twin plug ignition: 12mm or 14mm spark plugs
 

I'm in the process of getting a set of heads bored and tapped for dual plug ignition. I was under the impression 12mm is the standard for a second plug, but a quick search suggests 14mm is a viable alternative. Is this something that has changed over time, or is 12 mm still the way to go?

12 mm spark plug

Pro: less reduction in compression ratio
Pro: retain more mass in the head (strength and heat capacity)
Pro: avoid interference with head stud hardware
Con: reduced selection of spark plugs

14 mm spark plug

Con: may interfere with stud hardware (does using 5/8" drive spark plugs fix this?)
Pro: full selection of spark plugs
Pro: same dimensions as upper plug (same fit for COPs, boots, etc.)

What other reasons are there to select one over the other? My machinist has experience machining the heads for 12mm plugs.

Thanks,
Steve

One reason I have heard for using 12 instead of 14mm plug holes when dual-plugging is to leave more “meat” in the head between the valves, for structural reasons.

I have no personal experience with this however.

Strength is an issue. Apparently heads manufactured for race engines have used 12mm for both upper and lower plugs to retain as much metal as possible.

More discussion here.

12mm to avoid interference with head stud hardware. I went with 12 and still had to turn down my socket slightly in the lathe to get it firmly onto the lower plugs

I believe the factory used 14mm plugs on production 3.6 heads? How'd the get them to fit?

My 3.5 has 12mm lower plugs, and yes they are a tight fit (no barrel nuts on the studs right next to the plugs).

14mm work fine. I have never seen a failure based on this modification.
The compression loss is minimal but accurately dimensioning your engine will allow you to order your pistons to compensate for the chamber volume change.

One cure for head stud nut clearance is Supertec head studs. They offer improved head to cylinder stability,greatly improve engine assembly and the nuts offer clearance for lower spark plugs.

https://www.pelicanparts.com/More_Info/SPTCHSK1.htm?pn=SPTC-HSK-1&bt=Y&fs=0&SVSVSI=911M

The 964 and 993 engines do use 14mm plugs top and bottom. The Xtreme heads in my race engine use 12mm plugs top and bottom. Of course, with the heat and power my race engine creates, you want all the material you can get in the head.

964 and 993 use different head stud nut than the allen's used on earlier engines. 964 head nut.

http://forums.pelicanparts.com/uploa...1606767679.jpg

Head stud spacing is also slightly wider. so a little more clearance.

john

I had a parallel discussion with Al Kosmal (x-faktory) that cleared up several issues for me. Based on this, I elected to go with the 12mm lower plugs. One consideration is my machinist's experience with 12mm plugs.

Here are Al's comments (by permission):

This head nut won't help the plug clearance issue in the pre-964 heads.

I'm with Henry on this one.

All twin plug motors we have done are 14mm. Never an issue, and may only see the track. Plenty have been COP, again no issue.

A small bonus is that at plug change time, only one part number.

YMMV

Cheers