Correct Spark plugs?

[Freddo]

Hi,
I have owned this 1967 Chevy C 10 for several years. It is or at least appears to be mostly original. It has a 283 and I am getting ready to give it its first major tuneup. I ordered the tuneup supplies from rock auto and ordered the AC Delco R-45 plugs. I am removing the existing plugs and see that they are quite a bit different, the threaded area appears to have more threads and they are longer. I did a search on spark plugs here on this site but didn’t see anything that referred directly to this. I included a picture of the two different plugs. Can you tell me if it is OK to use these R-45plugs in place of these old plugs?
All help appreciated! Thanks!

[Mbeef61]

One is tapered seat and one is gasket seat. They cannot be interchanged. I have gasket seat on my 69 heads. You have to look at the thread seat in the heads to determine. If unsure post a pic that may help. Not sure if they switched from one to another for different years. Things get swapped so much on these rigs it’s hard to go by a catalog.

[Mbeef61]

If the ones that were in there were correct you need r45ts plugs

[71CHEVYSHORTBED402]

I'm reminded of the time I tried to put 402 plugs in 396 heads Knew I had a 402, forgot the heads were switched out.

[Freddo]

Thank you for the help on the spark plugs Mbeef61! I just went down and bought the correct ones. Looking forward to getting them in. I’m not sure what kind of hassle is involved with returning plugs to rock auto or if it is even worth it, but I guess I will check it out. I have been wanting to do a compression test on this motor for a while. I did the test while I had the plugs out and most of the plugs Landed in the 140 to 155 psi range which I thought was excellent except for cylinder number seven which came out at zero. (Ouch!) That was a surprise because it is a very smooth running engine. It uses some oil and is older so I expected less compression on all of the cylinders but not a dead one. I thought I would pull the valve cover off and see what was going on with the rocker. Are there any other easy to do tests that you might suggest? Thanks!

[Steeveedee]

Originally Posted by 71CHEVYSHORTBED402

I'm reminded of the time I tried to put 402 plugs in 396 heads Knew I had a 402, forgot the heads were switched out.

That's a lot of plugs! Did the 402 get the taper seat plugs in '71? Mine still has the N-type 14 mM long reach.

[RustyPile]

Originally Posted by Freddo

..........<snip>..... I have been wanting to do a compression test on this motor for a while. I did the test while I had the plugs out and most of the plugs Landed in the 140 to 155 psi range which I thought was excellent except for cylinder number seven which came out at zero. (Ouch!) That was a surprise because it is a very smooth running engine....<snip>....... Thanks!

Those two statements contradict each other.. Either your compression test is flawed, or your perception of a "smooth running engine" is flawed.. With zero compression that cylinder is completely dead, leaving a "gap" in the firing cycles.. The miss is most evident at curb idle.. I suggest you either conduct another accurate compression test or have an experienced person take a look/listen to the engine...

[71CHEVYSHORTBED402]

Originally Posted by Steeveedee

That's a lot of plugs! Did the 402 get the taper seat plugs in '71? Mine still has the N-type 14 mM long reach.

Factory 402 heads take the tapered plug. That's not to say GM never built one otherwise. So 71 is generally correct. If I recall, there was a slow transition 396 to 402, which occurred in some 1970 The trucks are badged 396, but have 402 blocks. Makes me ponder if GM assembled "396" heads on 402 blocks

I can't speak for all 396 heads, mine came off a 68 HP Camaro. They generally or always take the flat-seated gasket type plug.

[Mbeef61]

You can see if they will take the plugs back. If not just sell them on here. Those are the plugs my heads take. Fairly common. Good plugs too. You can squirt a teaspoon of oil or so down the plug hole and see if the compression increases, it may work but usually for slow building compression not zero

[Freddo]

Originally Posted by RustyPile

Those two statements contradict each other.. Either your compression test is flawed, or your perception of a "smooth running engine" is flawed.. With zero compression that cylinder is completely dead, leaving a "gap" in the firing cycles.. The miss is most evident at curb idle.. I suggest you either conduct another accurate compression test or have an experienced person take a look/listen to the engine...

I would have to agree with you. I have never seen an engine that had a dead cylinder that did not have a miss. This engine runs relatively smoothly through all RPMs. I ran the compression test three times on that cylinder and got zero pressure each time. It seems odd that the compression tester would give solid readings on the other cylinders but zero on this one. Maybe more testing is needed. Thank you for the advice!

[Freddo]

Originally Posted by Mbeef61

You can see if they will take the plugs back. If not just sell them on here. Those are the plugs my heads take. Fairly common. Good plugs too. You can squirt a teaspoon of oil or so down the plug hole and see if the compression increases, it may work but usually for slow building compression not zero

Thanks again for the info & advice!

[Freddo]

Well, the mystery of the dead cylinder is solved. I took a fresh look at it again, screwed in the compression tester and cinched it down fairly tight. When I tested it, the cylinder showed 125 pounds. All in all, for an older engine not too bad. One additional question: I installed new spark plug wires along with new plugs and a small packet of silicone grease came with the spark plug wires. Probably a dumb question, but does anyone know what this is used for?
Thanks!

[Ironangel]

The silicon is used to smear the inside walls of the plug boots to allow easy slip on and off...It also helps seal the plug boots...

[Mbeef61]

It’s used to keep the boots from sticking to the white porcelain. I apply it to the white porcelain and the inside of the boot. Just a light coat on each. It’s not for the connection area but some people put it there for some reason.

[Freddo]

Thanks for the info!