I have never seen this, and can't figure out why it is happening. It is off and on. I changed my plugs yesterday with NGK TR55 plugs to replace my old Autolites but the piston smashed the ground on 5 of the 8 plugs. I replaced them with the old ones and it ran fine until I tried to fire it up this morning and it smashed the Autolights. There is no water in the oil either. Any ideas? Thanks, Alex Sent from my SM-G960U using The H.A.M.B. mobile app
There are different plugs for Chevys, like AC R43 S or R45S with a washer and short threads and AC R45TS with a conus seat and longer threads... I have no idea what the NGK numbers are, but your plugs seem to be for the later heads, they still look pretty long threaded from here to me.
Try a spacer. My guess? Piston dome is smashing the spark plug. Could be the gap between the piston and head wasn't checked during build. Or the heads were angle cut for more compression, same result. Another thing to try, thicker head gasket.
Wrong plug for head used ? Would be my guess. The plus in photo are a bit longer thread then what many older heads take. Have you added or checked head ? Or maybe have pistons/pop -ups been added?
https://i.ebayimg.com/images/g/AUAAAOSwn~5fR6EX/s-l500.jpg these are the later ones, but they have shorter threads than yours.
Those plugs have an 18mm plug reach. Assuming this is a HAMB friendly Gen1 SBC, that’s a lot of reach. The plugs I run locally are only 9.5mm reach. Cheers, Harv
The electrode needs to just reach into the chamber, see how deep the thread holes go and compare to your plugs.
I think I used the wrong plugs. It has 3947041 heads. Sent from my SM-G960U using The H.A.M.B. mobile app
I see you indicated the heads..! You are using completely...the wrong spark plug..! I'll bet the threads in your heads are only about .375" long. I'd say...you've got the wrong plugs. Get the proper spark plug for the heads that you have...you should be fine..! Mike P.s. - Hopefully...you didn't break a piston..!
Your heads are late 60s early 70s 64 cc 350 heads, pretty good heads for factory heads. But I bet you´ll need The TS spark plugs like AC R45TS
Don’t those 041 heads use the big plugs with the sealing washer on them .??? But they are shorter on the threads !
How did it run, and for how long with da Autolites initially? Is this a new build or something changed or added like heads, timing chain? Not enough info?
After doing some checking , you got the wrong size, wrong kind, wrong length, plug . those are 5/8 dia hex. You want the old 3/16 dia hex plug with the sealing ring. I don’t see how they ever sealed to the head. ??? . 041 is a good horsepower head , but needs the old style plugs .....!
The reason that sometimes they contact and sometimes they don't is probably that there is one index angle where they don't touch and as soon as you remove the plugs and switch them around in a different order, they angle different in each hole and they hit. But regardless, they are too long. Get the shorter plugs as many have recommended. You may want to "scope" the piston tops to see if the witness marks go all the way through.
@Alex Gillman I did the same thing on a set of 041 heads...........felt sorta stupid, I used the wrong style of plugs, I finally figured out what the heads came on and then looked at the application chart. I don't have that handy or I'd tell you what number to use.
If you wat to stay with NGK plugs, try R5670-7's. They're .375 reach, 13/16 hex - gasketed, and non-resistor. Not sure what the resistor type number would be though. Got a pic of the spark plug hole? Send us a pic so we know what you're dealing with.
chevy changed plug type in 1970. If they are gasket type, use something like R45S. If they're tapered type, use something like R45TS. date code on the head might tell you, looking carefully at the hole the plug seats in will also tell you.
^^^^^ THIS!!!!!!. You have to make sure which spark plug seats the heads have but either way they use he short thread plugs.
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