Drawing for twin-plugging of heads (3L SC engine)
 

Hi

Is there such a thing? A drawing that shows angles/measurements etc.

Cheers!

/Johan

I don't have a drawing but here is a tutorial on how I machined my 2.4L heads

http://forums.pelicanparts.com/911-engine-rebuilding-forum/584651-twin-plugging-using-14mm-plugs.html

Great writeup neilca....balls of steel! :D

(loving that mill....I so need a mill in my life)

Not sure I have the balls to do that :)

Hoping for a drawing which I can't take to a machine shop in order to help them set it up

Here is a short vid of mine, I can get you the degrees I use on the mill but to position the head and set depth I use a jig so I am trying to figure out how I can translate the jig into a measurement?

<iframe width="420" height="315" src="//www.youtube.com/embed/br6R5bTS05k" frameborder="0" allowfullscreen></iframe>

Definitely not for the faint of heart. I documented the procedure so folks could understand what all goes into twin plugging. Now you won't feel so bad having one of the machinist on this board do your heads. ;)

Best to have a couple of scrap heads to set up on before you commit to the good heads.
Just an FYI 2.0-2.7 heads are different than 3.0-3.6 heads.
Frankly it might be worth the shipping to get an experienced shop to do this.

FWIW,..I've had all the fixtures to do this since 1976 and we do a lot of these for other people as we'll as shops. :) :)

I would never trust my random local machine shop to do this. I'd send them to Steve.

I twin-plugged two sets of 2.2-2.7 heads myself about a year ago with the help of cgarrs and neilca earlier posts.
Bought M12x1.25 roll tap and proper drill for that roll tap. Made fixture beforehand with correct angles and rest was a breeze.

Wouldn't want to imagine the cost of sending two sets of heads over to US and back...

cgarr is the man. Word apparently has gotten out because he's always got a set he's working on every time I talk/text him.

One may be surprised by the number of shops that have the fixtures and can do this work. We use a local shop that has done many sets for us. We have 2 shops to chose from here locally. My suspicion is that a fair number of large cites have shops that have the fixtures to do this. Out local machine shop has heads sent from all over the world, but plenty of choices exist. If you can find a local shop you save the shipping to/from and the risk of damage during transport... The catch is your local shop needs to do the work correctly. But Porsche heads are not any more difficult to machine than any other head once the fixtures are available.

I don't have the luxury of locations when I am on my phone. At my desk now.

Talk to Stig's Garage and Lars Schonemann. Excellent air cooled shops in Scandanavia.

24 and 12.5 degrees for the mill head, 1000 rpm using a ball end mill, no cutting oil.

Thanks Craig for the degrees, much appreciated.

Not a bad idea Matt, actually think Schonemann is not too far away.

Cheers!

/Johan

Twin plug in Sweden
 

Try Perrys Motor ,Strömsholm 0220-43558

Shipping my heads, rods, and Ps&Cs to Craig was about $105. So maybe ~$80 for just the heads?
He was cheaper and faster than the local machinist that everyone recommended, even factoring in shipping. And he does nice work.

I have shipped stuff to US and with the weight of two sets of heads the shipping is ~200 and then I would need to get them back too. Remember, I am on the other side of the pond lol..

Total cost for me when I did it myself was ~100 usd total for the roll tap and special drill for it plus my own free time.

Why did you use a roll tap?

Although when used correctly there may be some advantages in using Roll Taps they do have their problems.

Torque to tap is commonly 2-3 times greater than with a conventional tap and they need much better lubrication to avoid galling

The control of the hole diameter is much more stringent for a given Class of Fit compared to a conventional tap.

The recommendation for class of fit often changes from an H5 to an H7.

The Roll Tap is also much more likely to cause burrs at the entry to the thread and will almost certainly need a generous chamfer.

I read about using roll taps on this forum for cleaning out head stud threads on engine cases and bought an M10 x 1.5 Roll Tap with some enthusiasm.

I noticed that the fit of the studs in the engine cases seemed quite loose after using this tap and spoke to the manufacturer.

I now use an ARP thread chaser for heads and we cut plug threads in our Twin Plug heads using a conventional tap.

We always use a 14mm plug and cut the fins to suit a 964 plug with a reduced body.

I used roll tap mostly because it creates a stronger thread by deforming material instead of cutting it. Also with roll tap the plug sits tighter in the thread compared to the regular tap kit I have for M12x1.25.
As for requiring more torque, I tapped it using a mill where I twin-plugged the head, so neither torque nor orientation of the tap was not an issue.

Agree with the hole diameter, it needed a special diameter drill.

As for burrs, it did produce burr but plug opening in the combustion chamber needed a chamfer anyway so it was a moot point.

Its interesting that you find the thread rolling tap produces a better fit.

The data sheets for most roll taps show that the fit is not as good as a normal tap and this has been my experience with the roll tap I bought for studs.

