Milled heads? what spark plugs?

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63dartman

63dartman

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I never thought much about where my plugs are sitting in the head after I milled my heads. Now I'm talking about alot of milling. My j-heads cc'ed at 49 so you know there was a bunch taken off. My question is since the heads were milled so much should I be concerned about spark plug depth in the hole. I just thought about this and common sense tells me that I may need to run a plug that doesn't sit down in the hole as much. I may be thinking this all wrong. I just think my plugs may be shrouded or something to that effect, I have no experience or clue about proper sparkplug placement in the head so any help would be greatly appreciated. Where can I get a chart that would tell me the difference between plug depths?
 
63dartman,
The plugs only change by heat range and either a extended tip or a retracted tip. Generally milling the heads will only change the amount of timing that the engine can run and or make you run higher octane fuels due to compression ratio changing. As for the actual depth of the plug changing, it should be in the same place as the plug sealing washer is sitting in the same place and can only go so far. Milling the heads doesnt change the plug depth. If the tip is hitting the dome of the piston then a retracted tip plug is in order.

Most plug applications books have the differences in the back of the catalogs and a cross reference for changing tip locations and keeping the same heat range. Go to a parts store that still has catalogs and look in them and get the plug that you want or need.
 
thanks BJR, So everything I've read about heat ranges is true. In effect when I moved my plug closer to the piston from the milling didn't I also change the heat range? I'm just thinking I should be running a colder plug to make up the difference of the milling. Am I thinking this right?
 
No you didn't change the heat range of the plug unless you physically changed the plug. The heat range of a plug is the difference in the amount of openess in the bottom of the plug which is changed by the size of the porciline(sp) and cant be changed by the end user. The heat range is determined at the time of manufacture and then would have a different plug # like 64,65,66,67 are all autolite plugs and the 64 would be the coldest and 67 would be the hottest, all are extended tip plugs. they also have a range like this in retracted tip plugs. Different manufactures have different systems for rating there plugs and this is why I said that you have to get into the catalog of the manufacture that you want to use and find the plug that you want or need.
 
Another thing is just because you have a retracted tip plug doesn't mean that the plug is colder as it's just the difference of the tip location. Most high HP engines run cold plugs due to the compression ratio and the high output of the ignition systems and exotic fuels. In your case you may need to drop down a heat range or two, but I would try what you have first and go from there, just adjust the timing for the compression and look at the plugs after you drive it a bit.
 
63, If I read the question right, perhaps this might be clearer. You didnt change the plug...you probably have changed what the best plug type and range might be with your milling. BJRs covered the rest. It just seemed to me that you were perhaps thinking the right way, but looking to the wrong solution for the end result of the work. Hope that makes sense...lol
 
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