"Reach" is the length of the threads, and does not include the distance by which the electrodes project beyond the threads:



It is crucial to use only spark plugs having the correct reach (threaded length). Using a longer-reach plug to push the electrodes further into the combustion chamber is right out of the question, for the threads' sharp edges will incandesce and cause a lot of preignition (and you might well smack a piston). Furthermore, the threads will collect carbon and make the plug difficult or effectively impossible to remove! On the other hand, using a plug with too short a reach is also "no", because the electrodes will be recessed into the plug hole in the head, shrouding them and working against you, and carbon will build up in the head's plug threads, creating its own problems.

Other notes:

Suffix numbers on NGK plugs (such as -11 and -13 and -15) indicate wide, extra-wide and extra-extra-wide gaps, and no, you can't just close down the gap on a wide-gap plug to wind up at a standard gap.

www.ngk.com is not NGK's site, it is the site of a vendor of NGK spark plugs.

The '75-up slant-6 head requires plugs with 14mm thread, taper seat, and 0.460" (11.7mm) reach. No longer or shorter reach is acceptable!

LZTR5AGP has a 0.984" (25mm) reach. No go.

PZTR5A-15 has a 0.708" (18mm) reach and a 0.060" gap. No go.

LZTR4AIX-11 has a 0.984" (25mm) reach. No go.

PLZTR4A-13 has a 0.984" (25mm) reach and a 0.050" gap. No go.

There does not appear to be a taper-seat plug of the correct reach with the extra-long electrodes.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I would agree with the above statement.

Posted below are pics of the recent Plugs I pulled out of my peanut head.
I never had Iridium plugs so i thought i would try them, Otherwise I would have used the regular Flavor of ngk's (V power Tr5)
Stock plug for this head is a UR5 - .460 Reach - V power #5 Heat range.





Links if pics do not display
http://www.geocities.com/gao5498/carpics/plugs1.jpg
http://www.geocities.com/gao5498/carpics/plugs2.jpg
http://www.geocities.com/gao5498/carpics/plugs3.jpg


So I thought I would try The TR 5 - In the iridium Version - TR5IX
.708 Reach - Heat range 5 - Iridium Version - This Plug gets about 1/4" closer to the center of the chamber and I did check Piston clearance (stock Motor) No worries as the piston is -.200" in the hole, and The valve clearance was fine too.

I had no trouble pulling these as I thought I might due to carbon build up.



They give noticably more torque than the regular ur-5's There was a considierable amount of pre- ignition as dan said there would be. I also have some head work and Ign Advance work done to my motor, so the pre-ign issue might be less on a stock motor.

I am experinting with ur-6 and ur-7 and ur-8 Equivalants now to determine the correct heat range, then I will purchase another tr-? Version of the plug to try out.

The ngk chart seems to go into racing flavors of the plug after heat range 7.

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http://www.youtube.com/hyperpack

OK, now I'm curious: What if you were to chuck these plugs into a lathe and machine off the extra reach by removing the lower portion of the threaded metal shell? Seems to me this would preserve the performance characteristics of the plug (heat range, etc.) while removing the ping-causing extra threaded metal from the combustion chambers. You'd probably have to tweak the (new) first thread for ease of installation.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

How would you avoid machining off the ground electrode?

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

just skim off the threads.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Dan that's exactly what I thought of after I posted this.

Step on me kablowey - Yes just take off the threads..Leave the "core" alone and the ground electrode ramains intact.


I dont have a lathe, but maybe the shop staff at work can help me out.


Greg

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http://www.youtube.com/hyperpack

Dan that's exactly what I thought of after I posted this.

Step on me kablowey - Yes just take off the threads..Leave the "core" alone and the ground electrode ramains intact.


I dont have a lathe, but maybe the shop staff at work can help me out.


Greg

_________________
http://www.youtube.com/hyperpack

Dan that's exactly what I thought of after I posted this.

Step on me kablowey - Yes just take off the threads..Leave the "core" alone and the ground electrode ramains intact.


I dont have a lathe, but maybe the shop staff at work can help me out.


Greg

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http://www.youtube.com/hyperpack