Spark Plug Help

[Marine-58]

Hello Everyone,

Due to unfortunate circumstances, it has been quite a while since posting. Was hoping someone could help me determine which spark plug to use. The engine is a freshly built 454 Mark VI. The compression is 10.5. The heads are aluminum but since they were inherited and have no markings, I can't give much for the specs. All that to say, the question is what reach should the plugs have? The heads are already installed but can be removed if needed.

Just not real sure how to get an accurate measurement so the electrode isn't too close to the piston or too far inside the spark plug hole.

Jegs is recommending a NGK ITR4A15, but I just don't have much confidence in there on-line help.

I do appreciate any advice you can give!!

[Captainfab]

Have you looked to see if the heads are machined for a gasketed seat plug or a tapered seat plug? Are there any casting numbers or other figures on the top of the heads (under the valve covers)?

[Marine-58]

The heads will use tapered plugs. There are no casting Mark's whatever on the heads. I have thought to remove one head to measure the thread depth, but really don't want to have to buy a new gasket.

This is a head scratcher... No pun intended...

[Captainfab]

Well with no way to ID the heads, my guess is that they are a Chinese made head and finding out what the correct plug is without pulling one head will be a crap shoot.

[Marine-58]

You are probably correct... I'm going to try to take a thin wire, bend one end and see if I can mark it at the top of the plug hole. Maybe it can be accurate enough? I just hate wasting the head gasket. But if it comes down to that will probably pull both heads and take them to a porting shop while they are off the engine? Not sure at this point. Pulling the head is certainly the most accurate.

Oh well, nothing will happen until this weekend anyway...

Thanks everyone for the input...

[61K10]

Don't all BBC have the long thread plugs? 3/4" Should be easy to do with the wire.

[Captainfab]

Yes typically. But with what appears to be made in China heads, who knows.

Originally Posted by 61K10

Don't all BBC have the long thread plugs? 3/4" Should be easy to do with the wire.

[rpmerf]

Originally Posted by Marine-58

I'm going to try to take a thin wire, bend one end and see if I can mark it at the top of the plug hole. Maybe it can be accurate enough?

I was thinking something like this. First time you put it in, leave the engine a little bit before TDC. Put 1 plug in, spin the engine around to make sure it doesn't contact. Wondering if you can put the engine at TDC and measure the distance from the top of the piston to the seat on the spark plug so you know how much clearance you have in the cylinder.

[Marine-58]

Some good ideas, just not sure how to make the measurements with the head in place.

For the suggestion of spinning the engine to ensure no contact - that would work but you don't want anything but the electrodes in the cylinder. If the threads drop below the heads, then that leaves an area for carbon to collect.

If the electrode is too high inside the spark plug hole then you don't get good fire....

I'll try the "bent wire" method first and if that doesn't look to be accurate, then go ahead on pop off the head.

All this trouble and watch, it will wind up being the standard 3/4" reach...

Thanks to everyone for the suggestions and ideas...