coating and it is not the title of any of these five boxes. A yellow screw used on a fence in a review is still coated steel.
When is full thread the wrong thread?
Full thread is wrong when you are pulling two pieces together and the thread bites the top piece as well as the bottom. That bite can hold the joint open. The 30 mm and 50 mm boxes are full thread. The A2 4 x 40 mm box is partial thread, so the plain shank is there, and the coating is stainless rather than YELLOX.
Pozidriv Z2 or T-STAR plus if you already own one bit?
Match the recess. Z2 takes PZ2, not Phillips. T-STAR plus is the A2 box, titled T20, with UK reviews that also report T15 and a cross head. If you only own PZ2, the yellow Z2 boxes are the ones you can drive without a new bit, and they stay indoors. If you only own a star bit, check the screw that arrived.
Is a mixed case better than 200 of one length?
A mixed case is better when you do not know the next length. The 245-piece box lists several sizes and costs about £20.80. Two hundred 4 x 30 mm screws cost about £4.27 if that is the size you will use up. The about £21.49 kit adds bits on the bullets and disagrees about 199, 195 or 198 pieces, so it is not the automatic household case.
Which fields on these Amazon cards should you not trust on their own?
Do not trust a finish, diameter, origin, thread code, colour or piece count that fights the title. Powder coated against YELLOX, 8 mm against 4 x 30 mm, #0-80 on a wood screw, China against a German-made review, gray against yellow zinc, and 199 against 195 against 198 are all on these cards. The shared rank between the 30 mm and 50 mm ASINs is a listing fault.

Related pages

Best SPAX screws

SPAX screws

SPAX screws for chipboard: 30 mm, 50 mm, or SPAX-M

How to use SPAX screws: the bit, the pilot, the clamp

SPAX stainless or yellow zinc: which screw outside

Sources

Public UK sources from the research digest, used for this page in October 2026. Prices, stars and review counts are the amazon.co.uk lines in the display file. There is no laboratory test, no salt-spray test, and no pull-out value. A yellow assortment is not treated as ETA-12/0114.

  1. SPAX universal screw, WIROX page (retrieved 22 October 2026)
  2. SPAX technical lexicon (retrieved 22 October 2026)
  3. SPAX stainless A2 universal (retrieved 22 October 2026)
  4. SPAX UKCA (retrieved 22 October 2026)
  5. ETA-12/0114 (15 October 2024) (retrieved 22 October 2026)
  6. GOV.UK construction products (retrieved 22 October 2026)
  7. HSE wood dust (retrieved 22 October 2026)
  8. Screwfix screws buying guide (retrieved 22 October 2026)