Author Topic: Spark plug changing information  (Read 181 times)

Droneranger

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Spark plug changing information
« on: June 11, 2021, 12:51:35 PM »
Here is some information from NGK on changing spark plugs that should correct some advice given in earlier posts.


culzean

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Re: Spark plug changing information
« Reply #1 on: June 11, 2021, 01:19:14 PM »
Here is some information from NGK on changing spark plugs that should correct some advice given in earlier posts.

Never used a torque wrench on any spark plug ( or anything else except important things like big end bolts or cylinder head bolts where you need to actually stretch the bolt a fairly accurate amount when tightening it 'preload' ).  For normal car sized spark plugs the 'hand tight and then 1/2 turn' for a fresh crush washer and '1/8 turn for a used washer' ( if you are refitting a used plug) is all I do.  Used to use a light smear of copper grease on the older passivated zinc plated non-iridium plugs because they could give problems with  seizing up,  but with long life plugs they have a tri-valent coating that is specially designed not to stick in aluminium heads, so haven't bothered with lube for last 10 years+.

Torque values are always for a dry thread. 
« Last Edit: June 11, 2021, 02:56:51 PM by culzean »
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Droneranger

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Re: Spark plug changing information
« Reply #2 on: June 11, 2021, 05:18:35 PM »
Here is some information from NGK on changing spark plugs that should correct some advice given in earlier posts.

Never used a torque wrench on any spark plug ( or anything else except important things like big end bolts or cylinder head bolts where you need to actually stretch the bolt a fairly accurate amount when tightening it 'preload' ).  For normal car sized spark plugs the 'hand tight and then 1/2 turn' for a fresh crush washer and '1/8 turn for a used washer' ( if you are refitting a used plug) is all I do.  Used to use a light smear of copper grease on the older passivated zinc plated non-iridium plugs because they could give problems with  seizing up,  but with long life plugs they have a tri-valent coating that is specially designed not to stick in aluminium heads, so haven't bothered with lube for last 10 years+.

Torque values are always for a dry thread.

The problem with your advice is how tight is hand tight, and what tool are you using to get to hand tight, some people are stronger then others.
I used a torque wrench when i last changed spark plugs, I also use a torque wrench whenever I tighten my wheel nuts, also on my car brakes. I have tools, so why not use them. I have a few torque wrenches.
The second image in my post states that the car engine should always be cold when changing spark plugs, I mention this for those of you who have not read through the information.

culzean

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Re: Spark plug changing information
« Reply #3 on: June 11, 2021, 05:41:07 PM »
Hand tight is using either the box spanner or socket extension with no tommy bar or ratchet handle in place, just snug the spark plug down by finger pressure - you only want to get the crush washer touching the cylinder head - then fit the tommy bar or attach the ratchet drive and move it 180degrees.  Do you have you calibration checked on torque wrenches every year, are they quality maker.  A cheap or uncalibrated torque wrench can be very misleading. Do you always screw the wrench below the required torque and then take it up to required torque,  Do you store torque wrench with spring slackened off ?

Off spark plugs for a second -  the motor industry generally, and engine builders in particular use automatic torque+ angle to ensure correct tightening.  This means a small torque to make fastener face contact the seating  ( pretty much equivalent of 'hand tight' ) and then rotate the fastener through a known angle,  this is way more accurate than a torque wrench on its own.  All a torque wrench is measuring is friction between the threads, and once the faces are together it measures friction between the face of the fastener and its seat. Many fasteners in motor industry in engines do get lubricated with a specified lubricant before tightening, any deviation from lubricant affects friction which affects torque.  I rather use threadlock on brake fasteners than torque wrenches.

I worked for a supplier of assemblies to many car companies and we used the low torque + angle method on fasteners, many of them were automatic electrical powered devices that would lock out the assembly cell if correct angle was not achieved.

https://www.enginerepairshop.com/torque-specifications.html
« Last Edit: June 11, 2021, 05:56:20 PM by culzean »
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Westy36

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Re: Spark plug changing information
« Reply #4 on: June 11, 2021, 09:32:20 PM »
Copper slip on threads is bonkers. Good post!  :D Good point re cold engine too. Well worth remembering, although in winter it's tempting to change the plugs when the engine is warm so your hands don't freeze!

Fair point about how tight is hand tight? For me it has been down to servicing my own motors for 30yrs and experience. Touch wood never had a problem, but first turn of the key post plug renewal always comes with a little mild apprehension! Torque wrench is safest for sure though.

Wheel nuts. F. tight is how tight.  ;D ;D Hard to over tighten yourself I would imagine. For sure, wheel places with their air guns can be over zealous, but I doubt anyone would over tighten their own nuts.

What is worth remembering, is to check they are tight again after several miles.
« Last Edit: June 11, 2021, 09:35:01 PM by Westy36 »

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