Let's assume your engine is well running.
How often do you have to change the plugs if you drive only 4-5k mls per year?

Any difference between older air cooled and h2o engines?

Thanks for the inputs!

Every year, sometimes twice per year.

Usually that's due to inspection when it shows too much build-up or fouling, etc. They can be sand-blasted for minor normal build-up, but get a spark plug guide online with pix to show what the different deposits mean.

At that low rate plugs can go a long time, but probably are best changed at the owners manual recommended tune-up interval time or miles. However, they're pretty cheap for the OE recco'd copper plugs in the 914s, so not a wallet buster if you replace them more often.

We do them on the factory schedule for our 2 DDs (85 BMW 325e & 88 VW Westy) with 2-3000+/- mi/yr now that our kids are grown & gone, & what my guy did with my 914 when it was a DD at a more avg. 7-10k+ per year. He was factory trained on 914s & was Circle Porsche's specialist on them before he started his own shop in `72. Later this year will be 40 years with him this summer, since he PPI'ed several 914s for me & worked on my 914 & others since then - 40 years & he knows his stuff!

Cheers!
Tom
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Don't sweat the plugs. I would change then at the same time you change your cap/rotor/points. Around 15K miles unless there's a problem traced back to them (which is rare).

Every time you pull the plugs you needlessly risk X-threading them in the Aluminum head. This is more common than most folks will admit to. I typically change my plugs/cap/rotor every year or two. My car is a really good running L-jet car with over 350K miles on it But I typically put 15K-20K per year on my car too, so I typically go well beyond what the book says. They still look pretty good when they come out too. It more depends on the running condition of the car than anything else.

BTW, I have never gotten mileage like that out of a set of plugs from an engine I ran carbs on.

Prior to 1974, the recommended plug change interval was 6K miles. That changed to 10K in 1974. I suspect with modern lead-free fuels, 15K wouldn't be out of order. You should still do a maintenance service at the intervals recommended for your car because points and filters get used up, and changing the plugs at the same time makes good sense, as they need to come out for a compression test and the plug gasket shouldn't be reused (replacement gaskets used to be available). Sand blasting the plugs REDUCES their lifespan. If you don't know the difference between a "maintenance service" and a "tune-up", bad things can happen to your car and they'll be a total surprise.

The Cap'n

Do you guys use a smear of anti-seize on them? Id say the biggest reason to change them or atleast pull and inspect is to keep them moving and not seize up and strip out the heads. A few of my freinds have cars that theyve never messed with the plugs and they are very stuck. Iv also heard sandblasting is a no no. Soft wire brush and carb cleaner is all you need

I left the plugs in my Harley for 55K miles and they did not need replacing at that time. With modern fuels and mega volt ignition plugs last much longer than they used too.