Twin Plug DIY?
 

Anyone out there do their own head machining for a twin plug mod? As I look at the vast expense of my engine project, knocking off $500 for the twin plug machine work would be nice. I don't have a twin plugged head for reference and I'm a bit apprehensive about going off pictures so I'm looking for someone that's done it and that's not worried about giving away their trade secrets.

What equipment do you have? You cant do it with a makita and a tap set. You need some pretty special fixturing and tooling, which would cost you more than what you have been quoted to get it done. Spend the $500 and get it done properly, if you mess up, there goes the head.

I work in an extremely well equipped machine shop, so the only cost would be my labor. I'm still leaning toward sending them out to EBS for everything, but the idea of saving $500 makes it tempting to do myself. My main concerns are that in some of the pics I've seen it looks like the 2nd plug is at a different angle from the 1st (not just opposing angle) and maybe a different plug size.

Anyone have a trashed head they'd like to sell?

Here's a pic of the shop where I work from the the 100 ton crane, this is the center bay, the east and west bays house the smaller machine tools:

http://forums.pelicanparts.com/uploa...1089143228.jpg

Cool, then you can set up off the other spark plug angle, 65 degrees front to back, and 80 degrees side to side.

Thanks, I was actually getting ready to post the same question.

Do most people machine the second hole exactly the same as the first but on the opposite corner? Do they machine for the same spark plug dimensions?

some use a smaller plug,i went with same size.

When twin plugging a single plug head I prefer a 12 mm spark plug, stock is 14 mm.

It fits properly, a twin plug 911 was designed for this.
You can use short plug connectors that fit inside the valve cover.
Less metal removed (better cooling and maintains high compression.)

Best,
Grady

My twin plug machine work cost $350.

Tom

Tom

Did you use a local shop for the twin plug machining?

Email the shop info if you don't wish to post it here.

Thanks,
Cliff
Clifton_Brown@hotmail.com

German Precison did my twin plug (well Ted did a few twin plug machinings for me) conversion..

Ted is a stand up guy, and $350 seems like a good deal...

I do remember someone bashing GP awhile back... in all Ive dealt with him for almost 10 years and have not had a problem...

I've been staring at pics in books and it looks like some of the 2nd plug holes are at a different angle from the original. Does anyone have a twin plug head laying around that could take a visual check of how the second hole looks compared to the first?

Ted did mine as well. $350 was in addition to the top end work he did. The total, parts and labor for the top end work, with new springs/guides/seals and twin plug conversion came to ~$1450.

Ted's number is (408) 747-0728

The plugs are offset. The top set is angled in from the right, the lower set from the left. Here are the heads:

http://forums.pelicanparts.com/uploa...1060144468.jpg

And here is a good shot showing how the plug wires line up.

http://forums.pelicanparts.com/uploa...1089253510.jpg

Tom

Thanks Tom,

Could you tell if the 2nd hole angles are a mirror image of the first or are the angles a little different? The four heads in the bottom of your picture make it look like the angles are different, but the two on the top look like the same, but opposing angles.

You can set up using the same angles the original plug uses, I have seen others that deviate slightly from that as well.

In this picture from Wayne's book it looks like the 2nd plug is drilled at a much different angle than the 1st one:

http://forums.pelicanparts.com/uploa...1089313529.jpg

I have a trashed head if you want it, I'll just charge you shipping. The only damage is the combustion chamber had a valve pounding on it so it looks like someone hit it with a small hammer 1000 times. It should serve your purpose as a set-up piece.

Just PM me your address.

I think that it appears that way because the camera is off to one side a little bit, and tilted, relative to the head.

The term "Parallax" comes to mind, but I don't think that is the right word.........

Tom

I figured out what my confusion was. The hole for the 1st plug is closer to the exhaust side head studs, so my idea of lining up on the 1st hole and rotating the head about the head stud center would put the 2nd hole about 1/2" or so off alignment with the 1st. Now I understand why the 2nd hole recess is closer to the exhaust side stud recess. :D

On a side note, I picked up a 12mm plug out of curiousity. I bought an NGK, maybe others are different, but the porcelein is smaller than on the 14mm plug so I'd be concerned that a standard size plug wire boot would not seal out the elements.

Since I'm an amatuer machinist (I have made some intricate precision parts though), I alway consult with the pros at work and one recommended putting clay between the fins when machining to prevent damage. Another thought it wouldn't be a problem with mill at an angle, but I'll air on the conservative side and wedge some rubber between the fins.

David,
There are plug connectors designed for use with 12 mm plugs. The 12 mm plugs are what Porsche used in all the race engines.

Best,
Grady

Keep going boys--I have been thinking about this for quite a while but a sacrificial head is pretty scarce round these parts!
.I like the idea of a smaller dia.spark plug
might be able to run the lead thru a smaller hole in the cover, But I guess the plug tool would dictate that.

