I picked up a set of NGK iridium plugs for my 99 4.6,
when I pulled one of the factory plugs however I noticed that the stock plug is MUCH shorter. The NGKs (TR55IX) thread in at least a 1/4 inch further. When I went back to the store I looked at all the compatble plugs they list in the manuals and they ALL look way longer than the stock plugs.
Has anyone noticed this, can I trust the info from NGK, Bosch, etc.?

 

What you're looking for is plug thread diameter, distance from the last thread to the outer ground electrode (protrusion into the cylinder) and the length from the first thread to the 45 degree angle-cut sealing surface.

There is less than 1/2 inch of threads in the head shown by the OE plugs, many aftermarket plugs have threads that run up the plug but these threads would be in the air if you can imagine what I'm saying. The distance from the tip of the plug to the sealing surface is 3/4 or 1 inch but it should be the same for all plugs that you compare for your motor.

 

If the replacement plug will physically fit farther down the head I would NOT intall it. It the threads are longer then the plug will just screw down farther possibly allowing all 8 pistons to slap the plugs, depending on how much of a diff. there is. It probably would not happen, but with my luck I would not try it.

 

Thanks guys

It seems to me that ALL the aftermarket plugs that "supposedly" fit will thread in too far possibly hiitng a piston.
On the stock plugs there is a gap of about 5/16ths of an inch from where the threads end , and the seat. so it only threads in so far.
On all the aftermarket ones I've seen, there is NO gap between the threads and the seat, allowing it to thread in further. It's really hard to explain but maybe someone can help me ouy here.

 

O guess what I'm asking here is, do the aftermarket plugs that some of you guys have used look exactly the saame as the stock ones. All that I have seen are threaded upto the seat. But the stock ones aren't.

 

If you are in doubt just buy some OEM plugs. This good be an expensive mistake!

 

When I replaced my stock pluga at 67K they looked great, other then a little rust on some of the threads. I never had a misfire or other ignition problem. So I replaced the plugs with the stock Motorcraft plugs....if it isn't broke no reason to fix it. I don't think there is any reason to go to an aftermarket plug if the Motorcrafts are working for you.

 

I found Motorcraft plugs at O'Reilly's Autoparts for $2.59 each

 

I thought the same thing too when I got my Bosch + 4's. The threads went up the entire length of the + 4, while the threads on the Motorcraft were only on the bottom. Anyway, if you hold the two plugs side to side and align the bottom of both plugs to each other, then you 'should' notice that the seat will match as well.

If the seats DO NOT match, then the plug is not right. If the distance from the bottom of the plug to the seat IS the same, it'll work just fine. The threads going up the entire length of the plug will not make the plug go farther down the head. The seat will always stop it first.

Been running my + 4's for over 30k and not a problem yet!