How often do you change your spark plugs?

[jameshoward]

 

I ask because I'm about to change mine, which will be the first time in about 5 years. I was chatting with a chap at the weekend who owns a Mk 1 Ford Escort that he rallies. He changes his plugs every few races. So about 6 times a year. We discussed the merits and his point (no pun) was that he would rather spend the money than risk a plug failing. I've never had a plug fail, so to speak, and can quite believe that they last 30K miles. However, given the use my car gets, at that rate I'd probably change my plugs in The Year 2525.

So I wondered what others do, and why?

At about £10 for a set of 6 BP7s it's hardly an expensive issue, but I'm interested none the less.

James

[Cees Klumper]

I've replaced mine twice I think in 14 years of ownership and around 40 thousand miles. Once because one or more did fail, due to an incorrect fuel mixture, and the last time because I wanted to try out a different heat rating (didn't notice a difference). I carry a spare set with me so if a failure occurs I can swap them out roadside if need be. In a race that is a different matter so I understand that practice. But changing them very often 'just because you can, it can't hurt and it's not expensive' reminds me of some of my motorycling colleagues who change their engine oil every thousand km's, "because it's better than every five thousand like the owner's manual says".  It's just not necessary.

[mdsalemi]

http://www.ngksparkplugs.com/oreilly/tech_support/spark_plugs/faqs/faqchange.asp

That's a good explanation.  Pull a plug; clean it, and compare the electrode profiles to new.  If they look seriously eroded, perhaps it is time to change them.  If not, clean them up and put them back.

N.B. Always keep a spare set around...

[enochbell]

There may be a reason to replace at a reasonable (15k?) interval besides the electrodes.  On newer MBZs the interval in 50k, but these are platinum plugs.  I have replaced mine at that interval even though there was virtually no wear.  But I understand that they should be replaced or, at least, removed and replaced to make sure they don't get a chance to seize.  Could it be that on the older SLs that seizing is a bigger worry?

g

[UJJ]

I have to agree with enochbell,
since I had a scary experience the first time I changed my spark plugs after I purchased my 280 SL.
#2 cylinder plug was seized!     I used penetration oil and tapped the plug with a pipe I made to fit over the stem resting against the top of the hex -head. I repeated this procedure for about 1 hour and then I tried to loosen it again. I was able to give it a ¼ turn only.  Afraid that I break it off, I continued doing the same thing over and over again. I was able to get it out after hours of rotating it clockwise, counterclockwise, lubricating and tapping it; taking brakes in between to let the penetration oil (PB) do its job. 

Ever since I remove my plugs once a year, inspect the wear and their color then lubricate the thread with the proper high temp grease designed for that purpose. This puts my mind at ease.  I hate to take the head off of a nicely running engine just because a plug is seized.
Besides, it is a reason to use my beautiful fender covers I got for Christmas a couple of years ago!!
I love to receive gifts like that!  Who needs more ties, socks or under wear? 
Urban

[Rodolfo]

I wouldn't underestimate underwear, how much I like pagodes .

[garymand]

I used to worry about the the plugs sticking but not anymore.  After many alu heads and using anti-seise grease on each installation, I have never come close to a stuck plug.  Specifically on the SL , I haven't changed the plugs since about 1980: BP6ES.  I replaced the head gasket last Spring and put the old plugs back in.  They didn't look too worn.   

The only failures I've had with my plugs is with the standard ignition: worn plugs would miss at the top end and Bosche lugs had too narrow a heat range and would miss when cold at the top end.  ( hate reving to 5 or 6 k and getting push back from a set of plugs.)  With a CD ignition, I haven't experieced a plug miss.  Then too,  these plugs probably only had have 7 or 8k miles on them. 

Use anti-seise liberaly on the plug threads, be sure you don't miss-start the threads -use your fingers to know you have them started accurately, be sure they are all the way in and seated,  last, be sure to torque them so you know they are all the way in and sealed.  Its really touch and feel screwing plugs into alu heads -the angles can confuse beginners.

1) YOU DON'T WANT TO CROSS THREAD THEM.  If you feel ANY binding stop, start over.  There is a steel plug thread chaser that will clean-up slightly crossed -slightly miss threaded plugs.  I have had to use it on newly acquired cars or a few friends cars.  If you do get a plug stasrted wrong, I would just go get a thread chaser and clean them up - get the threads working really well before trying to get tha plug back in.
2) YOU DON'T WANT THEM LOOSE.  If they don't seal, combustion will come out around the threads and mske it very difficult to get them out.  You then wasn tot chase those threads clean and re-grease the plug.