For my H4 non turbo engine, I replaced the four factory OEM NGK spark plugs Subaru p/n 22401AA65A or FRP5AP-11 with the identical plugs at 125k. These NGK platinum tips are rated at 90K. The tips on two of them eroded from .043 gap to .071 gap. The factory manual recommends 60k intervals and based on my findings, I would change no later than 90k for my application the next time. The odd thing is there were no check engine lights or signs of misfiring or significant loss of mpg. I'm usually around 25mpg with mixed driving or 27mpg in the summer.

60k may not be bad advice because the tube seals also tend to harden up and let oil traces inside and could cause shorts from the ceramic insulator to the boot. Estimated time to replace plugs was about 1.5 hours or less if I rushed but you just need to have the right combination of extensions and patience to do the job.

I think it's kind of funny that just because the factory "recommends" .xxx" plug gap that all cars are set exactly at that when they roll off the production line. I can pretty much guarantee you that the factory does the exact same thing that people that aren't overly anal on things that don't matter do... they unbox them and screw them into the engine. Done.

So you can't say "they opened up .028" over 90k" or whatever unless you were personally the one that measured them new and then when they were changed. I promise you that 50,000+ volts cares not about a couple thousandths difference.

I can say they opened up to .028 for the worst one while the other were .052, .045., and .070. The fine wire style center electrode under visual inspection had two where the shape was an eroded blob. The other two still had reminisce of the step down tip shape. With the four brand new ones that were purchased and well within .041 +/-.002, I have confidence that the factory ones also were at that permissible recommended range.

I cannot decide how other people install their plugs but I will continue to measure mine and compare to specs and for the hundreds that I have measured, all were within vehicle recommended specs out of the box. The extra 5 seconds it takes gives me a measurable data point of what went into my engine.

CCS, you're likely the exception, and why I made the point. Only a few measure before they put them in; I don't know for certain that the vehicle manufacturer does but at the speeds of production it doesn't make sense that they would. As long as you have a sharp edge on the tip and the ground strap, and enough voltage to jump the gap, there should not be any really noticeable difference in performance.

I will agree with your statement that 100% gap check at plant doesn't make sense but probably disagree with you that automotive assy has more requirements than what the outside world knows about. Maybe a new posted thread should be created vs. hijacking this thread. Previously, I was a process mfg engineer where our company designs products like brake transducers, occupant weight sensors, heavy duty fluid pressure sensors as part of the portfolio for the big Asian and European vehicles. Their necessary demands and requirements upon us goes into the demands we have with our car makes.

In this example, what are the top three topics about plugs generally discussed? Gap, torque, brand??? So if you were part of the Subaru engine assy team and plug installation was under your ownership...gap would be listed as a key characteristic. There is a pretty tight tolerance for this spec so doubt if a precursor check is used. That means by definition this aspect must be checked more than just shift start or time check vs. output because now your into 5 or even 6 sigma quality level in which the plant's goal is striving for. Let's say realistically <250 rejects per 1M opportunities that can escape and still be called a stable robust process.

I am familiar with all the checks and balances that went into mfg of sensors and all the requested automation that goes along with it. For spark plug gap, that measurement can be made a mile away at way faster than engine plant assy thruput speed. I know these plugs are purchased in bulk trays and not individually like we buy them and staged where the gap is exposed and correctly orientated so a vision camera inspects/rejects thus 100% inspection before presenting to technician or even a pick and place robot arm staging queue that will zips them into the engine.

So to get back on track for OP topic, I checked my notes and made a slight err in that I did replace the plugs earlier along with all the timing and original roller hardware at 95K because factory recommended age exceeded well before mileage. So for my application and my findings suggests that yes...my model is hard on spark plugs.

Which year? My non turbo h4 also has those plugs, and Subaru says to replace the plugs every 30k.

2006 Outback with the EJ253 motor.