need help with spark plug choice and tapered or flat

[dmjlambert]

Here is a picture of a spark plug my engine builder put in, and one of the hole it fits into. I have a 1969 CST/10 with 350 CI engine, the original engine rebuilt for daily driver needs (nothing special done to it). I was told the heads were not re-used so are not original to this truck. Is this a taper plug in a flat seat hole? It looks that way to me, but I just don't know. The spark plug has Autolite 26 marked on it and I do not see any other markings on it.

I was getting ready to remove the plugs and check if they are OK and gapped ok for HEI, and I found the first 2 very loose.

[leftybass209]

Unless you have a unique set of heads, cast irons usually use short, tapered 14mm plugs. Aluminum heads usually use gasket style, though there's some small exceptions to both styles.

I think those plugs you have are fine, but it's hard to tell if the hole actually has a tapered seat or not. Either way, torque wrenches work with spark plug sockets too, that way you know they won't be backing out on you, once you figure out the correct plugs.

Here's some reading on the subject in case you just can't get enough spark plug info:

http://www.hotrod.com/articles/spark-plug-guide/

Best of luck!

[garyd1961]

I believe they changed from the flat hole to the tapered hole in 71 but I'm not sure. I do know 1970 came with the flat holes that uses a washer on the plug. Maybe try getting the casting numbers of the heads and look them up.

[dmjlambert]

I sprayed the hole with silicone spray, and put some putty on a stick and pressed it in there and down on the threads while holding the stick level like a spark plug. This is the indentation it makes. I'm thinking more and more it's a flat seat hole with tapered plug. So what to do now? R44 or R44S, something like that? I have an HEI distributor out of a later model Chevy. How much torque to do? I will need to buy a torque wrench.

[leftybass209]

It's good insurance to have a torque wrench anyways, but most plugs are about 20 ft. Lbs. If it is indeed the plug style with washer, you could just snug them up and be fine without the need to torque them.

Plugs are cheap, I'd say buy 1 of the other style and give it a test run. For your own personal information, you could pull a valve cover and run the numbers on the head. That would give you a healthy dose of information about them.

[dmjlambert]

Need more help folks, please. I bought AC Delco R45S plugs, and when I attempted to put them in I found they do not fit. The main body of the spark plug takes a 13/16 socket so it is fatter than the 5/8 plugs I was removing. So they do not go into the outer hole I have marked with arrows here in this picture. Is there an equivalent AC Delco spark plug that is 5/8 socket and therefore skinny, and which has the flat seat, and which has similar heat range to R45S?

[firedemon]

must be fairly old heads to use the fat plugs unless they got stripped and someone rethreaded them. I'm betting people are going to need the numbers to know for sure

[garyd1961]

If 13/16 don't fit they have to be 5/8 plugs. I have never seen a 5/8 plug with a flat seat and washer. Pull the valve cover and get the casting number.

[firedemon]

its possible they were stripped out and someone put heli coil inserts in it and they got removed when someone pulled the plugs and didn't notice , if neither plug fits

[dmjlambert]

Here is the casting number 10147898.

Any recommendations for a correct spark plug, preferably AC Delco?

[firedemon]

it comes bact to a 95 crate motor head 95 would be the small taper plugs not the flat fat ones I'm betting they had helicoil inserts at some point . you might have to have them re done if they don't fit

[garyd1961]

I don't think he had a problem with them fitting. He just wanted to know if they were the right plugs.

[dmjlambert]

Yes, I want to get the right plugs. And prefer AC Delco if possible. I will be doing city driving. I have an HEI distributor from a later model truck.

I also want to increase the ability of the spark plug to stay in, since it is seating against a flat and not tapered surface, and I found the spark plugs to be loose after a couple hundred miles.

[firedemon]

he said 2 were loose but your right I assumed he tried to tighten them and they still didn't fit right but 95 heads should be the small taper seat plugs . I believe ac delco r 44 plugs with 045 gap for hei

[dmjlambert]

Sorry, I probably wasn't clear and didn't let you know the full story of what is going on. All 8 plugs where loose, 7 about 1/4 to 1/2 turn out from seating. One was seated finger tight. I found the first 2 loose and then went in the house and wrote the first post in the thread, looking for info. Later on I examined all 8 to see if they were tight, and they were not. I can (and did) tighten them, but I thought since they are gapped at 035 I would just get the AC Delco plugs and gap them right and start fresh, but I don't know which ones to get now.

