Harley M8 Spark Plug Schedule
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Road Captain
Joined: Nov 2018
Posts: 614
Likes: 231
From: Hendeson, NV
Harley M8 Spark Plug Schedule
HD recommends changing spark plugs every two-years or 8k to 10k miles on an M8. I had a little over 10k miles on my SE spark plugs; so, I changed the plugs yesterday. (I like to do my own maintenance within the limits of my skills and tools.) Changing the plugs on an M8 engine isn't a difficult task, if you have a little patience; it took me about 30 minutes.
The spark plugs had the correct color for plugs in a properly tuned engine and looked in good shape. But, I had bought a set of NKG Iridiumix spark plugs; so, I went ahead and changed the plugs since I had the time. I then took my bike on a 25+ mile test run. I was surprised at the improvement in performance, particularly accelerating from a full stop and on low speed maneuvers. Overall the throttle response was more precise, smoother and quicker.
Moral of story. HD's recommendation is spot on. Spark plugs are another maintenance item, like fluid changes, for which owners should follow HD's service interval recommendation even if you feel the bike is running well.
The spark plugs had the correct color for plugs in a properly tuned engine and looked in good shape. But, I had bought a set of NKG Iridiumix spark plugs; so, I went ahead and changed the plugs since I had the time. I then took my bike on a 25+ mile test run. I was surprised at the improvement in performance, particularly accelerating from a full stop and on low speed maneuvers. Overall the throttle response was more precise, smoother and quicker.
Moral of story. HD's recommendation is spot on. Spark plugs are another maintenance item, like fluid changes, for which owners should follow HD's service interval recommendation even if you feel the bike is running well.
Seasoned HDF Member
Joined: May 2018
Posts: 5,814
Likes: 7,145
From: Las Vegas
HD recommends changing spark plugs every two-years or 8k to 10k miles on an M8. I had a little over 10k miles on my SE spark plugs; so, I changed the plugs yesterday. (I like to do my own maintenance within the limits of my skills and tools.) Changing the plugs on an M8 engine isn't a difficult task, if you have a little patience; it took me about 30 minutes.
The spark plugs had the correct color for plugs in a properly tuned engine and looked in good shape. But, I had bought a set of NKG Iridiumix spark plugs; so, I went ahead and changed the plugs since I had the time. I then took my bike on a 25+ mile test run. I was surprised at the improvement in performance, particularly accelerating from a full stop and on low speed maneuvers. Overall the throttle response was more precise, smoother and quicker.
Moral of story. HD's recommendation is spot on. Spark plugs are another maintenance item, like fluid changes, for which owners should follow HD's service interval recommendation even if you feel the bike is running well.
The spark plugs had the correct color for plugs in a properly tuned engine and looked in good shape. But, I had bought a set of NKG Iridiumix spark plugs; so, I went ahead and changed the plugs since I had the time. I then took my bike on a 25+ mile test run. I was surprised at the improvement in performance, particularly accelerating from a full stop and on low speed maneuvers. Overall the throttle response was more precise, smoother and quicker.
Moral of story. HD's recommendation is spot on. Spark plugs are another maintenance item, like fluid changes, for which owners should follow HD's service interval recommendation even if you feel the bike is running well.
Seasoned HDF Member
Joined: Feb 2008
Posts: 22,999
Likes: 11,439
M8 is 30,000 . My 2017 had first change at 29,500 plugs still fine . but once out new goes in.
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Road Captain
Joined: Nov 2018
Posts: 614
Likes: 231
From: Hendeson, NV
You are correct; my book says the same. But, several third-party reviews and local HD mechanics are experiencing that spark plug wear on the M8 engines is occurring faster than HD originally projected.
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Seasoned HDF Member
Joined: May 2018
Posts: 5,814
Likes: 7,145
From: Las Vegas
I hope you find the time to come back here and share your thoughts as time and miles rack up.