I've decided to try the extended tip ngk spark plug #zfr5n 3459. When I look at the existing spark plug this new one use about 3/8 of an inch longer than the original threaded area on spark plug. Looking at the two side by side my brain says "do not install", as it seems the ngk spark plugs will not only hit the piston but the threaded area will have protruded into the combustion chamber. Is this the right spark plug or not?? Thanks

What model year is your car? Does it have the original engine and cylinder head? If your engine has a cylinder head without aluminum spark plug tubes then you need short-reach tapered-seat spark plugs. If you have the older cylinder head with round tubes surrounding the spark plugs then you have the right spark plugs. 1976 is the crossover year and in the early 1990s when I sold auto parts I had a customer with a 1976 D100 pickup and someone had put tapered-seat spark plug in his early style head. He came back to buy a tap to get the carbon out of the spark plug holes.

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Joshua

Engine is a stock 1969 slant. This plug just seems too long in comparison to the champions in it now. Almost like the threads will be sticking into the combustion chamber, making it hard to remove if carboned up. Looking at other posts it states they will work but, getting over the length difference is a concern of mine especially with removing the metal washer.

What spark plug (number) did you remove? Should be RN14YC. That's a 3/4" reach spark plug just like the NGK ZFR5N. The NGK does have an extended tip, but the piston of a stock 225 is way down the hole and the combustion chamber is deep so there won't be any interference. If the Champions had the metal washer installed then there may be a tiny bit of carbon in the cylinder head, but not enough to cause issues.

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Joshua

It is a champion rn14yc. I will look again tonight but I believe the reach, the threaded part of plug, was a little longer on the ngk plug than the champion. Thanks for your reply and help. I will use these ngk's," under protest" and let you know how it goes. I take it leave the gap at .035?

Thread length of a Champion RN14YC and an NGK ZFR5N are the same. The ZFR5N's electrodes are longer—that's the reason for installing them. Remember to remove the metal ring washer from each plug before doing so.

If you're running stock ignition, stay with 0.035" gap.

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I have updated to electronic ignition. Thanks for the replies and help. This place is awesome!