Second sparkplug hole not machined deep enough?

[stownsen914]

Had the heads on my race motor reconditioned, and after I got them back I realized that when the second sparkplug hole was machined years ago, it doesn’t appear to have been sunk far enough in the combustion chamber. See first pic. Compared to the stock plug, the ground electrode on the second plug is almost 2.5 mm farther from the contour of the combustion chamber. Threads are also evident, as you can see in the first pic (no threads showing on the stock plug). Also including a pic of the stock plug for comparison. Both plugs are standard, non projected plugs.

Seems like the seating surface for the second plug should be machined by the 2.5 or so mm so the second plug projects into the combustion chamber like the stock plug. I’m getting different opinions on the value of doing this. Thoughts?

Second plug:





Stock plug for reference:

[icarp]

The exposed threads in the plug hole are a bad source of heat risers for detonation or preignition.
It should be a simple process to spot face the the top side of the plug seating land .
The goal is to have the same as your factory plug . No threads showing , either in the head or the plug.
The stock plug is the proper heat range , and the new one is too hot .
Just my thoughts
Ian

[stownsen914]

Thanks. Was thinking more about shrouding of the spark due to the recessed plug, but interesting point about exposed threads.

[Jonny042]

Those plugs do look quite different is there a chance the threaded portion of the lower (second) plug isn't the correct length?

I agree with Ian that it's fairly simple to spot face the plug seating area but will require the correct angles to be set. People who do twin plug machining all have fixtures made for this that may be ever so slightly different, it will require a bit of careful setup to get it right. This may be more time consuming and expensive than simply having heads machined for twin plugs in the first place.

[stownsen914]

The second plug in the pic is an NGK D8EA if I recall correctly. It's a standard 12mm plug. I had some older Bosch 12mm plugs around and tried them too, same result.

[icarp]

Jonny, I think the easiest way to set the heads up for a spot face would be to set the head up on a an adjustable angle jig. Then place a spark plug in the mill chuck , or custom threaded tool, hand thread the plug into the hole , making the necessary adjustment to the head for proper alignment . Pull the plug add the spot face bit and cut to the prescribed depth .
Just a quick thought , and yours?
Ian

plug range is too hot for that comp ratio, It's good for a fun street engine
The other plug looks like a 3 wr bosh correct for a racing engine
I am concerned about the difference in flame propagation and the hot spots
look at the b8 ground electrode the heat is past the 45* point and that is not good.

[winders]

Originally Posted by icarp

...plug range is too hot for that comp ratio, It's good for a fun street engine...

The NGK heat range "8" is the same as a Bosch heat range "4".

I run the NGK heat range "8" in my 12.5:1 race engine......

[icarp]

Scott,
look at the b8 plug , too much heat in the ground electrode
too much heat in the insulator
look at the bosh
fantastic
Ian

[winders]

Ian,

Look at their respective locations in the head.....get that 12mm plug where it is supposed to be and then see what it looks like...

[Jonny042]

Originally Posted by icarp

Jonny, I think the easiest way to set the heads up for a spot face would be to set the head up on a an adjustable angle jig. Then place a spark plug in the mill chuck , or custom threaded tool, hand thread the plug into the hole , making the necessary adjustment to the head for proper alignment . Pull the plug add the spot face bit and cut to the prescribed depth .
Just a quick thought , and yours?
Ian

I agree with what you're saying - was just giving the heads up (no pun intended) to the OP that the machining operation might be a bit more time consuming that he thinks.

Before I did all the machine work on my MFI monster I had little appreciation for how little I knew about machining and how time consuming it can be to make fixtures and do setup for what at first looks like a simple little job!

I'm actually preparing to do the machining on a set of heads for a second set of plugs so it's top of mind for me these days.

Hope all is well with you Ian!!!

[icarp]

Originally Posted by winders

Ian,

Look at their respective locations in the head.....get that 12mm plug where it is supposed to be and then see what it looks like...

Scott, you and I just disagree.

This Info is for any one else that might be interested in why i think this way:
The b8 is recessed further from the flame front and it is already to hot. the plug base is more narrow so it harbors more heat. The insulator is deeper and has more area so it stays hotter, the space around the insulator is greater , this has much to do with how hot the plug gets

the wr3 base is flat and thick so it transfers more heat, The insulator is smaller and not as deep, so it holds less heat.
I think having both plugs being the Bosh wr3 is the correct plug for this engine .
End of my thoughts
Ian

[winders]

I know how plugs are designed, Ian. It's kind of hard to stick a Bosch WR 3 in a 12mm hole....

[stownsen914]

Thanks for sharing thoughts, guys. Wanted to confirm that sinking the second plug closing to the combustion chamber is the right thing to do. I was surprised to get an opinion that it shouldn't matter.

Correct that the second plug is a 12 mm. Don't really want to get wound up about the plug itself. I'll most likely be using NGK D9 heat range (or possibly D8). I've noticed the 12 mm plugs do appear to be a little more projected than the 14 mm ones for a given heat range. Not sure why, I'll just get the proper heat range and call it a day. I think that's a W3DC in the stock 14 mm hole for the pic.

[Mixed76]

Originally Posted by stownsen914

Wanted to confirm that sinking the second plug closing to the combustion chamber is the right thing to do. I was surprised to get an opinion that it shouldn't matter.

.

You want both spark plugs in the chamber, not buried in little wells. If packaging allowed they would be square and flush to the hemi surface without the small pocket.

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