I got sent a link to a 'Ford' forum where they were discussing Zetec plugs.

As we know, the Ignition system on the Zetec engine is 'Wasted Spark', which means that two plugs are always firing (aka 'wasted spark') on the exhaust stroke.
With this approach, two plugs fire through the centre electrode while the other two fire through the side electrode.
So, to save a bit of money, Ford made plugs with the platinum tip on the centre electrode on some, and others with the platinum tip on the outside electrode.
Obviously, these plugs have to be fitted in the correct cylinders.
With replacement plugs, all of which are double platinum, you can fit them anywhere.

Weird!

More Info Here

When I bought my crate Blacktop from Raceline back in 2008 I spotted it had 2 spark plugs of one type and 2 of another. Spoke to Raceline and Pete said not to bother they go anywhere. Looked odd to me so I spoke to Alan at Scholar who had sold me some good Zetec kit before. He simply said that they fitted them in matching sets otherwise customers complain they have odd plugs.

Move on 6 years and having carefully looked at the Ford plugs again its obvious where they should be fitted. Mine are probably right now for the first time ever.

Suppose "double platinum" mean they have a coating on both electrodes.

Yep, all the OEM plugs, such as the NGK Platinum PLTR6A-10G plugs I now use, are coated on both electrodes, so can go anywhere

Having used NGK plugs on my x-flow Caterham since 1989 when I fitted the Silvertop back in 2001 I automatically bought a set of NGK plugs from our local Ford factor. Fitted them, fitted leads and the boots did not fit down into the cam cover. Removed plugs and they were about 6mm longer than the Motorcraft plugs from the seat to the tip. Returned plugs and the chap was not in the least bit surprised, simply said they never use NGK in the workshop for that reason, they used Denso. Exchanged NGK for Denso and no problems. 6 years later the factor had stopped selling Denso, now sold Motorcraft, obviously they fitted with no issues. Current Blacktop has the original "odd" Motorcraft plugs and they fit fine using a Silvertop cam cover instead of the plastic fantastic Blacktop one.

Eugene
What is the difference between PTR5A-10 plugs and PLTR6A-10G plugs? (2L Zetec)
Cheers,
David

[P] Platinum ground and centre electrode
[T] or [LT] Taper seat type (* Reach is increased by 0.4mm if letter is preceded by 'L')
[R] Resistor
[5] or [6] Heat rating - higher value denotes a cooler running plug for use in higher performance (hotter) engines
[A] Design series (I guess A is the first design of this plug?)
-
[10] Spark Gap (pre-set) - 10 = 1.0mm, correct for Zetec (was originally 1.3mm, but Ford reduced it due to HT lead failures)
[G] Design Feature - G = Copper core for ground electrode

So basically, the PLTR6A-10G is a higher performance plug... if that's what your engine needs?

Cheers Steve
How much more reach does the 'LT' have? Obviously no problems with Piston2Plug clearance!!?
david

Not actually sure, but the idea as I understand it is the extended reach means the 'spark' is closer to the centre of the combustion chamber, giving improved efficiency and power...

Interestingly, I changed my plugs a short wile back as I was having some starting issues in the colder weather.
I swapped the PLTR6A-10G for PTR5A-10 to see if that helped.
It seems to have done so...

Maybe the PLTR6A-10G are just too much for my engine?

I imagine that the change was down to the cooler plug due to conditions? but what do I know!!!!!
Thanks Steve, good info as always

Oh - and I think Ford standard is Motocraft AYFS22F1 - that gets good reviews as well...

I have just found out that the 'Longer Reach' is an additional 0.4mm - so should not be an issue!
Have edited the original post above...

I had a low rev misfire on mine a couple of years ago, it really felt like the carbs were out of sync but mines fuel injected, after changing the plugs you'd have thought I'd spent £300 on a tune up.
I have in my notes for plugs either, pt16vr10 or PTR5A-10 or13u not sure where that info came from, probably Dunnell 17 years ago.

As an update, I was suffering a misfire again, and on inspecting the plugs found this:

As it happens, I still had the old set of PLTR6A-10G plugs, and on inspecting them I see one has a crack in the insulator.
So, I've gone with a set of PLTR6A-10G again, rather than the standard PTR5A-10 plugs...

Plugs