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A generator that will not start when the power is out leaves the fridge warming and the lights off, and the spark plug is the part to look at first. Generator spark plug size is the plug number that matches your engine, and it comes down to thread diameter, thread length, hex size and heat range. The listings in this guide cover 4 plugs: a 4-pack with a wrench, a plug with stated thread measurements, a genuine Honda plug (on Amazon) and an NGK plug (on Amazon), and the steps below show how to read the old plug and match a new one.
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The quick answer
The size of a generator spark plug is the plug number, and the number encodes the thread diameter, the thread length, the hex size and the heat range. Read the number off the old plug or the generator manual, then buy the plug with that same number. The TORCH F6TC (on Amazon) in this guide states a 14 mm thread diameter, a 19 mm thread length and a 20.8 mm hex size, and the GREPSPUD F7TC (on Amazon) states a 21 mm hex on its wrench. Cross-references help too: the TORCH F6TC is listed as a replacement for NGK BP6ES, and the Honda plug is listed to fit NGK BPR6ES.
Generator spark plug size: the 4 measurements
4 measurements decide whether a plug fits, and the listings in this guide state them in different amounts of detail.
- Thread diameter. The width of the threaded part, in millimetres. The TORCH F6TC states a 14 mm thread diameter.
- Thread length. How far the threaded part reaches into the engine. The TORCH F6TC states a 19 mm thread length.
- Hex size. The socket size that turns the plug. The TORCH F6TC states a 20.8 mm hex, and the GREPSPUD wrench states a 21 mm hex with a 180 mm length.
- Heat range. How hot the plug runs in the engine. The GREPSPUD listing states a wider heat range than ordinary plugs, with no heat range figure stated, and none of the other listings states a heat range figure either.
When a listing states no thread size, the plug number is the only match to use: the Honda plug is listed as part number 98079-56846 to fit NGK BPR6ES, and the NGK plug is listed as part number 6578, type BPR4ES.
The spark plugs that fit these engines
Each plug below covers one part of the matching job: the GREPSPUD 4-pack with a wrench for several engines, the TORCH plug with stated thread measurements, the Honda plug with the exact OEM part number, and the NGK plug for listed Argo, Case IH and John Deere models.
| # | Product | Solar Score | Buy |
|---|---|---|---|
| 1 |  GREPSPUD F7TC 4-Pack | 9.7/10 | Buy on Amazon |
| 2 |  TORCH F6TC Spark Plug | 9.6/10 | Buy on Amazon |
| 3 |  Honda BPR6ES 2-Pack | 9.6/10 | Buy on Amazon |
| 4 |  NGK BPR4ES 2-Pack | 9.6/10 | Buy on Amazon |
GREPSPUD F7TC 4-Pack
4 F7TC plugs with a spark plug wrench, listed for Honda GX engines, Generac portable generators and most 4-stroke engines.
Pros
- 4 plugs plus a wrench in the box
- Wrench stated at 180 mm long with a 21 mm hex
- Compatible with Honda part numbers 98079-56846 and 98079-56855
- Compatibility list covers Honda GX engines and Generac portable generators
Cons
- No warranty term is stated
- No thread diameter is stated
- No thread length is stated
- No heat range figure is stated
Detailed Review
The listing states 4 F7TC spark plugs with a spark plug wrench, a removable terminal nut, corrosion resistance and a wider heat range than ordinary plugs. The wrench is stated at 180 mm long with a 21 mm hex. Compatibility covers Honda part numbers 98079-56846, 98079-56855 and 98079-56876-H0, Honda GX120, GX160, GX200, GX240, GX270, GX340 and GX390 engines and Generac portable generators, plus most 4-stroke engines for generators, lawn mowers, tractors, chainsaws, rototillers, water pumps, go-karts, mini bikes and snow blowers.
Who it's for: an owner with several 4-stroke engines who wants spare plugs and a wrench in one box. Who should skip it: anyone who needs 1 plug for one engine, or a stated thread size in the listing.
Its Solar Score of 9.7/10 comes from performance 30/35, build 21/25, value 22/25, safety not scored (non-powered gear) and warranty 0/5: 73 of 90 points, placed on our 9.2 to 9.8 scale.
Key Specifications
| Plug | F7TC |
|---|---|
| Pack | 4 plugs and 1 wrench |
| Wrench | 180 mm long, 21 mm hex |
| Terminal | Removable terminal nut |
Figures as published on the retailer listing, collected 12 November 2026. Performance figures are manufacturer rated.