Bob
76 & 77 2.7's

I've just about finished the first head. After starting to machine through the fins I decided to go with the 12mm plug. It was just too scary cutting with an endmill large enough for the 13/16" socket. I cut the pocket with a 3/4" endmill and then followed with a 1" endmill to make room for the 5/8" socket. I should be set up to crank out the remaining 5 pretty quick. Then I'll change the set up and relieve the combustion chamber side with a ball end mill.

Is it worth the trouble? NO, I don't recommend this as a DIY job. Every cut I make makes me feel like I'm taking the final critical cut on something I've spent 20 hours machining.

As for the plug wires, does anyone know a source for matching plug wires to fit the 14mm and 12mm plugs?

David,
You are right, this is not a DIY process.

Now that you are set up, fill the fins with wax to prevent vibration.

Best,
Grady

David:

You can use the Beru, Bosch, etc ones (I prefer Magnecor 8.5mm wires & connectors) as they fit very well on all those 14mm and 12mm plugs (W-series Bosch/B-series NGK and X-series Bosch/D-series NGK)) that you might use with a 13/16" hex. I've used them on all twin-plug motors for 25 years. I also use plug retaining clips on the lower 6.


They do not fit well on the 5/8" plugs (FR5DTC) that the 993 uses,..... :)

EDIT: Grady, thanks for the wax tip, it works great.

Steve, David,

There is a nice short Beru (resistor and non-resistor) connector where the wire is 90 deg that will fit a 12 mm sparkplug. The great part of this is it fits under the lower valve cover!

Once you have a twin plug that is all settled in and you rarely have to change plugs, this is a great solution for a street car. The connectors and wires are all protected by the valve covers. Yes, to change the plugs you have to remove the valve covers. That can be a pain. However with a good running engine it works great. It keeps the wires and connectors out of the debris and water path from the rear tires.

If you want to appear original single plug, this hides the second ignition.

You would not do this for a track only/race car.

Of course if you want to show off your twin plug installation; long connectors, bright retaining clips, and bright red/yellow wires work great.


Best,
Grady

After putting wax between the fins things went very smooth. Here's a few pics:

http://forums.pelicanparts.com/uploa...1089581304.jpg

http://forums.pelicanparts.com/uploa...1089581315.jpg

http://forums.pelicanparts.com/uploa...1089581326.jpg

http://forums.pelicanparts.com/uploa...1089581336.jpg

Here's some pics of the ports where I opened up the intake for the Carrera intake and the smooth plug for the exhaust air injection:

http://forums.pelicanparts.com/uploa...1089581621.jpg

http://forums.pelicanparts.com/uploa...1089581627.jpg

David,

Beautiful work.
The Factory machinists would be proud of you.

Best,
Grady

Nice....

Thanks for the compliments.

After looking at the new holes for a while, I think the 12mm plug is the way to go. The combustion chamber pocket is so much smaller and I sure don't need any less compression.

Grady, do you or anyone else have pictures or more information on the above-mentioned set-up. Pros/Cons?

Thanks
Scott

Hey what's that guy doing in my garage??

Very nice! What EFI is going to be used?

Sorry, I couldn't resist the urge for a commercial plug (no pun intended).

These work well to prevent the spark plug cables and connectors from melting. Exhaust tubing surface temps are right around 400 deg. F.

This is a pic of Thom's 3.6 but they work fine for all 911s.
http://forums.pelicanparts.com/uploa...1102022000.jpg

Sherwood

I've been moving pretty slow on this project. Hopefully I'll get the cams in and seal it up this weekend.

http://forums.pelicanparts.com/uploa...1102042703.jpg
http://forums.pelicanparts.com/uploa...1102042747.jpg

New parts:
Carrera intake
Autronic ECU for injection and ignition
Autronic CDI to fire 6 waste spark coils
Twin Plug (still haven't looked for wires for 12mm plugs)
Three angle valve job with new valves, guides, and springs (done by EBS)
JE 3.4l, 7.7:1 CR pistons
Pauter rods
ARP head studs
Patrick lightweight flywheel
Centerforce aluminum pressure plate
8:39 r&p
a ton of expensive odds and ends :)

Existing parts (installed by PO):
K27 with 1 bar wastegate spring
B&B exhaust
B&B dual outlet muffler ( I may build a stainless open pipe exiting on the right)
B&B intercooler (I may get a custom large intercooler to fit the Carrera intake)

I was wondering about this, too. My parts are on the way to the machine shop and we have been talking about twin-plugging the heads and valve covers. I'd like to nail down my approach ASAP. I'm already thinking of 12mm.

Ooops. Might be a thread hijack in progress. :eek:

David, just curious, what kind of wax did you use between the fins ?

I used Gulf parifin wax from the grocery store. I guess it's used for canning. I used some clay to block off the ends and the head bolt holes and poured in the melted wax.

Very nice work David. Very nice!