I’m sure a crush washer could be replaced, but have you ever tried to find one? I haven’t, like you I just replace the plug.
Being curious, I actually searched around a bit, and found virtually no one saying it was a problem to reuse a crush washer. I didn’t find anything very definitive though (such as what a manufacturer has to say about it.
Honestly, I don’t think it is an issue to reuse a crush washer. The issue is HOW to reuse it. Most FSMs (factory service manual) have a torque spec for the spark plugs and that is ok as long as the threads are good and clean. If you add a little anti-seize, the clamping force on the crush washer will actually be a little higher than design, about 10% but that could be good as it will give the crush washer just a little more crush to insure a good seal.
Crush washers have a design compression to give maximum sealing, so most plug manufacturers just give a torque angle spec instead of a torque spec. This is based on the amount of crush times the thread pitch. So if the plug mfg says 7/8th turn after contact, then 87.5% of the thread pitch will be the ideal crush for the washer. Once it crushes, it does not rebound much like a rubber seal would. There is some rebound from the modulus of elasticity of the metal in the washer, maybe 1-3% based on the metal.
So basically, you can’t reuse the plug mfg recommendation on a used crush washer. The vehicle mfg torque spec should be OK.
However, I still don’t like reusing a plug, especially on my Subaru which requires jacking up the engine to get to the plugs.
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I always went by the adage, if you take a plug out, you put a new plug in. Almost no car manufacturers recommend just inspect the plugs anymore. OTOH if you’re looking at the plugs for a running issue, inspect, correct any issue (eg. rich/lean) and still replace the plug.
A friend has to replace a single plug frequently in his Avalon. Previous owner didn’t maintain it well.
Just a couple comments. Yeah I buy from rock auto but their stuff is shipped from multiple supplier warehouses. How tightly the inventory is checked is unknown but it’s not like they have their own staff working these locations.
I’ve done the oil analysis. The instructions say to take the sample from the mid point of a drain. So if you drain half the oil out just to get a reliable sample, what’s the point. Might as well just change oil at that point. Might be nice to know the condition of the oil and the metals for wear information, but I’m not sure it make sense to use as a guide to extend oil changes, after paying the $30 for a sample.
Would this not indicate a problem with one cylinder? Why the need to frequently replace just the one plug?
California did oil analyses when they changed the oil in their fleet vehicles. They would review the data and set an extended interval, then review the data and make changes as needed. This led them to extend the change intervals for all vehicles. The only one I remember is the Cavaliers in the fleet. They ended up at 10,000 miles for the change interval instead of the recommended 7500 miles. For you and me this might not mean much, but for a large fleet it saves a lot of money and labor, allowing the techs to do other maintenance or repair work more quickly.