Stainless steel washers are small but critical: a washer spreads bolt load, protects the joint surface, and (when it is a lock type) resists vibration loosening. This guide covers flat, spring and lock washers, how to match them to your bolt, and what to specify on an RFQ.
Flat washers: load distribution and surface protection
A flat (plain) washer is a simple annulus that sits under the bolt head or nut. Its main jobs are to spread clamping pressure over a larger area, protect the paint or finished surface from damage, and bridge oversized or irregular holes. A plain washer by itself does not lock the fastener against rotation.
Spring and lock washers: resisting vibration
- Split spring lock washer: a helical ring with a gap; its spring tension keeps the joint snug and resists moderate vibration loosening.
- Internal-tooth lock washer: teeth point inward and bite near the bolt, good for small fasteners and finished or recessed heads.
- External-tooth lock washer: teeth point outward, giving a larger bite radius; common in general sheet-metal and equipment assembly.
Toothed lock washers grip by digging into the mating surface, so avoid them on soft, polished or coated surfaces where marring matters.
Match the washer to the bolt grade and thread
Washers should match the corrosion grade of the fastener: use A2 (304) with A2-70 bolts and A4 (316) with A4-80 bolts for marine or chemical exposure. For grade selection detail, see our A2-70 vs A4-80 fastener grade guide.
The washer inner diameter should match the nominal thread size, and the series matters: SAE (regular), USS (wide/flat) and heavy-duty sizes differ in outer diameter and thickness. For stud and rod joints, see our guide on threaded rod and stud bolts.
Common installation order
For a bolted joint with locking, the usual order from nut outward is: nut → split/tooth lock washer → flat washer against the surface, or flat washer under the bolt head and lock washer under the nut. Confirm the arrangement with your engineer rather than stacking several washers arbitrarily.
Buyer’s RFQ checklist
- Type: flat plain, split spring, internal- or external-tooth lock.
- Thread size (M6, M8, M10… or #10, 1/4″, 3/8″) and series (SAE / USS / heavy / DIN).
- Grade: A2 (304) or A4 (316) stainless.
- Quantity, finish, and any standard (e.g. DIN 125 flat, DIN 127 spring).
- Material certificate on request.
Send us your bolt size, grade and joint condition, and we will recommend the matching flat or lock washer.
SOROYA PTE. LTD.
Email：nancy@soroya.com.sg
WhatsApp：+65 8058 3416
