Reach (thread length)
Reach is the simplest spark-plug spec to picture and one of the easiest to get catastrophically wrong. It is the length of thread that screws into the cylinder head and pokes into the combustion chamber. Get it wrong and, in the best case, you carbon up your threads — in the worst case, the plug hits the piston. Here is what reach really does and why it must match your head exactly.
What reach actually is
Reach is the distance from the seat of the plug (where it seals against the head) to the tip of the threads. In other words, it is how deep the threaded portion travels into the head, and how far the firing end ends up sitting inside the chamber.
It is almost always quoted in millimetres, but the classic values come straight from imperial fractions:
- 19 mm = 3/4" — the long reach, common on most modern car and motorcycle engines;
- 12.7 mm = 1/2" — a medium reach, found on many older or smaller engines;
- 11.2 mm = 7/16" — a short reach, typical of small engines, mowers and some vintage units.
Why reach is critical
Reach does two jobs at once, and both have to be right.
First, it positions the spark. The flame has to start in the right place to travel cleanly across the chamber. A spark sitting in the correct spot lights a strong, even flame front; a spark buried too deep or sticking out too far ignites poorly.
Second — and this is the non-negotiable part — the firing end must physically clear the piston and the valves. The plug protrudes into a space where metal parts are moving at speed, millimetres away. Reach is what guarantees the plug stops short of that moving hardware.
Too long
If the reach is greater than the head's threaded bore, the extra threads stick out into the chamber. Those exposed threads collect carbon and run hot — a glowing hot spot that can trigger pre-ignition. When you finally try to remove the plug, the carboned threads fight you, and the next plug goes in cross-threaded, wrecking the head. And in the worst case, the protruding plug strikes the piston crown or a valve. That is not a fouling problem; that is bent valves, a holed piston, scrap engine.
Too short
If the reach is too short, the spark sits recessed up in a pocket of the head. The flame has to climb out of that pocket before it can spread, so ignition is weak and combustion incomplete. Meanwhile the lower head threads sit exposed in the chamber and carbon up — so the next correct-reach plug may not even thread in cleanly.
Fitting a too-long reach in a short head is among the most destructive mistakes you can make on an engine. The plug can bottom out against the piston on the very first crank. If you ever find carbon built up in the head threads from a previous short plug, chase the threads with a thread-chaser (not a cutting tap) before fitting the correct plug — otherwise it will jam and you will read it as "too long" when the real problem is dirty threads.
Reach and the seat type
Reach never travels alone — it is locked to how the plug seals against the head. There are two seat types:
- Gasket (flat) seat: a crush washer seals the joint. Tightening compresses the washer, and the reach is measured to that flat seat.
- Taper (conical) seat: no washer; a 60° cone seals metal-to-metal directly against a matching cone in the head.
The two are not interchangeable. A taper-seat plug fitted to a flat-seat head — or vice versa — changes where the firing end ends up, even if the nominal reach looks similar. Always match seat type and reach together.
Common reaches at a glance
| Reach | Inch equivalent | Typical use |
|---|---|---|
| 19 mm | 3/4" | Most modern cars, motorcycles, performance engines |
| 17.5 mm | ~11/16" | Many older cars and bikes |
| 12.7 mm | 1/2" | Smaller and older engines, some industrial |
| 11.2 mm | 7/16" | Small engines, mowers, vintage units |
| 9.5 mm | 3/8" | Very small / specialist engines |
The common mistake — and how the code tells you
The classic error is swapping a plug for one with the same thread diameter and hex but a different reach, because it "looks the same" and threads in by hand. It does thread in — until it bottoms out or leaves threads exposed.
On NGK plugs, a suffix letter encodes the reach. Take BPR6ES versus BR6ES: the P denotes a projected (longer) reach. Drop the P and you have shortened the reach — the firing end now sits further back in the chamber. Same threads, same heat range, but a different geometry entirely. Always read the full code, never just the heat-range digit.
In short
- Reach = the length of thread reaching into the combustion chamber, measured from the seat.
- It positions the spark and must clear the piston and valves.
- Too long: exposed threads carbon up, hot spot, seized/cross-threaded removal, and at worst the plug hits the piston — engine-killing.
- Too short: recessed weak spark, poor flame travel, and the head threads foul.
- Reach is tied to seat type (gasket vs taper) — match both, never one alone.
- Read the whole part code (e.g. the NGK suffix), not just the number — the letters carry the reach.