MTFCA Forum

I'll be replacing the plugs in my '26 Fordor. The plugs are so old that I believe they've since been discontinued. They're currently Bosch Platinums. If it matters, I'm using a distributor, (forgive me, please). I don't have the car handy to check the numbers right now.

Regardless of what's in it now, what are your favorite "go to" 14mm plugs for use with a "Z" head?


Thanks!!

My choice would be platinum plugs that looked exactly like early Champion X plugs. The platinum wire type plug would facilitate starting on magneto.

Been a long time since I ran a Z. I believe I ran Autolite platinum AP5405 in the Z.
I still run these in a lizard head.

speedytinc wrote: ↑

Tue Jan 27, 2026 12:05 pm

Been a long time since I ran a Z. I believe I ran Autolite platinum AP5405 in the Z.
I still run these in a lizard head.

Yes, I see other references to AP5405 in other forum threads. Also, some Champion 16YC plugs are mentioned.

TXGOAT2 wrote: ↑

Tue Jan 27, 2026 11:59 am

My choice would be platinum plugs that looked exactly like early Champion X plugs. The platinum wire type plug would facilitate starting on magneto.

Is that what you use, Pat? What number & make would those be?

Plugs are cheap really, so play around with them and see which works best. But word of advice, my opinion (others have seen it too); if you are running coils look for plugs with no "R" in the number. The resistors don't seem to like the huge swings in current/voltage esp. if running the magneto.

They're made of Unobtainium by the Nonesuch Corporation.

TXGOAT2 wrote: ↑

Tue Jan 27, 2026 1:48 pm

They're made of Unobtainium by the Nonesuch Corporation.

Thanks for that valuable input...

Mark Gregush wrote: ↑

Tue Jan 27, 2026 1:41 pm

Plugs are cheap really, so play around with them and see which works best. But word of advice, my opinion (others have seen it too); if you are running coils look for plugs with no "R" in the number. The resistors don't seem to like the huge swings in current/voltage esp. if running the magneto.

Thank you, Mark. I'll most likely try the AP5405 and see what happens. Even though I run a distributor, I'll still stay away from the resistor plugs, (since I've had good experiences with the non-res. plugs so far).

I run NGK BP5ES can find them at most auto parts stores.

I run Bosch Platinum 4003. FR9HP
Whatever you use, make sure the top unscrews.

I replaced the original nuts with some period-ish looking thumb nuts.

: ^ )

Keith

I don't think resistor plugs or wire give good results with systems not designed for them. If you have a modern type coil and distributor system, you can probably use them if you care to. They are intended to reduce radio frequency (RF) interference radiated by the high tension side of the ignition system. Most Model Ts have no radio, and apparently microwave cell phones are not affected. I have a ' 27 T with a stock system. I was surprised to find that the car's engine caused virtually no RF interference on an old portable AM/FM/SW radio, either near or in the car. This was in a fringe area with no strong local stations. I suppose that the steel firewall and steel coil box and very short high tension wires prevent a lot of RF radiation. The old portable radio runs on its own batteries and is in a vinyl-covered cardboard case. An earlier T with a wooden firewall, coil box, and comparatively long plug wires might be a different matter.

I connected a T coil up to a 50 foot outside antenna wire and set up a 1/4" spark gap. I used a Morse key in the primary supply. I could send Morse code to any AM radio in the vicinity. A neighbor 4 blocks down the street could hear it clearly on a typical table top clock radio. I think it may have affected nearby TVs with antennas. That lasted about one day until the complaints began to come in.

KWTownsend wrote: ↑

Tue Jan 27, 2026 4:43 pm

I run Bosch Platinum 4003. FR9HP

20260127_133748.jpg

Whatever you use, make sure the top unscrews.

I replaced the original nuts with some period-ish looking thumb nuts.

: ^ )

Keith

Keith,

I believe the 4003 are what I've used in the past, but I think they were discontinued.

I still have the instructions from when my Z head was new (around 2009), the two plugs recommended were Champion N16YC and Bosch Platinum 4220. Both are obsolete, the Champions were easy to find NOS on the big auction site so that is mostly what I have been running with rebuilt coils.

