I have a '65 A100 and was treating the slant as a '65 as well. Well I got a peek at the top of the block and the date is April 21, 1976. The plugs in the engine are Autolight 65's, and what I was going to put in were NGK UR5's. The Autolight's sit much deeper into the combustion chamber (16mm of threads) NGK has 10mm, Which do you prefer? Thank you!!

NGK UR5 is a taper-seat plug suitable for use in '75 and newer cylinder heads. It is physically not suitable for use in '74 and older cylinder heads.

Autolite 65 is a gasket-seat plug suitable for use in '74 and older cylinder heads. It is physically not suitable for use in '75 and newer cylinder heads.

All Slant-6 heads fit on all Slant-6 blocks, so the date on the block doesn't necessarily tell what kind of head you have. If your head has spark plug tubes (stamped aluminum tube, flared and O-ringed at one end, about 3" long) surrounding the spark plugs, that's a '74-down head that requires gasket-seat plugs, in which case the plug you want to run is the NGK ZFR5N, after removing the metal ring washer from each spark plug (unless you have a '60-'62 head, then leave the ring washers on).

If you do not have spark plug tubes, that's a '75-up head that requires taper-seat plugs, then use UR5 spark plugs.

Also see tune-up parts and technique suggestions in this post.

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

Thanks Dan. So I have a early head on a post '84 block. I'm learning.

Post 84? You said yourself that it was a 1976 block in the 1st post.