Piston Dome Hitting Spark Plugs?

Stretch32 · 05:39 AM

Is it possible for the dome of a high compression piston to hit the spark plug and close the gap so it won't fire? The engine in question is a 2.3L (85mm x 66mm) twin plug with 10.5:1 CR JE forged pistons and I'm using NGK Iridium BKR 8EIX plugs. We just finished installing and starting the engine and we found that number 1 and number 5 cylinder are not igniting. The inside of the cylinders are completely clean of carbon and no ignition of the air / fuel mixture appears to have occurred.

We checked that the cylinders are getting spark and fuel and we did a compression test as well which confirmed there is spark / fuel / good compression. Upon further investigation, it looks like the piston domes of 1 and 5 are just touching the spark plugs (all 4 of them) causing the gaps to close up and preventing the air / fuel mixture from igniting. We're going to run a borescope in the cylinders today to see if there's any damage and to confirm this isn't happening to the other cylinders just to be sure.

Assuming there is no damage, what should my follow on path be?

Is it possible the plugs are too long for the pistons I'm using and I should use a different plug for this engine setup?

Did I miss something during the assembly of the engine (even though the other 4 cylinders seem to be fine)?

Thanks in advance for the help.

Stretch

Neil Harvey · 01-05-2023, 06:02 AM

Quote:

Originally Posted by Stretch32

Is it possible for the dome of a high compression piston to hit the spark plug and close the gap so it won't fire? The engine in question is a 2.3L (85mm x 66mm) twin plug with 10.5:1 CR JE forged pistons and I'm using NGK Iridium BKR 8EIX plugs. We just finished installing and starting the engine and we found that number 1 and number 5 cylinder are not igniting. The inside of the cylinders are completely clean of carbon and no ignition of the air / fuel mixture appears to have occurred.

We checked that the cylinders are getting spark and fuel and we did a compression test as well which confirmed there is spark / fuel / good compression. Upon further investigation, it looks like the piston domes of 1 and 5 are just touching the spark plugs (all 4 of them) causing the gaps to close up and preventing the air / fuel mixture from igniting. We're going to run a borescope in the cylinders today to see if there's any damage and to confirm this isn't happening to the other cylinders just to be sure.

Assuming there is no damage, what should my follow on path be?

Is it possible the plugs are too long for the pistons I'm using and I should use a different plug for this engine setup?

Did I miss something during the assembly of the engine (even though the other 4 cylinders seem to be fine)?

Thanks in advance for the help.

Stretch

I'll jump in here.

""Did I miss something during the assembly of the engine (even though the other 4 cylinders seem to be fine)? ""

Yes.

From what you say, I assume the ground straps on the plugs has closed from hitting the pistons? If this is the case and all 4 plugs have closed their gaps, its not the plugs, its the pistons that are wrong. If it was just the lower plugs, one could argue the plug depths were cut to low, but the stock plugs should not hit, even with projected plugs. When you measured the chamber volume, and having fitted plugs to measure this, you would have seen the plugs too low.

When changing pistons, certain checks are required. Especially pistons with high domes. Piston to head clearance and plug clearance checks have to be done. In your build notes you should have recorded the piston deck height to obtain your CR number. After calculating your CR number, checked to see what the other clearances were. If they were too low, you would have machined the piston to clear or added clearance shims and recalculated the CR number.

There is a number of other factors that could be in play here too. The head heights could be short, and the chamber volume would show this. If the pistons used are a common profile sold as a "drop in" then your parts are probably where the problem lies.

Do as many checks you can before you tear the engine apart. Go back into your notes and see if the problem shows up in your numbers.

This is exactly why its important record every possible dimension and number when building an engine. It not only gives you info when something goes wrong, it reminds you to check all the critical dimensions.

There are plugs that have less protrusion into the chamber but their performance will change.

Stretch32 · 01-05-2023, 10:04 AM

Neil,

Thanks for the reply and the help.

I was wrong in my first post and it was only the lower plugs that were contacted (not the uppers / original holes). There is no clearance issue with the upper plugs on looking again.

When I did the measurements while building, there was ~1.00mm - 1.25mm of clearance between the cylinder heads / piston edges and valves / piston domes. We also cc'd the heads and they all measured ~68cc's with a computed CR of 10.3:1.

With what you've suggested its possible the lower plug holes could be cut too deep allowing the lower plugs to protrude into the combustion chambers too far. When I was building the engine I did not notice this but I also wasn't looking for it since I've never built a twin plug engine (the heads were already cut when I got them).

