Spark plugs are hitting the pistons. Help!

[Bill D]

Running very rough. Pulled the plugs and the electrode is pushed against the other electrode. Virtually no gap. What’s happening?

[spdbrake]

Bill, From an earlier post you listed your build being a 331SBF with Trick Flow 205cc heads.

From the Trick Flow catalog they list an Autolite 3924 as the plug if that helps.
https://static.trickflow.com/pdf/catalog_2018.pdf (page 41)

[Bill D]

.thanks. Upon closer analysis, it appears the exhaust valve is kissing the spark plug, flattening the electrode

[spdbrake]

That is strange. I would think not possible that they'd design their highest flowing heads, which folks would put high lift cams in and then place the spark plug in the path of the exhaust valve travel?

It could be detonation.

I'd make sure you have fresh gas, plug gaps @ .035". Whats your Base and full advance set at?

[Bill D]

Would detonation push the electrode flat? Seems weird. Timing hasn’t been set yet. Haven’t noticed any detonation issues.

I plan to call trick flow tomorrow

[spdbrake]

Smashed electrode http://4.bp.blogspot.com/-c0RcgGHOXC...detonation.jpg

Trickflow 205cc heads combustion camber
https://cdn2.bigcommerce.com/server5...0.1280.JPG?c=2

[Bill D]

Thanks a bunch. I believe the timing is too far advanced. I’ll reset it tomorrow.

[undy]

Ignition timing isn't going to affect PTV contact. The engine need to be torn down, at least head removed. "Possibly" a thicker head gasket will fix the problem. It sure sounds like some mismatched parts (pistone/rods).

[Ron61]

Possibly if the exhaust lobe on the cam is higher than the intake and this doesn't happen at lower RPM then you may need a stronger valve spring. I did see this happen on a car that some guys were building to drag race. It ran OK at idle and lower RPM but when they started to open it up the valves hit the pistons and completely wrecked their motor. They went to the cam manufacturer and they told them the valve springs they used were way to light and they needed much stronger ones for that cam.

And as Undy said, there could be some mismatched parts. Anyway I hope the fix turns out to be a simple and easy one.

Ron

[Danr55]

It's been awhile since I've torn down a small block motor, but I don't believe that the valves can hit the spark plug. If anything is hitting the spark plug it would seem that it would have to be the head of the piston. That tells me that possibly the spark plug is being used is wrong was too deep ahead or the engine was built wrong. What am I missing?

[LMH]

Valves hitting the plug sounds impossible. The plug would have to be WAY down inside the chamber. Something else is going on there.
Larry

[Bill D]

After analyzing the situation more, I’ve come to the belief the piston domes are kissing the plug electrodes.

I just got off the phone with the tech support guy at NGK spark plugs. Told him my situation and they were able to find a shorter spark plug with a better heat range for my compression. Going to get those today install them tonight and crank it over without the coil wire attached. Then I’ll pull a couple plugs and see if that fixes the problem.

[Karl Bebout]

Bill, had you been using the 3924 or its equivalent?

If so, hope the shorter plug does the trick.

Mighty strange, if the proper parts have been used.

[Bill D]

I think my problem stems from using a domed piston with zero deck height, allowing the dome to move further up into the combustion chamber. The shorter plug should be the fix. I sure hope so

[Tom Kirkham]

If you are talking about the threaded portion of the spark plug, you just can't use a shorter plug. The threaded portion of the plug length has to match the threaded portion of the spark plug hole in the head. If the plug is too short then you will have exposed threads from the spark plug hole. If the plugs are too long then you will have the threads from the spark plug exposed. Exposed threads in the combustion chamber will cause pre-ignition. That being said you might be able to index your spark plugs so they clear. You also might be able to find spark plugs with shorter electrodes. Or something maybe a surface gap plug like this: https://www.summitracing.com/parts/n...BoCHSgQAvD_BwE

[Bill D]

Thanks Tom
The new spark plug still has the .750 threaded area but the tip is not projected. According to NGK tech this reduces the length .100 which hopefully does the job

[Danr55]

If that fixes your problem, would it make sense to pull the heads and check the pistons for damaged crowns?

[Bill D]

Dan, I just don’t want to do that. I really don’t.

Here is a picture of the new (left) and the old (right) plugs

[cycleguy55]

Originally Posted by Bill D

Thanks Tom
The new spark plug still has the .750 threaded area but the tip is not projected. According to NGK tech this reduces the length .100 which hopefully does the job

0.100 shorter should be plenty - if you need more than 1/2 that your current plugs would show a lot more damage than just closed electrodes.

[spdbrake]

A cheap endoscope will allow you inspect the piston tops and camber areas.
You may need to use the mirror attachment (please use a good quality tape as insurance it does not pop off inside the camber)
I use mine quite a bit to inspect areas, retrieve part numbers off parts, etc...

https://www.amazon.com/Endoscope-Wat...ndoscope&psc=1