I guess you can control this with very carefully sizing the hole diameter.

In the end in the end I don't suppose it shouldn't make much difference as the load on the plug isn't too high and in real terms.

How much of a chamfer do you use on the lead for the plug?

How I did mine.

I cheated, I borrowed a twin plug head and made/used a spark plug that I broke out and modified for a long rod, I chucked up the rod in the mill and then carefully clamped the head down so it was repeatable/indexed.
I used just a regular 14mm tap, good fit no issues.
I also have a scrap head made up (was the 1st practice head) in case I have to ever do more.

I also waxed my fins, a bit of a PITA but I didn't break a single fin.

OK, you guys doing this work at home are seriously badass well beyond normal DIY. Great write up and pics nelica, made me realize how much I will enjoy parting with real money to let someone like Steve machine my heads for me! :-)

Why use a 14mm plug here? Why not a 12? Gives you more room for the spark plug wrench so it doesn't bang up against the head stud barrel nut. Or a better, reduced wrench size nut. Plus you have a smaller amount of the combustion chamber surface removed, so 12mm would reduce CR less.

I have not found having two sizes of plugs to be a problem.

I thought about that and figured it was no big deal.
I used Supertec head studs so I had absolutely no plug clearance issues at all. Plus all my plugs are the same.
The CR lost would be very slight as you still have to drill a hole.

Porsche used 12mm plugs on their race twin-plug engines iirc.
I think using 12mm plugs is more important on 2.2-2.7 heads where there is less material compared to later heads.

Here's a topic regarding using 12mm plugs: http://forums.pelicanparts.com/911-engine-rebuilding-forum/449930-why-12mm-spark-plug.html

I had thought Porsche used 2 x 12mm plugs as they must have had easy access to bare castings and could make both plugs identical.

Having only converted SC heads we made the decision to use a 14mm plug to keep both plugs identical in terms of heat range and electrode position as in the 964.

We machine the fins to use a small plug spanner and use reduced hexagon plugs so the spanner size is still small.

Has anyone measured the increase in combustion chamber volume created by using a 14mm plug as opposed to a 12mm plug?

If you model it on a CAD system it comes out to 0.35cm^3 which reduces CR from a nominal 10.3 to 10.23:1.

Clearly the impact on a 2.2litre would be greater.

Yes I can see that, my engine is a 3.0, I think 14mm is fine on 3.0 and up.
I'll look into the 12mm more if I ever build my 2.7 backup core.

I use plugs with a smaller hex on my regular 14mm single plug heads. Really nice with the extra room for a normal wrench.

In about 1994 I purchased a race motor. It was twin plugged - 14s all around. A machinist tapered the tops of the barrel nuts so I could more easily get at the lower plugs. It worked, but when I built (assembled is perhaps a better description) my own first race motor, the machinist and I decided to use 12mm plugs. Now I have Supertec studs (though I'm not sure the studs change the plug clearance part) and the smaller nuts which do away with the socket clearance issue. But habits die hard, so the next heads I had twin plugged got the 12mm threads.

The heat range consideration is certainly a valid one, although I can't say that I saw any difference between the two plugs, and I just used the same heat range for both.

I guess if you use reduced plug wrench sizes, you can run the same plugs both top and bottom. But aren't your choices of plugs reduced somewhat? Though perhaps the same could be said for 12s vs 14s - I've never researched this.

The 2.8 LS race motor I bought has 14s all around. Works fine, and the heads are 2.2-2.7 in origin, so I don't think that 14s cause problems like being too close to something (other than barrel nuts) or to weaken areas of the head between penetrations or the like.

I've not had the $ to have a set of heads made from the ground up (love to be able to, though), so haven't had to wonder if I wouldn't order them with 12s in both places.

I didn't know there are such things as reduced wrench size 14s (but chance learning is one of the great things about discussions like this - now I do).

All interesting.

When I discussed plugs with NGK Technical in the UK several years ago they told me that there were no direct equivalence in heat range between different diameters of plugs as it is thermal capacity of the electrode and insulator interface that affects this area of the plug.

They did give advice on good combinations but I decided to be cautious and use matching plugs.

So far we have only used Twin Plugs for Rally spec engines and the choice we have had by using reduced hex 14mm plugs has been OK.

It is obvious that running a 12 and a 14 works well as many people do so its probably one of those unimportant decisions.

I tend not to like small plugs as I once ran a Lancia Fulvia 1600HF which had such large valves that the factory changed from 14 to 12mm plugs.

The car a pair of Twin Choke 42mm Solex cabs and used to foul its plugs for a pastime - it was a nightmare.

My colleague had a 1300HF with 14mm plugs and it was just fine.

I guess it is probably irrational but I just developed a dislike of small plugs.

We have always used 14mm for both plugs. Never had issues, only need one plug. I am sure 12mm lower plugs work well too, but never saw the need to go that route.

Cheers