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minimal gap between piston and spark plug

Engine Building Fundamentals

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hi Guys! is there a recommendation for the gap between the piston and the spark plug? In my BMW M50B25 project, the gap is only 0.6mm wide. Makes me a little worried. Thanks in advance

That is a very good question about something that is so often overlooked, as almost all commercially produced spark plugs are of the projected design as they're more resistant to fouling, have a more central spark position, and they have other advantages. However, I know of one instance when a chap changed his spark plug brand only to have a misfire when he started the engine - the new brand had slightly more projection into the cylinder and the pistons had just contacted the earth electrodes enough to almost close them up. Fortunately, no damage and going back to the original type fixed it.

Your engine will be susceptible to the same issue, but there are some options available to you as spark plugs are available in a range of designs, from very projected tips to flat, even recessed, surface.

I would suggest having a look through this rather good guide to see what's available - https://www.ngk.com/ngk-ground-electrode-designs

Some may not be available from your favourite provider as there are many other physical considerations, such as thread diameter and length, seat seal design, insulator length, terminal design, etc. But having some idea of your options should allow you to find something suitable, and aid you in your enquiries with your supplier(s).

Thanks first of all! Good info but not quite what I had hoped for;) A more precise specification for an under limit would be good. something like: "stay over a millimeter and that fits in most cases." or did I overlook the topic in the course? :/

Our pistons have such a high dome for high compression. (12.4: 1) but cannot find a spark plug (heat range 8 NGK) with a spark position below 3mm.

With the BKR8EIX i get the 0.6mm gap. Is this too narrow or not? :/

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These are very much my thoughts, and NOT definitive answers.

Assuming this was actually verified with a dummy build, then you 'should' be OK provided you don't change the plug type, or brand, for one with a slightly 'taller' earth electrode position - it may pay to physically check each one before installing and after running to check they hadn't closed up.

For a little extra clearance, you can actually purchase different thickness spark plug washers - https://www.moroso.com/spark-plug-indexing-washers71910/ - these were introduced for 'indexing' the ground electrode position as some tuners believe it affects the burn - however, they also offer the ability to 'lift' the spark plug in the head for more piston clearance. I don't recall the normal crush thickness for stock washers, but it will be very close to used washers - if you install, and torque, a spark plug with the stock washer and mark the position, then install and torque the aftermarket and mark it, you can work out the difference in installed depth from the pitch, 1.25mm, and the angle difference.

Rather than trying to find the 'correct' spark plug type, and heat range, I would suggest directly contacting NGK to see what they can offer - you may also suggest they add the dimensions of their various tip designs, as their site seems to have everything but that on it.

If you're still on the dummy build stage, you could also weigh the pistons and remove the same amount of material on each where the pistons approach the spark plugs.

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