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I am my 911's PO
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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 ratio14 mm spark plug Con: may interfere with stud hardware (does using 5/8" drive spark plugs fix this?)What other reasons are there to select one over the other? My machinist has experience machining the heads for 12mm plugs. Thanks, Steve |
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Slippery Slope Expert
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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.
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“As new technologies become indistinguishable from magic, and I can no longer tinker, the magic goes away for me.” |
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I am my 911's PO
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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. |
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Registered
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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
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80SC (ex California) |
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It's a 914 ...
Join Date: Jan 2008
Location: Ossining, NY
Posts: 4,706
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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). |
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Try not, Do or Do not
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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
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Henry Schmidt SUPERTEC PERFORMANCE Ph: 760-728-3062 Email: supertec1@earthlink.net |
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Racer
Join Date: Oct 2010
Location: Franklin, TN
Posts: 5,885
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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.
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Scott Winders PCA GT3 #3 2021 & 2022 PCA GT3 National Champion 2021 & 2022 PCA West Coast Series GT3 Champion |
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Registered
Join Date: Feb 2004
Location: denver
Posts: 1,147
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964 and 993 use different head stud nut than the allen's used on earlier engines. 964 head nut.
![]() Head stud spacing is also slightly wider. so a little more clearance. john |
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I am my 911's PO
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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): Quote:
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Try not, Do or Do not
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This head nut won't help the plug clearance issue in the pre-964 heads.
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Henry Schmidt SUPERTEC PERFORMANCE Ph: 760-728-3062 Email: supertec1@earthlink.net |
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,002
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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
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Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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