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