Spark Plug (NGK D8HA) | Honda CT90 / Trail 90
The NGK D8HA is the correct spark plug for the Honda CT90 / Trail 90 from 1966 through the final 1979 production year. Copper-core construction, 18mm hex, 12mm thread. The CT90 is a single-cylinder engine — one plug per bike.
Why You Need This
Spark plugs are the cheapest, most ignored part on a vintage Honda. After 50+ years of bike flippers and shade-tree mechanics, there is no telling what has been screwed into your CT90 head — wrong heat range, wrong reach, wrong thread, fouled, or gapped wrong. A fresh D8HA gapped to spec is the fastest tune-up money you can spend.
Copper-core plugs like the D8HA are also ideal for carb jet tuning. Grab a few extras, pull a fresh plug after a wide-open-throttle run, and read the porcelain tip color to confirm your jets are dialed in.
Specifications
| Spec | Detail |
|---|
| Replaces OEM | 98066-58714, 98066-58717, 98066-58723, 98066-58727 |
| Fits Models | CT90 / Trail 90 (1966–1979) — all K-codes plus 1978–1979 |
| Plug Type | NGK D8HA — copper core, 18mm hex, 12mm thread |
| Gap | 0.6–0.7 mm (0.024–0.028 in.) |
| Qty | Sold Individually — one plug per CT90 (single-cylinder engine) |
Gap and Installation
Set the gap to 0.6–0.7 mm (0.024–0.028 in.) before installation using a spark plug gap tool. Adjust the gap by bending only the outer ground electrode — never bend the center electrode or you'll crack the porcelain.
Apply a light dab of oil to the threads, then finger-tighten the plug all the way before snugging it down with an 18mm long-reach spark plug socket so the gasket compresses evenly.
Terminal nut vs. terminal stud: The D8HA ships with a terminal nut threaded onto the top. CMC spark plug boots connect to the stud underneath, not the nut. Use pliers to unscrew the nut — it can be very tight. See our terminal nut vs. stud guide for details.
Service interval: Pull and inspect the plug every 1,000–1,200 miles as part of the CT90 tune-up — same cadence as oil change, points adjustment, valve clearance, and ignition timing. Replace whenever the plug is worn, fouled, or out of gap spec.
Reading Your Spark Plug
After a 5–10 mile ride at normal operating temperature, pull the plug and check the porcelain tip:
- Light tan to chocolate brown — normal, healthy mixture
- Black, dry carbon deposit — rich fuel mixture, dirty air filter, or prolonged idling
- Oil fouling (wet, glossy black tip) — worn rings, worn valve guides, or ignition timing off
- White crust or blistered porcelain — lean fuel mixture or heat range running too hot
If a fresh plug fouls quickly and your carb is correctly jetted, the next things to check are the ignition components: contact points, ignition condenser, and ignition coil.
Cleaning a Used Plug
A spark plug cleaner blaster is the best way to clean a used plug — it strips deposits without damaging the porcelain or electrode. If a cleaner is not available, use a thin needle or stiff wire to scrape deposits off the electrode, wash the plug in gasoline, and blow it dry with compressed air. Re-gap before reinstalling. Cleaning is a stopgap; a fouled plug usually points to a carb or ignition problem worth chasing.
Frequently Asked Questions
How many spark plugs does my CT90 need?
One. The CT90 is a single-cylinder engine, so one D8HA per bike. The vintage Honda twins (CB175, CB350, CB450) use a different plug per cylinder — see the BR8ES for the CB350/CB360/CB450 family.
What gap should I set on the D8HA?
0.6–0.7 mm (0.024–0.028 in.). Some older guides and aftermarket boxes list .030" as a rule of thumb — that is too wide for the CT90. The correct range for every CT90 model year is 0.6–0.7 mm.
Why won't my spark plug boot fit on the new plug?
Unscrew the terminal nut on top of the plug to expose the terminal stud underneath. The boots connect to the stud, not the nut. Use pliers — it can be very tight. See our terminal nut vs. stud guide.
Does the D8HA fit my CB175 or CB350?
No. The D8HA is specific to the CT90 single-cylinder. The CB350 / CB360 / CB450 / CB500T twins use the NGK BR8ES, and the CB175 / CL175 / SL175 twins use a different D-series plug sold separately. Do not substitute — wrong heat range or reach will damage the engine.
How often should I replace the spark plug?
Inspect every 1,000–1,200 miles as part of the CT90 tune-up. Replace whenever the plug is worn, fouled, or out of gap spec. A copper-core plug is cheap insurance — keep a spare on the shelf.
You Might Also Need
Last updated: May 2026