The Model T vendors that sell 14mm plugs used to sell NGK BP5ES. Not sure why, but they switched at some point and started selling Autolite 66. I've tried both of these and they seemed to do OK. The Autolite 66 is a resistor plug with a 4k resistor, I don't know about long term effects but I can't seem to tell if there is any difference in performance due to the resistor. I think the Bosch Platinum plugs above are also resistor but I've never tried those.

Do you use stock T ignition coils? I'd stay away from resistor plugs with a stock ignition system or with antique magnetos. Some plugs are designed for aluminum heads and the thread area is plated with a material that prevents the plug threads from seizing in the aluminum. A non-resistor platinum electrode plug with a fine wire type electrode should work well with a stock T system or any other type of system. The ideal heat range depends on how your engine is set up and how you drive. The plug's thread length should be appropriate for the head you are using. A plug that places the spark gap a short distance into the combustion chamber usually works well in low compression engines with large combustion chambers, assuming clearance is available and that the threads do not project into the chamber.

Jerry VanOoteghem wrote: ↑

Tue Jan 27, 2026 11:55 am

Regardless of what's in it now, what are your favorite "go to" 14mm plugs for use with a "Z" head?


Thanks!!

I run older/original Autolite AG42 plugs, using a Z-Head, Hi-Volume aluminum intake and a Straight Through Holley NH. An older discontinued plug as well, but I got a lifetime supply from Ebay. You should be able to find your discontinued plug there, with any luck.

I have had an ongoing "issue" with the number 1 plug fouling and what worked to stop it was using an AG32 (hotter plug) just on the number 1 cylinder, so 3 AG42 plugs (2, 3,and 4 cylinders) and 1 AG32 plug, not ideal but it works.

Been running Autolite 66 for twenty years
With no issues they are copper core non
Resistor. Have run them in 2 stroke race
engines @ 13,000+ rpm so I think they do
well on coils. Most Auto parts store have
them or can get them asap the price is
usually in the $2 range.
Craig.

Craig Leach wrote: ↑

Wed Jan 28, 2026 5:39 pm

Been running Autolite 66 for twenty years
With no issues they are copper core non
Resistor. Have run them in 2 stroke race
engines @ 13,000+ rpm so I think they do
well on coils. Most Auto parts store have
them or can get them asap the price is
usually in the $2 range.
Craig.

Thanks, Craig!

If you can find them, a Bosch 4003 has a long reach and gets the electrode close to the top of the piston. We have also machined off the hex head on the adaptors to get the electrode even closer. Have not changed a fouled plug with this set up in years, using both Z and Prus heads. The only downside of removing the hex on the adaptors is you have to destroy the adaptor getting it out of the head. If the head was set up using 14 mm holes , then not an issue. Using the 1/2" hole offers options as to plug options. Any new heads will be 14 mm because they just don't foul. Both of the hi comp heads don't foul plugs if the ignition is set up correctly.

Hello all... I swapped out the Bosch WR70P, says 4020 on the box, and put Champion RN12YC in, hoping these will work. Or maybe it wasn't really necessary , any thoughts on it? Wingnut

Model T engines that are anywhere near stock and in fairly good condition and driven under anything approaching normal use are very tolerant of what spark plugs are used.

Well, here's my progress... I ordered 4 Autolite AP5405 plugs from Amazon. Good price, free shipping, bought & shipped by Amazon, (no shady 3rd party seller). So, what I received were 4 used, random brand plugs! Thank you, Amazon What a joke! They're being returned today. Going to buy them at a "brick & mortar" seller.

IMHO, platinum plugs are a waste of money for a T, unless you imagine you’re going to run them for 100,000 miles !
Any good quality copper plug is fine. Rockauto has them in Champion and other brands. A Champion 38 is a non resistor plug of what wingnut used. N12YC, $1.88 each, his is an RN12YC. He is also running a dizzy. I don’t think a resistor would make a big difference especially with a dizzy.