Spark Plug threads in Aluminium heads

[stevehuckss396]

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

 

[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

 

[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.

 

[2manyhobbies]

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