I know about the NGk # ZFR5N for the slants with tubes. What's the number for the later year slant? Is this it NGK VPower #UR4?

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82 D150 Slant6 Shorty
http://www.cardomain.com/ride/658846

For standard plugs, GR4 is the one for drool tube heads, UR4 is for later heads.

ZFR5N is an extended tip plug for early heads. I don't think theres a direct equivalent for the later heads. Check this post:

http://www.slantsix.org/forum/viewtopic ... gap#116016

By the way, I just stumbled upon www.ngk.com - seems like good place for info, says they do some international orders too.

"For standard plugs, GR4 is the one for non drool tube heads, UR4 is for later heads."

Which is for the 1975 to 1987 heads?

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82 D150 Slant6 Shorty
http://www.cardomain.com/ride/658846

Err oops post edited to fix typo

drool tube heads (phrase coined from the idea that the seals leak and drool oil) = early, non drool tube = the later 1975+ heads.

You can go to any parts store site such as rockauto.com or kragen.com or the one I gave you, plug in an original application, and get listings for stock replacement parts. For basic parts like this they are fairly accurate.

In terms of stock slants, gr4 and zfr5n are interchangeable but the zfr5n is preferred because it reaches further into the combustion chamber.