Twin plugging using 14mm plugs

[neilca]

It can be done, I just did 12 heads for my race car. After much searching, reading, and posting I decided I did not want to use two different kinds of plugs. Tadd and Cgarr gave some really good starting points on how this is done, I am adding this for archival purposes because the next time I won't remember how I did it.

First I hade a jig to hold the heads. I used a scrap piece of aluminum about 1/2 inch thick. Scribed a line that was the diameter of the inside of the head. I then used two fender washers per location and drilled and tapped the plate for a bolt. When picking the three points I was sure to clock them in areas away from the head bolt holes, this keeps the washers on the head diameter. I had enough slop between the bolt and the washer so I could fine tune the fit. Once the position was fixed I needed to define the rotation position. I placed a flat bar on the jig nominally 0.100" away from the head. The fins are used for this location and they vary so the gap was needed. Simply equal space the fins across the bar to get alignment. I then added two holes to hold down the head where the head bolts are located. Last I had to grind the heads of my bolts to clear the combustion chamber.



I used a simple table set up squaring the mill to the rotation bar. The mill was set to 12.5 degrees in nod and 24 degrees in rotation.



I placed a head into the jig with the original plug hole on the side of the cutter. I placed a transfer punch into the plug hole and a 1/2" collet in the mill. I then aligned the mill to the transfer punch. Be sure you have the z axis set to what you want when you do this because changing it afterward changes the vector you are going to drill. Once aligned the head is turned around and ready for drilling. EXCEPT you now have to establish the offset. Tadd suggests .375" in X toward the exhaust port and 0.060" toward the center in Y. I tried this on one head and decided to tweak it a little to .350" offset in X only.

I used a 3/4" end mill to provide a flat area for drilling the plug. This also removed a lot of the fins without breakage.



I am using a 1 1/4" ball mill with six flutes. I tried a two fluted cutter and broke some fins. Getting the depth is trial and error. At this point the final depth is not important just don't go to deep.



I used a 3/8" drill to start the hole and finish with a 1/2" drill. My drill chart said to use a 33/64" for a 14mmX1.25 thread. I did this on one had and thought the hole was too sloppy. I will Helicoil that head. I went to the 1/2" final size and liked the fit. I think if you use a split point dril you could go with the 1/2" straight away. Using the intermediate step does give you more precision.



I found my plug depth was variable due to the fact a plug is .780 in diameter and my hole was .750 so I took a 1 1/8 cutter and set the final depth providing a nice wide flat area to seat the plug.

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Threading the plug hole I used a spiral fluted tap. These taps are much stronger than normal taps and will produce a curl of cut material rather than chips.



I started using Tapmagic for all metals because it was right next to me at the mill. I decided not to be lazy and go get my can that is made for aluminum. To my surprise the Tapmagic for all metals was much better to use. The force required to tap the hole as about half of the fluid for aluminum.



I used a tap handle with a pilot to keep the correct vector of the hole. Crooked holes will not let the plug seat flat.



Here is a picture after some hand working of the threaded hole on the inside.



I measured the volume of the heads prior to the machine work and after. The extra plug increases the volume by 2 cc's.

Well that is what I did. Again I post his for archival reasons and perhaps for those that are going to be paying a machinist to do this to their heads. This shows how much work goes into an operation like this.

[jcge]

Great post neilca - thanks for sharing your setup and process
John

[cgarr]

So do you kind of hold your breath when you flip that first head over to see where the hole came out?

[neilca]

LOL! You better believe it! When I drilled the first it kinda sqeaked at the end I thought I may have nicked a seat.

[Tippy]

Originally Posted by cgarr

So do you kind of hold your breath when you flip that first head over to see where the hole came out?

I LOL'd.....

[Tippy]

Great job neilca!!!!

[tadd]

Very nice! Congrats.

I orginally made a threaded 'bar' that threads into the original plug hole and a 3/4" collet for alignment.

It also works backwards (threaded inside) allowing one to "hang" the Porsche head from the mill head then clamp in a regular vise with the mill head normal. Then you can go in with a 5/8 or 3/4 ball mill to do the combustion chamber plug pocket.

I had a bear of a time getting the correct tap (tolerance). All my plug holes the first time were way sloppy . I finally went to a speciality tap supplier for a special order.

t

[cgarr]

neilca, Just a note if your using original head barrel nuts with 14mm 2nd plugs. I have never done it but others have said there is a clearance problem getting a 13/16 14mm plug socket past the barrel nut. The 12mm plug uses a 5/8 socket but you can get 14mm spark plugs which use a 5/8 socket too if its a problem using the original head nuts.

[neilca]

I will have to check this. I mocked up the engine to check valve clearance yesterday and found I need to clearance the cam tower a little to clear the socket.

[kent olsen]

So what is the advantage to 14 mm plugs?

[nocarrier]

Having only 1 type of plugs to buy.

[ajwans]

It was recommended to me to use 10mm plugs which allows a large
range of motorcycle plugs to be used. Any opinion?

andy

[Walt Fricke]

I had plug to head nut clearance issues even with 12mm plugs. In addition to getting the plug in and out, you also need to have room for the spark plug connector ends.

The first solution was to put six barrel nuts in a lathe and taper their tops. Then I used a grinder to taper the end of a plug socket. This worked for both problems.

A better solution is to use reduced head flange nuts, which work great. I don't see why Porsche used the barrel nuts at all (maybe flange nuts weren't available?). Henry Schmidt uses flange nuts with his super head stud kit. In addition to taking care of stud depth issues (ones that stick out a bit too far, that is), they also provide better air flow. And you are much less likely to end up rounding the Allen socket bits.

All around better mousetrap, seems to me.

I get my 12mm plugs from a local motorcycle shop, which carries NGK in both diameters. Not that big a deal to have a spare set of each size on hand.

[neilca]

Walt,

Thanks for the nut idea. I do need to buy new nuts. I will go with the flanged nuts.