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Discussion Starter #1
I bought a car with these Holley Systemax heads and they had Champion RV17YC plugs with 0.460 reach tapered seat in them. Holley's catalog calls for Autolite 3924 which is a gasket seat plug 3/4" reach. Here's a pic of the seat. To me it looks like a tapered seat, so now I'm not really sure what plug to run.
398354
 

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70 Nova, 427, TKO600, Vintage Air, Terminator EFI
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yeah so that does look like someone torqued a set of tapered seat plugs into a flat gasket designed surface. I would think the spark plug called for by Holley would still be the correct plug and it shouldn't have any issue sealing or tranfering heat. Well anyway the R45TS or Champion RV17YC plugs have a short thread design and most likely are not going all the way down flush with the combustion chamber. So in effect, the short plug is creating a "Pre" combustion chamber that may actually be ok for emissions reduction but you really want the tip of that plug, down in the combustion chamber with the threads flush at the end of the hole. I think if you look down inside the spark plug hole you'll find some of the threads further in have not been used (dirty or dark). If that was the case, I think I would chase the threads (don't use a tap, use a chase), by hand being careful not to chamfer the seal area any further and use the Holley recommended spark plugs. And be sure to torque them to Holley's spec's. The gasket of the plug should still crush on the flat surface and maybe even get squeezed down into the tapered chamfer that was previously created. I don't think the seal area is damaged so badly the called for plug will be hindered. I also use a tiny amount of Anti-Sieze on the plug threads in aluminum heads (no lubricant on magnesium heads).
 

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70 Nova, 427, TKO600, Vintage Air, Terminator EFI
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This is a standard looking Chase for spark plug holes.
Lisle 20200.jpg Lisle 20200
Any auto parts store would have one like it.

Also I find PB Blaster penetrates the carbon and turns it mushy fast. so if you spray a little in each hole and then chase them, the job should go smoothly. When you pull the chase back out of each hole most of the carbon chased off the threads will be collected on the tool, so be sure to clean it before each hole.
 

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Agreed, looks like someone mushed the wrong spark plug in at some point and caused that tapered seat. Those are definitely meant to be gasket seal spark plugs.
 

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DRM, Brodix cast those heads for Holley and all Brodix SBC heads use 14 mm, .750 thread reach, gasketed seat spark plugs.
As said, clean 'em up and you should be good to go
 

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Discussion Starter #6
Great info! Thanks very much. Dean
 
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