The reason I am getting into this is because my engine is running very rough when cold, during the first 60 to 90 seconds of running and then it smooths out. During the rough running, manipulating the choke plate does not have any effect of making it better. I can feel it missing when warm, just occasionally, nothing severe. When warm, the engine runs pretty well, but still misses some.

I'm eliminating all the usual crud that could be wrong by replacing plugs, spark plug wires, distributor cap and rotor, and I've already replaced the carburetor with one that was remanufactured and dyno tested by National Carburetor. I would like to get this stuff done and then take the truck to somebody more knowledgable than me for timing and carburetor adjustments and to give the truck a look over.

When I tightened the old Autolite plugs I have and drove it today, it was still rough running when cold, and I can feel it missing when warm, so not much of a difference. I'm not expecting miracles out of a new plug, just figured I would gap the plugs I want to keep, and since I am removing the plugs anyway it should not be harder to just replace them, and I would like to know the correct plugs.

[dmjlambert]

The rough running while cold is difficult to troubleshoot, because I only have seconds to troubleshoot and then I have to wait at least 8 hours, because the truck has to be completely cold such as sitting overnight in order to again experience the rough running. When it is rough it is extremely rough and difficult to keep running long enough to warm it up, but it only takes a minute or maybe 2 and then it smooths out and runs just fine. If I take the truck somewhere, such as to the store or to work, even if I leave it sitting for hours and come back out after work, it still starts up and runs nice and smooth right away. It fires up in about 1/2 a crank usually. Anyway, I know I'm getting off the subject, I just wanted to let you know why I'm interested in the plugs, wires, distributor cap, etc. I would like to eliminate the easy stuff before I take it to a skilled mechanic and pay big bucks just to have him say "hey look here at this, dummy." :-)

[randy500]

Those heads are tapered seat and take the plug you took out. Run the engine at night in pure darkness and look around the plug wired for arcing, listen for snapping, could be high voltage leaking in the plug wires causing it to miss. Also look inside the distributor cap and make sure its very clean, while your in there check the mechanical advance under the rotor. Those HEI caps can crossfire inside and cause a miss and it's very hard to detect visually, I usually keep some cheap ones onhand from rockauto and just change em out.

[dmjlambert]

Thanks. I started it last night when it was dark, probably around the time you were posting, to do just that. I did not see or hear any evidence of arcing at all. I have a new AC Delco HEI cap and rotor that I'm going to swap out anyway because I have no idea of the age of the current ones, they are probably dozens of years old.

[firedemon]

ya I would run the r44 plugs and set the gap to 045 for hei not 035 . change the cap rotor and wires just for cheap insurance . as far as running rough for only a few seconds while the choke is on that's fairly normal I have never seen one run perfect while choked . think of it this way your closing the butterfly causing less air to be brought into the motor and it in turn sucks more fuel because of the increased vacume so its running rich the increased gap to 045 will help with that but still its only a short time its on choke I don't believe its anything you should worry about unless you start foweling out plugs or something

[tdangle]

As far as spark plug selection, You should probably use an AC Delco R45TS. The TS stands for tapered seat which per the year of head it should be a tapered seat. It's best to use a torque wrench on the plugs, avoids stripping the threads or not tight enough. Flat washer plugs which used a larger socket also was phased out in late 70, 71 I believe.

[72kool]

if they are for sure '95 350 heads, the information im bringing up in my shop says to use ac delco cr43ts spark plugs. that is oem copper plug. if you want a little better plug, you oculd use the double platinum plugs for a little more money but not much

[davepl]

Originally Posted by garyd1961

If 13/16 don't fit they have to be 5/8 plugs. I have never seen a 5/8 plug with a flat seat and washer. Pull the valve cover and get the casting number.

Just installed Autolite 3922s which are a 5/8 washer plug. That was in an aluminum head, but they do exist... this is a cold plug for a turbo motor, 3924 would be the normal heat range I think.

[Harley03joe]

Originally Posted by davepl

Just installed Autolite 3922s which are a 5/8 washer plug. That was in an aluminum head, but they do exist... this is a cold plug for a turbo motor, 3924 would be the normal heat range I think.

Dave Is The Spark Plug In My 87 R 10 Chevy Silverado 5.7 K Eng 14 MM Thread ??
Thank You
Harley03joe