# Spark Plug threads in Aluminium heads



## Niceonetidy

Hello,

How do you Engineers fit spark plugs in Aluminum cylinder heads? Do you thread the head and fit the plug, or fit a bronze insert in the head to make it more reliable and a better life?

Thanks in Advance for any replies,

Colin


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## Longboy

.....You can just bore, tap and thread your plugs dirrectly. I use some anti -seize on the plug threads. This should last till around......2073 or longer.


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## enfieldbullet

seen it done both ways.

also seen on old VW's a nut inserted in the head and crimped in place.


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## Coilmotorworks

Use a good quality tap and die (if you make your own plugs), a little lube on the threads and things will be just fine. Pull the plug(s) just before you store your engine, dry out and lube the cylinder(s) and leave the plug loose until you run the engine again.


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## lohring

I race model boats that use Zenoah industrial style engines.  They have plug threads tapped directly into the die cast aluminum cylinder.  I've not heard of a stripped plug thread.  I bet this is the same with most low cost gasoline engines.

Lohring Miller


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## ConductorX

On VW engines it was common for spark plugs to strip out.  We would buy a special tool to ream out the old threads and then the same tool would tap new larger threads.  Afterward we would screw in a spark plug with a "Heli-Coil" (brand name) insert.  The insert would stay in the head and provide a new steel lined threaded hole with the proper 14mm threads.

"G"


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## Tin Falcon

in the air force we leaned about slimserts. 

Everything and more you want to  know about them .

http://www.electronicfasteners.com/pdfs/linecard_rosan_slimsert_inserts.pdf

Tin


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## Philipintexas

Also, don't remove the plugs from a hot engine.


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## Philjoe5

Or an engine that's running :hDe:

Phil


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## robcas631

Tin those inserts work!


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## robcas631

Colin,

I'd try a threaded steel insert.


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## gartof

On aircraft engines we take the plugs out hot every year or 50 hours very rarely is there a problem. When they go back in we use a grafite based antisize sold by Champion and torqued with a calibrated tool. I think the special spark plug antisize is the key.
Gary


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## wakeup

My dear old Dad was an engine fitter in the RAF during the war.  He suggested a really soft pencil, something like a 2B or thereabouts, just rub the lead of the pencil up and down the thread, depositing a little bit of graphite, screw the plug in tight, slacken and nip up.  It needs to be tight enough to seal, that's all, many plugs are overtightened.  This process worked for me for 50 odd years in alloy heads.

Don't use any wet lubricant, grease or oil etc.

cheers
Bill


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## gus

Tin Falcon said:


> in the air force we leaned about slimserts.
> 
> Everything and more you want to  know about them .
> 
> http://www.electronicfasteners.com/pdfs/linecard_rosan_slimsert_inserts.pdf
> 
> Tin




Thanks and I did not know this inserts exist.

When installing spark plugs into motorcycle engines ,do take care to ensure
plug is well position and aligned before screwing in to avoid stripping threads.
A friend of mind was stranded in the middle of no-where in the Malaysian Jungle with his bike kaput.Lucky for him a truck passed by before it go too dark.A night in the jungle is very frightening with wild boars and predators after them.Did my jungle training in survival school. Was told to finish all food and bury used food cans deep to avoid attracting wild pigs.


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## Wizard69

Niceonetidy said:


> Hello,
> 
> How do you Engineers fit spark plugs in Aluminum cylinder heads? Do you thread the head and fit the plug, or fit a bronze insert in the head to make it more reliable and a better life?
> 
> Thanks in Advance for any replies,
> 
> Colin



We use these at work extensively: http://www.timesert.com/.   It is a slightly more tedious installation than Heli-Coils but they are also more reliable in my estimation.   In fact I've grown to hate Heli-Coils.  If you look through Timeserts catalogs you will find a number of special purpose inserts, including 1/4-32 glow plug inserts.  

As for your engine I'd drill and tap a naked thread and worry about thread inserts later.   The problem is you will spend a lot of money for the kit even though your threads will likely hold up for years.  By the time you will need new threads the engine will likely need a rebuild already.   Honestly I'd be more worried about the head bolts and other highly stressed threads.


