What a Knife Bolt Has to Get Right
A chipper knife bolt does one job: hold a knife flat to a drum spinning at over a thousand revolutions a minute while wood hits it. It is a countersunk high-strength bolt, and every feature on it is there for that job. EKINSUN LTD is a custom parts manufacturer in Guangdong, China, that machines knife bolts to the old bolt or to a drawing for the machines a dealer no longer supplies, from one set of 4. The table gives what each feature has to be and what goes wrong when it is not.
| Feature | What it has to be | When it is wrong |
|---|---|---|
| Head angle | 82° flat head on inch bolts (ASME B18.3), 90° countersunk on metric (ISO 10642); the knife's countersink is cut to one of them | an 82° head in a 90° countersink seats on a ring, not the cone: the knife cracks at the hole or the head shears |
| Drive | 12-point, hex socket or Torx, deep enough to take the torque in the manual without rounding | a rounded drive means the knife comes off with a chisel |
| Grade | SAE Grade 8 (about 1,034 MPa) or ISO 10.9 (1,040 MPa) / 12.9 (1,220 MPa), marked on the head | a Grade 5 or 8.8 bolt stretches under impact and the knife walks |
| Shank and length | plain shank through the knife, thread starting inside the drum or at the nut, length so the thread does not run out in the knife | thread in the knife hole is a fatigue notch at the highest-stressed point |
| Thread | 1/2-13 or 5/8-11 UNC on most US machines; M12, M14 or M16 on European ones; the odd pitch on an old machine | a 1/2-13 bolt in an M12 hole starts, binds and strips the drum |
| Torque and reuse | torqued to the manual's figure; the makers' manuals call for new bolts with each knife change | a reused bolt torqued a second time is the one that lets go |
Common Threads and Head Angles — Check the Old Bolt
Two things identify a knife bolt before anything else: the thread and the head angle. Inch bolts have an 82° flat head, metric bolts a 90° countersunk head, and a chipper's knife is countersunk to one of the two. Count the threads over an inch or over 20 mm, and look at the grade mark on the head.
| Thread | Head | Where it turns up | Check |
|---|---|---|---|
| 1/2-13 UNC | 82° flat, 12-point or hex socket | smaller US drum chippers and shredders | 13 threads per inch on a rule over 1 in; 82° head |
| 5/8-11 UNC | 82° flat, 12-point | larger US drum and disk chippers | 11 TPI; 82° head; often a Grade 8 nut on the drum side |
| M12 × 1.75 | 90° countersunk, hex socket or Torx | European drum chippers, flail mowers | 10 threads over 17.5 mm; 90° head |
| M14 × 2 | 90° countersunk | mid-size European chippers | 10 threads over 20 mm |
| M16 × 2 | 90° countersunk, 12-point | large European drum chippers, stump grinders | 10 threads over 20 mm; 16 mm shank |
| 9/16-12, 7/16-14, BSW, fine pitches | as on the old bolt | older and discontinued machines | send the old bolt: the odd size is the one no kit carries |
From the Dealer, or Made From Your Old Bolt
For a current model of a major brand, buy the bolts from the dealer with the knives. The gap is a model the dealer no longer supports, a set of bolts without the knife kit, a thread or head angle no kit carries, and a length for a thicker knife or a re-ground drum.
