Do not remove sparkplugs when engine is hot?
 

Makes total sense, might bring threads with you....I had never heard that until recently though....hence no hot leak down test.

What do folks think? I was searching and saw that one regular backs the plugs out cold and then leaves them in finger tight for the warm up prior to a leak test...not sure that is a good idea...

I have taken plugs out hot on previous cars a few times..guess I wont do that anymore.

According to what I've read and heard, this is the way to go. Loosen carefully, tighten finger-tight ( or a bit more than just finger-tight) and loosen when warm to do leak down test.

Loosening in advance prevents damage to threads.

Makes sense, just a touch past finger tight and then a gentle drive to get it warmed up....I am just surprised with all the cars I have messed with that have aluminum heads that I never heard/thought of this??

crikey, i do it all the time. never had an issue. you get one now and then that is seized solid and it wouldn't have mattered if it was hot or cold. must be an old wife tale.

That's what I typically do. I don't see any problem with it. I like to break loose the plugs when they are cold. Even if there's no difference, it's easier to work on a cold engine, and you don't have that option of burning yourself.

-Wayne

How did I know JW would say that :-)

Good to know.

It probably comes down a lot to maintenance on the motor and has it been 5 years since the plugs were out...

This info came from a Pro Indy who has been working on air cooled P cars since 75. A lot of his work now is on race cars....

They must wait around a lot in the pits for the cars to cool off... :-)

So if I have a stuck bolt in aluminum I should freeze the aluminum and heat the bolt to help release it without damaging the threads?

Hello,
25 years working on aircraft engines, never had a problem removing plugs hot or cold. The standard industry treatment is to put a small amount of Champion anti-seize on the 1st couple of threads. Costs about $8.00 at your local airport shop or from Aircraft Spruce. Been using it on everything that has sparkplugs from chainsaws to aircraft.
http://www.aircraftspruce.com/catalog/eppages/antiseize.php
Hola
eric
P.S. But I also use a torque wrench to tighten to factory specs.

++1,......:)

I've been pulling hot plugs out of engines (aluminum or iron) since the early sixties without any issues, unless someone else touched the thing and created a pre-existing problem.

John is right; just another Old Wive's Tale, and there are many,...:)

I prefer removing them when they are.. warm.
If thats possible.
Like after the car has sat for an hour or so after running. Of course that is not practical for a mechanic working on cars all day..

Not cold and not hot, but somewhere in between they seem to come out with the least effort and are less likely to bring some threads or a helicoil if there's one in there with them.

I put a little anti-seize on the threads but not so much that the sparkplug has a harder time grounding in the head.

Checking you plugs when pipping hot is a great way to test your mixture if you dont have a CO reader.

I take my cars out for VERY spiritted runs and pulling over to pull a plug o check its color when hot. Been doing this for years.

May depend on how long they have been in and if a little anti-seize was used. Typically track cars have plugs checked/changed more often, so they will tend not to seize up and rip out the threads. My $.02

+ 1 to what John and Steve said. Been taking them out hot for years to do hot compression tests, and never had any problems.

+2...

No way. One might think that freezing the aluminum thread will make the metal "pull away" from the hole due to contraction and make the opening bigger but it's the opposite. A hole in a material with a positive coefficient of thermal expansion, i.e. metals, will contract when cooled, and expand when heated. Aluminum has higher CTE than steel so heating both should make removal easier. Going by this, a spark plug in hot/warm AL head should have a looser fit than it does when cold.

Won

if the aluminum expands more than the steel...wont the hole get smaller making it a tighter fit?

This is a common misconception, one that even a lot of engineers struggle with. If the material expands, all the holes in it get bigger. The exception would be something physically constrained. If say you have a rubber seal captured in a gland. If the rubber swells, the hole in it will get smaller as ther is no place left to go.

Remember your high school physcis class when the teacher demonstrated a metal ball could not pass through a metal loop? Then the teacher heated the loop over a burner and in a few seconds, the ball was able to pass easily through.

Think of the hole in the cylinder head as a loop. If the metal in a loop expands, the opening enlarges, same with the head.

I haven't noticed much of a difference between a hot and cold leakdown test. Personally I would rather work in a cold engine bay. Plus I do leakdowns on engines sitting on pallets all the time too. Hard to get them warmed up. I am not saying one way is right or wrong, just my experience. SmileWavy