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Discussion Starter #1
It seems like my Weisco 11.5:1 pistons are hitting the spark plugs when hand cranking the engine. I haven’t started the car, so hopefully the pistons aren’t damaged, but anyone know if these would need shorter plugs and which ones?
 

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How much clearance do you need??

Take a look at this. https://www.sparkplugs.co.uk/spark-plug-symbol-codes/

Standard plug
https://www.ngk.com/product.aspx?zpid=10210

ZFR5F-11

Z = Extended Gap
F = 19mm reach (3/4in), 16mm hex (5/8in)
R = Resistor
5 = Heat range
F = Firing end construction
-11 = 1.1mm gap

Instead of F, the only other option is T = 17.5mm reach

PTR5F-11

P = Platinum
T = 17.5mm reach, 16mm hex
R = Resistor
5 = Heat range
F = Firing end construction
-11 = 1.1mm

I cannot guarantee anything, ie. whether this plug will work for you.. I would compare the two plugs, measure the resistance... etc

Not cheap!

Good luck!!
 

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Discussion Starter #3
How much clearance do you need??

Take a look at this. https://www.sparkplugs.co.uk/spark-plug-symbol-codes/

Standard plug
https://www.ngk.com/product.aspx?zpid=10210

ZFR5F-11

Z = Extended Gap
F = 19mm reach (3/4in), 16mm hex (5/8in)
R = Resistor
5 = Heat range
F = Firing end construction
-11 = 1.1mm gap

Instead of F, the only other option is T = 17.5mm reach

PTR5F-11

P = Platinum
T = 17.5mm reach, 16mm hex
R = Resistor
5 = Heat range
F = Firing end construction
-11 = 1.1mm

I cannot guarantee anything, ie. whether this plug will work for you.. I would compare the two plugs, measure the resistance... etc

Not cheap!

Good luck!!
Ok thanks for the info, I’ll look into that and heard about Miata plugs as an option too. BTW do you know of anyone running a 11.5:1 KLDE?
 

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Sorry, no.

OK, I have done a little more research on the plug PTR5F-11, it has a 5k resistor (instead of 10k) and a tapered seat (instead of gasket) ... so it looks like it is NOT a good match!!

The Miata plugs have the same reach as the MX6, i.e. 19mm ... also 5K not 10k resistor (not sure if resistance really matters, because other upgraded plugs are 5k)
https://www.ngk.com/product.aspx?zpid=9308

I would use the original plugs and put a custom M14 washer or two to lift them up. (custom = professionally made so the fit without fouling, i.e. a thin edge)

With the spark plugs removed and when you hand crank the engine, are there any interference issues?

What cams have you got? Custom grind?

Who built the engine?

The KL-ZE had 10:1 compression and high lift cams to achieve 200hp. Were you planning to put in custom high lift cams?
 

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Discussion Starter #5
Sorry, no.

OK, I have done a little more research on the plug PTR5F-11, it has a 5k resistor (instead of 10k) and a tapered seat (instead of gasket) ... so it looks like it is NOT a good match!!

The Miata plugs have the same reach as the MX6, i.e. 19mm ... also 5K not 10k resistor
https://www.ngk.com/product.aspx?zpid=9308

With the spark plugs removed and when you hand crank the engine, are there any interference issues?

What cams have you got? Custom grind?

Who built the engine?

The KL-ZE had 10:1 compression and high lift cams to achieve 200hp. Were you planning to put in custom high lift cams?
With the plugs removed there doesn’t seem to be any interference, so I guess the 11.5:1 dome pistons are closer to the heads and spark plug electrode than OEM.

I have the basic race grind from Colt Cams.

My engine was built by a reputable local builder who has been doing it since 1948, but I guess they didn’t rotate the engine with the plugs installed to avoid compression and no one thought about interfence due to spark plugs.

I wonder if the head gaskets should have been thicker and also noticed that NGK seems to have thicker washers on some of the spark plugs.

Any other suggestions?
 

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You cannot go wrong with someone who has been doing it that long. LOL

I just edited my response.. but thought of something better....

I think if you got a couple of say M10 washers drilled them out to M15, to make the outer edge thin, and just use the original plugs.

Put copper grease on the thread of the spark plug (of course), and on the washers to keep them in place.

Torque them to 18 - 21.6 lb. ft and hopefully they seal well!
 

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Familiar with indexing the plugs so the spark kernel's flame front is directed towards center of combustion chamber? Indexing shims for std plugs are available in many thicknesses. Each shim added moves neg electrode deeper into plug boss.
 

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Familiar with indexing the plugs so the spark kernel's flame front is directed towards center of combustion chamber? Indexing shims for std plugs are available in many thicknesses. Each shim added moves neg electrode deeper into plug boss.
The only issue is - these are expensive, just for a few washers.. see link
Indexing shims
 

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Not as expensive as replacing damaged pistons. More likely having to replace plugs every day because plug gap closed to 0. No sparkie no runnie. Know what playdough is good for? Put a cone shape on end of one plug. Be sure it is suck good to plug. Install plug and roll engine past TDC for that hole and remove plug. thickness of playdough on bottom of neg electrode is piston clearance.
 

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Late to the party, but bkr spark plugs should do the trick, bkr6e11 are what I run, I don't remember the length difference, but they have a shorter ground strap reach with the same body as the zfr plugs
 
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