14 mm Spark Plug Recommendations?

Rick_Benjamin · Post by **Rick_Benjamin** » Sun Jun 08, 2025 12:32 pm

Greetings Friends:

My 1923 touring car has a modern head with 14 mm spark plug holes. I'm having some ignition problems, and since I last changed the plugs 15+ years ago and the car has been driven a lot, I think it's time to for a new set of plugs. The car currently has Bosch FR9 HP (R3 488) Platinum 14 mm plugs in it. (These were highly recommended on this forum back then.)

My car ran well with these plugs for many years and I would just buy another set, but Bosch has apparently discontinued them.

So: Can someone please recommend a good, available 14 mm plug for the T? I've made several searches here and elsewhere in the net, but I haven't come across an answer.

BTW, I am otherwise running "traditionally": 6 volt and timer & coils. Also - the plugs have to be "stud" or "loose terminal" type because I'm using the T-era wire connectors.

Many thanks!

Sincerely,

Rick B.
Lewisburg, PA

Post by **speedytinc** » Sun Jun 08, 2025 12:37 pm

Autolite AP5405 platinum.
They are long reach & work great with spark plug adapters.

Post by **JTT3** » Sun Jun 08, 2025 1:13 pm

Try eBay, they come up often or try the Bosch #4003

ModelTWoods · Post by **ModelTWoods** » Sun Jun 08, 2025 1:46 pm

A long time club member from California, known for having a pretty good performing speedster, recommended Champion RN 10YC to RN 16YC plugs. He said use the coldest plug that won't foul. If you have a cylinder or cylinders that are letting oil past the rings, use the hottest plug. 10YC is the coldest plug; 16YC is the hottest.

Post by **Moxie26** » Sun Jun 08, 2025 2:00 pm

Hey Rick... Are you using adapters to use with the 14 mm plugs?..,.. or can you use the regular plugs ?

Post by **RajoRacer** » Sun Jun 08, 2025 4:16 pm

He stated above that it's a 14 mm head.

Post by **Moxie26** » Sun Jun 08, 2025 5:19 pm

Steve, I've had some friends that said all they use are 14 mm plugs and they neglect to mention that the adapters stay in the head. No intent on any negativity, I just wanted clarity.

Rick_Benjamin · Post by **Rick_Benjamin** » Sun Jun 08, 2025 8:31 pm

speedytinc wrote: ↑
Sun Jun 08, 2025 12:37 pm
Autolite AP5405 platinum.
They are long reach & work great with spark plug adapters.

Thank you!

Rick_Benjamin · Post by **Rick_Benjamin** » Sun Jun 08, 2025 8:40 pm

Thanks for all of your replies!

To clarify a little, I need 14mm plugs that have the old-style threaded stud or removable nut type terminal on top. I don't want to have to change all my spark plug wires too.

Oddly enough, plug manufacturers (and retailers) don't seem to put this info into product descriptions.

Thanks again,

RB

Rick_Benjamin · Post by **Rick_Benjamin** » Sun Jun 08, 2025 8:48 pm

Moxie26 wrote: ↑
Sun Jun 08, 2025 2:00 pm
Hey Rick... Are you using adapters to use with the 14 mm plugs?..,.. or can you use the regular plugs ?

Hi Bob:

It's great to hear from you. I'm using an aftermarket 14 mm head.

I do have some standard Ford heads on hand, but I'm trying to avoid "mission creep." I just want to get my car back to running as well as it had been.

On that note: are you still doing coil rebuilding/tuning work? It's probably time for a refresh of mine sometime soon.

Thanks!

Sincerely,

R

Post by **speedytinc** » Sun Jun 08, 2025 9:07 pm

Those Autolites do not have a threaded top. I run them on distributor motors.

Norman Kling · Post by **Norman Kling** » Sun Jun 08, 2025 10:34 pm

Since the head is not standard,It would not be judged as "original" so it really doesn't make a lot of difference if you have the thumb nuts. You can put the spark pug wires from the coils directly on the top of the spark plugs and screw on those connectors. It will hold the wires just fine. I have that kind of connectors on one or more of my Model T's.
Norm

Post by **Craig Leach** » Mon Jun 09, 2025 1:07 am

Hi Rick,
All my engines run 14mm plugs. I don't know what reach you need, I use Autolite 66 plugs they are a 1" reach ( that is plug seat to electrode end)
& they have a removable end. I run distributors so I have no experience with them using T coils but I also used them in KT-100 2 stroke kart race
engines turning over 13,000 rpm so I think they will handle the multi spark OK. I make my own adapters out of the old Autolite & Champion
crimped plugs no one likes so they look like 1/2" pipe spark plugs.

Craig.

Rick_Benjamin · Post by **Rick_Benjamin** » Mon Jun 09, 2025 8:59 am

Norman Kling wrote: ↑
Sun Jun 08, 2025 10:34 pm
Since the head is not standard,It would not be judged as "original" so it really doesn't make a lot of difference if you have the thumb nuts. You can put the spark pug wires from the coils directly on the top of the spark plugs and screw on those connectors. It will hold the wires just fine. I have that kind of connectors on one or more of my Model T's.
Norm

Thanks Norm:

I've enjoyed your posts for years and appreciate your advice.

You're right, I don't show my T (I do enjoy looking at other's cars at shows). However, I'm trying to keep mine as "period" as possible, within reason. So I'd prefer to use plugs with the old-style screw-on top terminals, if possible.

Also - I don't quite understand your post about the modern connectors. Is there another bit of hardware that I can attach to the ends of my old-style spark plug wires that will slide over/lock on the terminals of modern solid-top plugs? (Please excuse my general automotive ignornance. This T is the only automobile I've ever worked on, although I've been at it since 2001!)

Again, thanks.

Sincerely,

Rick

Norman Kling · Post by **Norman Kling** » Mon Jun 09, 2025 10:21 am

If you have the original wires, they will fit on the plug as shown in the picture. Then screw on those snap on connectors and they will hold the wires. If the thread is the same you could use the original type nuts. You would need to try them to see if the thread fits.
Norm

Post by **RajoRacer** » Mon Jun 09, 2025 11:23 am

Rick - go over to Restoration Supply and get yourself a set of RAJAH terminals.

Post by **Mark Gregush** » Tue Jun 10, 2025 12:13 pm

If you are running coils try and stay away from plugs that have an "R" (resister) in the number. These plugs do not seem to like the wild fluctuation of voltage esp if running off magneto.

Post by **Moxie26** » Wed Jun 11, 2025 3:43 pm

Rick.... Lang's has 14 mm spark plugs that they use with adapters for the standard Ford head. Being that you have is cylinder head with 14 mm, I would call them and ask if there would be any clearance problems if you use the 14 mm plugs in the brand of cylinder head you have on the car. I believe using those plugs with the adapters for the 1/2-in pipe thread standard would not put the spark plug as deep into the combustion chamber area..... Worth the phone call to Lang's or any other supplier you choose.

Post by **Bryce S.** » Wed Jun 11, 2025 5:24 pm

The speedster that I have takes the same exact Bosch 14mm plugs that you use. If you go on ebay and look there are normally deals for a box of 10 for usually $19.99. I consider that a screaming deal. If you don't see any send me a PM and I'd be glad to get ya some of my stock. Anything to help in this hobby.

-Bryce

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14 mm Spark Plug Recommendations?

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