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## wings_of_fire

Niceonetidy said:


> Hello,
> 
> How do you Engineers fit spark plugs in Aluminum cylinder heads? Do you thread the head and fit the plug, or fit a bronze insert in the head to make it more reliable and a better life?
> 
> Colin



Apart from the suggestions others have given, I would like to point out that you also need to use a thin copper washer between the spark plug seat and the aluminum head. Copper is pretty ductile and it creates a good air tight seal under squeezing pressure created by torqued spark plug and aluminum head.


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## Niceonetidy

Hi,

Thank you everyone that has contributed.  Some good stuff here.  I like the thin inserts.  And, npt too keen on Helicolis.

Regards

Colin


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## Walltoddj

Niceonetidy said:


> Hi,
> 
> Thank you everyone that has contributed.  Some good stuff here.  I like the thin inserts.  And, npt too keen on Helicolis.
> 
> Regards
> 
> Colin



Reading the different post I thought I'd put in my two cents. When racing go-karts back in the 70's the first thing we did was insert a Heli-coil insert due to the threads striping out from high compression and a 3/4" length. In 93 I moved to GM Powertrian in Buffalo, NY and all the aluminum heads they used a forming tap, they told me that the form thread is stronger because the aluminum is compressed not cut. I have almost all the different sizes steel inserts, heavy steel inserts, Heil-coil(Recoil), and aluminum I find if done properly all have their advantages but I tend to use the wire inserts the most.


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## Walltoddj

Just a point of interest I found CM6 or 5812 plugs at AutoZone today $2.99 each 10mm compact size for Lee's Radials. I bought what they had in stock in my store.


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## Goldflash

When I was a young airman I bought a BSA Bantam for about $50 and just used it for riding around base. the spark plug thread in the head was a bit stripped. Used to wrap a bit of aluminium foil around the threads on the plug and screw it in. was good for about 10 miles then it would blow out with a bit of a bang. Didn't keep it long before I gave it away so it became someone else's problem.


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## stevehuckss396

Niceonetidy said:


> Hello,
> 
> How do you Engineers fit spark plugs in Aluminum cylinder heads? Do you thread the head and fit the plug, or fit a bronze insert in the head to make it more reliable and a better life?
> 
> Thanks in Advance for any replies,
> 
> Colin




Hello Colin!

I have made a few models and some have there fair share of "miles on them and I have never had a problem with threads in aluminum. My heads are 6061 aluminum and not some exotic species. The plugs in the V8 are #8-40 and no trouble. Tap the aluminum, you will be fine.

1/4-32








#8-40


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## Tin Falcon

Colin: like steve said you should be fine drilling ad tapping. 
that said do your homework on tapping. 
Do not use the same tap drill for aluminum as you would for steel. A typical tap drill for steel gives 50 % thread engagement. For brass and aluminum use a slightly smaller tap drill to give 75% thread engagement. . 
You should probably go for a class 3b fit and select a tap accordingly Ie the proper h limit of the tap. full explanation herehttp://www.cutting-tool-supply.com/TechTips/Tapping/HLimitsClass/HLimitsClass.htm or read the section on hole tolerances and fits in your machinery's handbook. 
And last but not least consider a forming tap as Todd mentioned. Forming taps require there own tap drill size as metal is only being moved not cut away. 

While slimserts are great for production and repair the cost of a pack of inserts and the required tooling are not likely justified for one or a few holes. 
If in the future you find the plug hole worn then you can always drill out the damaged thread and install a slim insert of your choice. 
Tin


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## Fabrickator

I've worked extensively on motorcycles (dirt and street), and for many years on Chrysler Hemi Top Fuel engines.  When I made repairs I used Timeserts.  Generally, the std aluminum threads last forever unless someone cross threads the hole. Or, in the event of TF, a rod breaks going through the timing lights and hand grenades the head. Here are some sequential pics of typical head repairs I used to do.

Heli-coils are a last resort.  If they go bad, you have to mill out a large hole and plug it or weld it to start over.


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## 2manyhobbies

Nice job repairing that cylinder head, any idea what when wrong to cause all of the damage?


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