Safety and Warranty
What this listing states. This product does not plug in or run on a battery, so safety is not part of its score. No warranty term, so warranty scores zero. See How We Pick.
Follow the manual. Connect and use the product as the maker describes, stay within every stated rating, and read the manual before first use.
This is general buying information, not professional advice. Follow each product manual and local rules, and see our Safety Disclaimer.
TORCH F6TC Spark Plug
One F6TC plug with a stated 14 mm thread, 19 mm thread length and 20.8 mm hex, listed as a replacement for NGK BP6ES.
Pros
- Thread diameter, thread length and hex size all stated: 14 mm, 19 mm and 20.8 mm
- Listed as a replacement for NGK BP6ES, Bosch W6D and CHAMPION N9YC
- Listed for Honda GX140, GX160 and GX200 5.5hp and 6.5hp engine generators
- Maker states longer service life, fast starts and stable performance
Cons
- No warranty term is stated
- 1 plug in the box
- No electrode gap is stated
- No heat range figure is stated
Detailed Review
The listing states a TORCH F6TC plug with a 14 mm thread diameter, a 19 mm thread length and a 20.8 mm hex size. It lists the plug as a replacement for NGK BP6ES, Bosch W6D, CHAMPION N9YC, DENSO W20EP-U, Honda 98079-56841, AUTOLITE 53 and Brisk L15YC, and for Honda GX140, GX160 and GX200 5.5hp and 6.5hp engine generators. The maker states longer service life, fast starts and stable performance.
Who it's for: an owner of a Honda GX140, GX160 or GX200 generator who wants the thread measurements stated before buying. Who should skip it: anyone who needs a pack of plugs or a stated warranty term.
Its Solar Score of 9.6/10 comes from performance 30/35, build 21/25, value 21/25, safety not scored (non-powered gear) and warranty 0/5: 72 of 90 points, placed on our 9.2 to 9.8 scale.
Key Specifications
| Thread diameter | 14 mm |
|---|---|
| Thread length | 19 mm |
| Hex size | 20.8 mm |
| Replaces | NGK BP6ES |
Figures as published on the retailer listing, collected 12 November 2026. Performance figures are manufacturer rated.
Safety and Warranty
What this listing states. This product does not plug in or run on a battery, so safety is not part of its score. No warranty term, so warranty scores zero. See How We Pick.
Follow the manual. Connect and use the product as the maker describes, stay within every stated rating, and read the manual before first use.
This is general buying information, not professional advice. Follow each product manual and local rules, and see our Safety Disclaimer.
Honda BPR6ES 2-Pack
2 genuine Honda 98079-56846 plugs that fit NGK BPR6ES, listed for Honda GC and GX engine series.
Pros
- Genuine Honda part 98079-56846 that fits NGK BPR6ES
- 2 plugs in the pack
- Fits Honda GC135, GC160, GCV160, GC190, GX120, GX160, GX200, GX240, GX270, GX340, GX390 and GXV160 engines
- The listing names the exact Honda part number
Cons
- No warranty term is stated
- No thread diameter, thread length or hex size is stated
- No electrode gap is stated
- No heat range figure is stated
Detailed Review
The listing states a 2-pack of genuine Honda 98079-56846 spark plugs that fit NGK BPR6ES. It names Honda GC135, GC160, GCV160, GC190, GX120, GX160, GX200, GX240, GX270, GX340, GX390 and GXV160 engine series. No thread size, heat range or warranty term is stated.
Who it's for: a Honda engine owner who wants the genuine OEM plug number for the engine. Who should skip it: anyone who needs thread measurements or a heat range figure in the listing.
Its Solar Score of 9.6/10 comes from performance 29/35, build 21/25, value 20/25, safety not scored (non-powered gear) and warranty 0/5: 70 of 90 points, placed on our 9.2 to 9.8 scale.
Key Specifications
| Part number | 98079-56846 |
|---|---|
| Pack | 2 plugs |
| Fits | NGK BPR6ES |
| Engines | Honda GC and GX series |
Figures as published on the retailer listing, collected 12 November 2026. Performance figures are manufacturer rated.
Safety and Warranty
What this listing states. This product does not plug in or run on a battery, so safety is not part of its score. No warranty term, so warranty scores zero. See How We Pick.