| What you need | Available? | At EKINSUN |
|---|---|---|
| Knife bolts for a current model of a major chipper brand | Yes — the dealer, usually with the knives | Buy the OEM bolts with the knives if the dealer lists them; a stocked set is cheaper than any one-off. |
| A bolt set on its own, without buying the knife kit | Often kit only | Made as a set of the bolts alone, from 4. |
| A chipper, shredder or flail mower whose maker is gone or no longer supports the model | No | Copied from one old bolt: thread, length, head angle, drive, grade. |
| 9/16-12, 7/16-14, a Whitworth or a fine-pitch thread | No | Machined to the old bolt, from one set. |
| A longer bolt for a thicker aftermarket knife, or a shorter one for a re-ground drum | No | Length under the head to your measurement. |
| A corrosion-resistant finish for a machine that lives on road salt | Rarely | Zinc-flake or black oxide on the alloy-steel bolt. 304 (1.4301) and 316L (1.4404) stainless reach about 700 MPa at best and 6061 aluminium far less, so neither is a knife-bolt material; the corrosion protection goes on the alloy-steel bolt as a coating. |
| The knives themselves | Yes — knife makers and the dealer | Buy the knives there; we make the bolts and the counterknife (anvil) bolts. |
Buying chipper knife bolts? Quoted per set of the bolts for one knife change, from a set of 4. The first set is the sample; sets for the season or a yearly quantity get a price ladder, and a repeat is one email against the drawing on file. About 5 working days to make, 5–7 working days by courier, DDP to your door with duty paid; quote in 12 hours. Send one old bolt, or the thread, the length under the head, the head angle and the bolts per change.
Price per Set, Lead Time and Delivery
Knife bolts are quoted per set — the bolts for one knife change, 4 to 16 depending on the drum — within 12 hours from one old bolt. A first set is the sample; if it fits and torques to the manual, the drawing stays on file and every later set is the same part at the same price tier. Sets for the season or a yearly quantity are quoted as a ladder: the more sets of one drawing, the lower the price per set. Production is about 5 working days, then 5–7 working days by courier; every quote is DDP to your door with duty and VAT paid, nothing collected at delivery. If your current supplier has dropped the model or sells the bolts only inside a knife kit, send one bolt and we quote the set as a second source.
What to Send
One old bolt in an envelope is the best input; a worn one is fine, because the thread, the length under the head, the head angle and the grade mark are all still on it. Without a bolt: the thread (or ten-thread length on a rule), the length under the head, the head angle (82° or 90°), the drive, the grade mark, the machine make and model, and how many bolts one knife change takes. A dimensioned drawing comes back for approval before the bolts are cut.
Related: security and special-drive screws · bolt grade chart · replacement bolts from a sample · obsolete fasteners
Chipper Knife Bolts FAQ
The countersunk high-strength bolt that holds a knife (blade) to the drum or disk of a wood chipper. It has a flat head that sits flush in the knife's countersink, a drive in the head (12-point, hex socket or Torx), a plain shank through the knife and a thread into the drum or a nut, and it is made in a high-strength grade: SAE Grade 8 (about 1,034 MPa tensile) or ISO 10.9 (1,040 MPa) or 12.9 (1,220 MPa).
The chipper makers' manuals say yes: fit new bolts with new or resharpened knives and torque them to the figure in the manual. A knife bolt is tightened to near its yield to keep the knife from moving under impact, and a bolt that has been through that once and been shocked for a season is not the same bolt. That is why they are bought in sets, one set per knife change.
US-built chippers usually carry 1/2-13 or 5/8-11 UNC bolts with an 82° flat head; European machines M12 or M16 with a 90° countersunk head, sometimes M14 or a fine pitch. The head angle matters as much as the thread: an 82° head in a 90° countersink, or the reverse, seats on a ring instead of the whole cone and cracks the knife or the head. Send one old bolt, or the thread, the length under the head and the head angle, and the bolt is copied.
Yes. From one old bolt, even a worn one, we copy the thread, the length, the head angle, the drive and the grade, and machine the bolt in 42CrMo4 quenched and tempered to the 10.9 or 12.9 level, or in an alloy steel to Grade 8, from a set of 4 up to a year's supply. The drawing stays on file, so the next set is one email.
Knife bolts are quoted per set within 12 hours: a set is the bolts for one knife change, usually 4 to 16 bolts depending on the drum. A first set is the sample; sets for the season, or a yearly quantity, get a price ladder. Production is about 5 working days, then 5–7 working days by courier, DDP to your door with duty paid.
EKINSUN, a custom parts manufacturer in Guangdong, China, machines countersunk high-strength knife bolts to the old bolt or to a drawing, for chippers, shredders, flail mowers and stump grinders whose bolts the dealer no longer lists or lists only as part of a kit, from one set.