Pending any other issues, I'm planning to get a spark plug shim / index kit to move the lower plugs away from the piston domes. I'm also going to borescope all the combustion chambers to make sure there's no damage to any pistons and to see how deep the lower plugs are protruding in the current configuration as a point of reference (before putting in shims).

Stretch

stownsen914 · 01-05-2023, 03:51 PM

Couple things. Neil knows way more than I do, but I’ll add thieve thoughts:
BKR 8EIX Looks like it has a more projected tip than the NGK 9’s I’m putting in my race engine. I think they’re B9EG’s? This could be a factor.

When I took my engine apart for rebuild, I found the second sparkplug holes (12mm) were incorrect depth. I had a machine shop sink them about 2mm deeper so the sparkplug ground strap just kisses the arc of the combustion chamber and no female thread show on the chamber side, like the stock plug holes when using non projected tip plugs.

Another thing - while investigating the above, I found that the projection distance is not the same for different manufacturers for roughly equivalent, non projected tip plugs. I found a difference between Bosch and NGK plugs, for example. If you can get comfortable that there isn’t any damage to your engine, it might be worth looking at other plugs before deciding to tear down.

Stretch32 · 06:57 PM

Thanks again for the help.

I was able to get a good look at all the plugs today when I stopped by the shop and to borescope the piston tops. All the top plugs are untouched as I mentioned earlier and all the bottom plugs have some closing of the grounding strap due to piston contact. The worst one is on number 1 where the grounding strap was pushed off to the side of the diode (after being pushed into the diode first) but it did not touch the insulator. All the other lower plugs just had the gaps closed down but none were closed so far that they touched the diodes.

The only issue I see is a mark on each piston top under the lower plug holes where the piston and grounding straps met. With the borescope, it’s difficult to see anything more than a slightly shiny spot at the contact occurred. The worst of the marks is on number 1 where the ground strap on the plug was bent the furthest over after contacting the diode.

Based on that observation, I’m thinking there is no real damage that would cause an issue to function since the ground straps don’t require much force to bend and it doesn’t appear there’s any knicks or gouges. Additionally, the engine was spun over on the starter a number of times before the engine actually started so the engine wasn’t running when the plugs were struck and bent. For the engineers, does this mean there was less force to the plug to piston dome contact or would it be the same as a running engine?

I ordered more spark plugs of the same type along with a plug index washer kit that will allow me to to get another 1mm - 1.5mm of clearance with these plugs. If there’s another type of plug with less protrusion that works with this engine configuration then I’ll order those if that’s better than using the index shim kit.

Stretch

stownsen914 · 01-06-2023, 04:42 AM

For reference, here are some pics of B8EG and BKR8EIX sparkplugs. Granted these are pics off the internet, but the BKR8EIX looks to project possibly 1 mm or more beyond the B8EG.

B8EG:

BKR8EIX:

gsxrken · gsxrken's Garage

It’s also puzzling why cylinders 1 and 5 did not fire at all.

Neil Harvey · 01-06-2023, 12:27 PM

Generally speaking, your engine will run better with a projected plug than one that is recessed. The further into the chamber you can start the ignition sequence the better. The rule of thumb here is, the more projected the cooler the plug needs to be and vise versa with a recessed plug. Also, be careful if the shop that did the 2mnd plug allowed for a 13/16" socket. They may have cut only for the 5/8" hex. This changes your plug choice.

I think you can now accept that the projected plug with your Pistons work, but that the machine work for the 2nd plugs is too deep.

My advice is to shim the plugs and stay with the projected plugs. Make yourself up a dead stop and fit it to the upper plugs. Measure how deep you have to go until the piston just touches. Subtract 3/4" and this will be your clearance. Then do the same for the lower plugs. The difference will be how much you need to shim. I would do this even if you decide to use another firing ended plug.

stownsen914 · 01-06-2023, 02:47 PM

I think he said some of the stock position plugs also hit the pistons?

Either way, as suggested some washers of appropriate thickness ought to solve the issue.

Stretch32 · 01-06-2023, 05:11 PM

Thanks again for all your help.

A leaky crank shaft due to a faulty freeze plug is going to require a complete tear down to send the crank back to Ollie’s. With that, I’ll have an opportunity to pull the heads off and visually inspect the pistons. I’ll also be able to get a good look at the lower plugs and have a good idea of how thick of a shim I’ll need. I will do the measurements as suggested above just to make sure everything is good once it all comes back together. Most likely, based on my work schedule, this won’t be until later this month or late next month.