NOT good......

If a shorter reach plug in desired heat range cannot be found, I would just machine a small smooth relief into the piston.
Yes, that will increase the CC volume a bit.
But a shorter reach plug, or recessing the plug with washers/spacers will also increase the CC volume a bit.

If a shorter/washered plug is used, inspect that this does not expose bare spark plug hole threads in the CC. Sharp, pointy little projections like that are ideal for causing pre-ignition.
Trim them away.

erohslc wrote:

EvanB wrote:

erohslc wrote: I believe the OP's second picture is the piston sitting with dome in the head, not the block: "The piston is resting on the spark plug electrode.. "

And?

tuna55 had posted: I am missing this as well. Without a mock up piston/rod/crank/headgasket that you are maybe just not showing, we can't be sure of anything. I suggested that the OP had shown that such a mock up wasn't needed in the picture.

It is VERY necessary. You have no way of knowing what the clearance is. So what if the spark plug hits first? Something has to hit first. If you put any piston on a cylinder head, something is going to hit, but it doesn't mean it's close at all as assembled, there could be 1/2" of clearance.

To quell some of the side issue here... the deck* on these pistons allegedly is less then 1/2mm (about .020") that is why I just inverted the pistons into the chamber.

Deck - distance from pistons edge(+ or -) to the edge of the block

Since the head is flat, where the piston is flat(green hash marks), the squish should be about 1mm (.020" gasket + .020 deck)

oldeskewltoy wrote: possible solutions?

Buy a miata?

Have you dry assembled the the reciprocating assembly and verified deck height with a dial indicator?

Until you have you're guessing.

motomoron wrote: Have you dry assembled the the reciprocating assembly and verified deck height with a dial indicator? Until you have you're guessing.

Yes, exactly this.

Short block is in the caribbean... but that (measured deck)is exactly what I asked him to get me with one of the other 3 pistons.

oldeskewltoy wrote: Short block is in the caribbean... but that (measured deck)is exactly what I asked him to get me with one of the other 3 pistons.

I really really doubt you have that mocked up properly. You'll need measurements. From that the the piston crown, from that to the deck, then you can shim the piston from the head appropriately for gasket thickness and piston-deck clearance.

speedbiu wrote: I may be missing something but without the piston in the block you have no idea how high its going to come up.It might clear.The dome does not go flush with the head.IMO :)

Exactly.

Ranger50 wrote: Find out the piston top/deck thickness and put in a fire slot in while you are deburring and smoothing the piston top.

This is what I immediately thought, if there's enough thickness to do it.

BTW, O/T but I did look into shipping you my head for work...not quite doable on my budget. About $250US round trip, I'd need to hire a customs broker and 100% duty would be charged on the work as well.

motomoron wrote: Have you dry assembled the the reciprocating assembly and verified deck height with a dial indicator? Until you have you're guessing.

Another +1 on trial assembly before doing ANYTHING.

One trick I've used is to take some modeling clay and put it in strategic areas on the piston, assemble the motor, rotate through 720 degrees(1 full cam rotation) , then peel off the clay, cut in half, and measure. That's your x to piston clearance.(where x is plug, valve or head depending on location).

Also don't forget it not only has to clear, it needs some extra clearance to account for elastic deformation. How much extra is dependent on RPMs, rod, valvetrain design and engine geometry.

rotary plugs, especially if its in the Caribbean, the should be plenty to gather from PR lol.

curtis73 wrote: I would definitely try a washer under the plug first. Conical or flat seat? Either way its not too tough. Perimeter-electrode plugs are another possibility. They've been used in 2-stroke marine applications since the late 70s. They are generally a very cold plug, but from the looks of those domes it shouldn't be too much of a problem. They look like this: Of all the GRM solutions, the extra washer under the plug seat is the best. Cutting pistons for a plug electrode can make hot-spots especially given its proximity to the flame front. Adding head gasket thickness lowers compression by adding volume all over just to get a small distance of clearance.

Those are also known as 'surface gap' plugs. The only drawbacks: they tend to be very 'cold' heat ranges and the gap is not adjustable.

I'd stay away from using a real short plug in the plug hole or spacing them up with washers. The threads will quickly fill with carbon which can glow, leading to preignition.

Custom pistons... info from manufacturer- deck = .005" on a stock uncut block. Head gasket the manufacturer recommends. 040".

Based on measurements, the piston will need additional clearance.