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Index of spark plugs

Posted: Sun Feb 14, 2016 9:28 pm
by spro
I thought of this and then it was ignition into the chamber. With lower type engines it wouldn't matter. Perhaps it does. I saw some guys explaining how the electrode leg of a spark plug may shield the anode at the intake valve. If the engine was in balance, there wouldn't be equal firing if all were mismatched. So there are washers which allow the plug to be oriented to the correct position. .....or another position.
In a way, many could be configured to the lowest... to be equal.
I think this could be useful to know with smaller engines, especially 2 cylinders where the ignition of one plug has a firing pattern opposite the other. I know there are the newer plugs with surrounding electrode path for automobiles but am unaware that they are available for small engines. So there may be a time when someone changes a plug and it fixes a problem. The plug measures well and actually the problem was the orientation compared to another.
The fellows at hot rod, rebuilding engines, places are or were into this. There are at least, three different thicknesses of the compressible sealing washers, by which the electrode tang doesn't interfere with the ignition pattern, or balances them.

Re: Idex of spark plugsI

Posted: Wed Sep 07, 2016 1:36 pm
by Mark D
I have gone to great lengths to properly index spark plugs in some very high performance engines, one being an engine that finished within the top ten two times at the Engine Masters Challenge competition a few years ago.
In dyno testing, it was found that there was / is / can be, a difference to the positive by indexing the plugs. However, it is very small and thus is only worth the cost and time if it is an engine to be used in serious competition where a split millisecond or one or two horsepower can make a huge difference. I don't bother with my daily driver cars. No point in it. Lower grade engines as found in daily driver cars probably would not benefit at all because the burn rate is so much slower anyway.

Mark D.

Re: Idex of spark plugsI

Posted: Wed Sep 07, 2016 4:16 pm
by tornitore45
How do you know where the GND electrode ends up? Short of taking the head down.

Re: Idex of spark plugsI

Posted: Wed Sep 07, 2016 4:22 pm
by warmstrong1955
tornitore45 wrote:How do you know where the GND electrode ends up? Short of taking the head down.
Mark the OD of the plug, and use different thickness shims to get it indexed to where you want.

Used to be standard procedure back in my motocross days.

Bill

Re: Index of spark plugs

Posted: Thu Sep 08, 2016 5:44 am
by tornitore45
Gee why did not think of that?

Re: Index of spark plugs

Posted: Thu Sep 08, 2016 9:01 am
by SteveM
The indexing washers are a packages, IIRC, of three: one that changes the position by 90 degrees, one by 180 and one by 270. You don't need four because that would just be no washer.

If the thread pitch were 20, then you would want a washer that was 25% of 1/20th of an inch. In practice, that washer might be too thin, so they might make it one thread turn plus 1/4 turn (then 1/2 and 3/4 turns).

As to how you do it, Bill has it correct, but I came up with a different method:

When you have the head off, find a plug that indexes to where you want it in cylinder 1 (or just put one in and mark it). Take it out and screw it into a block of aluminum with the same threads as the spark plug hole. Scribe a line where it is pointing and stamp "1" on that line. Do that will all cylinders. Note that you COULD do this with the head on, it would just take a little longer.

Once you have that block, you can pick any plug, screw it in and see in which cylinder it will properly index WITHOUT USING A WASHER.

You could also see, for ones that don't index directly to a hole, which would work where when using a washer.

Steve

Re: Index of spark plugs

Posted: Fri Sep 09, 2016 1:22 am
by Harold_V
SteveM wrote:As to how you do it, Bill has it correct, but I came up with a different method:

When you have the head off, find a plug that indexes to where you want it in cylinder 1 (or just put one in and mark it). Take it out and screw it into a block of aluminum with the same threads as the spark plug hole. Scribe a line where it is pointing and stamp "1" on that line. Do that will all cylinders. Note that you COULD do this with the head on, it would just take a little longer.

Once you have that block, you can pick any plug, screw it in and see in which cylinder it will properly index WITHOUT USING A WASHER.

You could also see, for ones that don't index directly to a hole, which would work where when using a washer.
I'd like to give you something to think about.

Assuming all holes have threads with the entry point clocked in the same orientation as the first hole, yeah, your idea would work just fine. However, that may or may not be the case, and I expect it is not. While it's true that you can determine registration of all plugs as they refer to the test block, or the first hole, they may or may not align the same way in other cylinders.

Harold

Re: Index of spark plugs

Posted: Fri Sep 09, 2016 7:24 am
by SteveM
Harold_V wrote:While it's true that you can determine registration of all plugs as they refer to the test block, or the first hole, they may or may not align the same way in other cylinders.
While I had some spelling mistakes, the idea I was discussing was to have marks on the block for EACH cylinder - you have a V8, you have 8 marks.

That way when you take any spark plug and thread it in, you can see if it has an optimal cylinder or not.

Steve

Re: Index of spark plugs

Posted: Fri Sep 09, 2016 3:10 pm
by Harold_V
That makes sense, and would offset the fact that threads most likely are randomly started unless all holes are tapped using a rigid tapping system with a CNC, tapping each hole with the same tap. In that case threads would be identically clocked.

Harold

Re: Index of spark plugs

Posted: Sat Sep 10, 2016 1:50 am
by BigDumbDinosaur
Back in my racing days (methanol-burning big block Chevy), I used 0.05mm thick round shims to get the plugs correctly aligned. Engines fired on methanol do best with very high compression ratios and in the case of the engines I ran, if the plugs weren't correctly aligned the outer electrode would be smacked by the piston crown. Ergo I spent a fair amount of time getting the plugs accurately indexed. The shimming method proved to be the best way to do it.