In one line:
M35 = 5% cobalt — the everyday choice for stainless and tough steel.
M42 = 8% cobalt — more heat and wear resistance again, for hardened and high-alloy steel and heavy production.
Both handle stainless. M42 is the one you step up to when the material is genuinely hard or you’re drilling all day.
M35 and M42 are both cobalt drill bits — high-speed steel with cobalt alloyed in for heat resistance. The number is really about the cobalt content, and that changes how hard and how heat-resistant the bit is.
M35 vs M42 at a glance
| M35 cobalt | M42 cobalt | |
|---|---|---|
| Cobalt content | 5% | 8% |
| Best for | Stainless steel, steel, cast iron — the everyday step up from HSS | Hardened, heat-treated and high-alloy steels; sustained production drilling |
| Heat & wear resistance | High | Higher again — keeps its edge where M35 starts to struggle |
| Hardness | Hard | Harder |
| Toughness | Slightly more forgiving | A touch more brittle — wants a rigid setup and steady feed |
| Price | Lower | Higher |
Which should you buy?
Choose M35 (5%) for the vast majority of tough-metal jobs — stainless steel, general steel and cast iron. It’s the sensible everyday upgrade from HSS and it’ll handle most of what a workshop or site throws at it.
Step up to M42 (8%) when:
- the steel is hardened, heat-treated or high-alloy — the jobs that eat ordinary bits;
- you’re drilling hole after hole and heat builds up over a shift;
- you want maximum edge life and the bit to stay sharp longer between sharpenings.
Both are alloyed all the way through, so both re-sharpen back to full performance — you’re not just paying for a coating.
M35 and M42, every size, from stock
Single sizes and sets, plus standard HSS for everyday work. Over a million bits in stock, same-day dispatch, free next-working-day UK delivery on orders over £50.
Get the speed right too
Even M42 will blunt if you run it too fast. Cobalt tolerates a little more speed than HSS, but the rule still holds: slow down for stainless and for larger diameters. See our drill speed chart, and if you’re weighing cobalt against plain HSS in the first place, our cobalt vs HSS guide.
Frequently asked questions
What is the difference between M35 and M42 cobalt drill bits?
Cobalt content. M35 has 5% cobalt and M42 has 8%. The extra cobalt in M42 gives greater heat and wear resistance and a harder edge, which makes it better for hardened and high-alloy steel and for heavy repeat drilling. M35 is the everyday choice for stainless and tougher steels.
Is M42 better than M35?
For hard, heat-treated or high-alloy steel and sustained production drilling, yes — it lasts longer under heat. For everyday stainless and steel, M35 is perfectly capable and costs less. M42 is also slightly more brittle, so it prefers a rigid setup and a steady feed.
Do I need cobalt at all, or will HSS do?
For mild steel, wood, aluminium and plastic, standard HSS is fine. Step up to cobalt for stainless, cast iron and hard steels, where HSS blunts quickly. See our cobalt vs HSS guide for the full picture.
Can M42 drill bits drill hardened steel?
M42 handles hardened and heat-treated steel far better than HSS or M35, drilled slowly with firm pressure and cutting fluid. For genuinely through-hardened material (like security fixings) you may need TCT (tungsten carbide tipped) instead.
Are M35 and M42 bits available in single sizes?
Yes — both are available as single sizes and in sets, so you can buy exactly what the job needs.
UK Drills has supplied drill bits to UK trade and industry since 1985, from our trade counter in Hindley, Wigan. Not sure which grade for your job? Email info@ukdrills.com — we’re happy to advise.