Follow the manual. Connect and use the product as the maker describes, stay within every stated rating, and read the manual before first use.
This is general buying information, not professional advice. Follow each product manual and local rules, and see our Safety Disclaimer.
NGK BPR4ES 2-Pack
2 NGK BPR4ES plugs with a copper core and trivalent plating, listed for select Argo, Case IH and John Deere models from 2010 to 2024.
Pros
- Trivalent metal plating for anti-corrosion and anti-seizing
- Corrugated ribs prevent flashover
- Pure alumina silicate ceramic insulator for strength and heat transfer
- Copper core aids in heat removal
Cons
- The warranty description gives no term
- No thread diameter, thread length or hex size is stated
- No heat range figure is stated
Detailed Review
The listing states NGK 6578 BPR4ES plugs with trivalent metal plating, corrugated ribs to prevent flashover, a pure alumina silicate ceramic insulator and a copper core. It lists compatibility with select Argo, Case IH and John Deere models from 2010 to 2024. The title states a quantity of 2 while the spec table lists 1 item, so the title is the figure to trust.
Who it's for: an owner of a listed Argo, Case IH or John Deere model who wants the NGK BPR4ES plug. Who should skip it: anyone who needs thread measurements or a stated warranty term.
Its Solar Score of 9.6/10 comes from performance 28/35, build 20/25, value 20/25, safety not scored (non-powered gear) and warranty 0/5: 68 of 90 points, placed on our 9.2 to 9.8 scale.
Key Specifications
| Part number | 6578 |
|---|---|
| Plug | BPR4ES |
| Pack | 2 plugs |
| Fits | Argo, Case IH and John Deere, 2010-2024 |
Figures as published on the retailer listing, collected 12 November 2026. Performance figures are manufacturer rated.
Safety and Warranty
What this listing states. This product does not plug in or run on a battery, so safety is not part of its score. No warranty term, so warranty scores zero. See How We Pick.
Follow the manual. Connect and use the product as the maker describes, stay within every stated rating, and read the manual before first use.
This is general buying information, not professional advice. Follow each product manual and local rules, and see our Safety Disclaimer.
As an Amazon Associate I earn from qualifying purchases.
How to match a plug to your generator
Start with the old plug and the generator manual, not the shelf.
- Read the old plug. The plug number is printed on the ceramic insulator. Match that number exactly: a TORCH F6TC replaces an NGK BP6ES, and a GREPSPUD F7TC is listed as compatible with Honda part 98079-56846.
- Check the manual. The manual names the plug number for the engine. The Honda plug here is listed for Honda GC135, GC160, GCV160, GC190, GX120, GX160, GX200, GX240, GX270, GX340, GX390 and GXV160 engines.
- Match the measurements. Where the listing states them, compare thread diameter, thread length and hex size with the old plug. The TORCH F6TC states 14 mm, 19 mm and 20.8 mm.
- Count the plugs. The GREPSPUD box holds 4 plugs plus a wrench, the Honda box holds 2 plugs, and the NGK title states a quantity of 2 while its spec table lists 1 item, so the title is the figure to trust.
Run the generator outdoors only, far from doors and windows, because of carbon monoxide. See the best 4000-watt inverter generators with CO detect for units with carbon monoxide detection.
FAQ: power away from the grid
What size spark plug does my generator take?
The size is the plug number in the generator manual or on the old plug. That number covers thread diameter, thread length, hex size and heat range, so buy the plug with that same number. The TORCH F6TC states a 14 mm thread diameter, a 19 mm thread length and a 20.8 mm hex size.
Can I use an F6TC plug instead of an NGK BP6ES?
Yes, if the listing states it as a replacement. The TORCH F6TC is listed as a replacement for NGK BP6ES, and also for Bosch W6D, CHAMPION N9YC and DENSO W20EP-U. Match the plug number to the old plug and the manual before you buy.
What does the hex size on a spark plug mean?
The hex size is the socket size that turns the plug. The TORCH F6TC states a 20.8 mm hex, and the GREPSPUD wrench states a 21 mm hex with a 180 mm length. Use the hex size the listing states for the plug you buy.
Do I need a different plug for each generator?
Yes. Match the plug number to the engine: the Honda 98079-56846 plug fits NGK BPR6ES and the listed Honda GC and GX engines, while the NGK BPR4ES fits the listed Argo, Case IH and John Deere models from 2010 to 2024. A plug that fits one engine does not fit every engine.
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