4.0l OHV and 4.0l SOHC use different length spark plugs but the recommended gap is the same, 0.054
Recommended gap is not written in stone, it is determined for overall performance and life.
As an engine gets older it will get lower compression and some cylinder oil intrusion, from valves and/or rings, this is normal, welcome to the physics of friction, lol.
General rule of thumb for spark plug gaps is that you can change the recommended gap by +/- 0.01 to suit your driving habits and engine condition, so 0.054 could be gapped 0.044 to 0.064
The larger the gap the stronger the spark but the shorter the time between regapping or replacement.
Larger gap will give better cold starts and low RPM performance.
Smaller gap will have harder cold starts and better high RPM performance.
There is a heat and recovery time frame at work here, wide gap produces a stronger spark BUT at high RPM, 4,000+, you will see power curve dropping off because coil can't charge fully for larger gap, narrower gap has a "weaker spark" BUT a better spark above 4,000 RPM than wider gap, so power curve stays higher longer.
If you are dealing with higher mile engine that struggles cold then going from 0.054 to 0.059 would be a good test so see if spark is the reason for the struggling.
Last edited by RonD; 03-29-2014 at 02:50 PM.