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AC 26 spark plug


New member
The morons at AC-Delco could not/would not identify their own old product :mad: so I turn to you guys. This plug was in a FM 1.75hp flywheel throttler. Is there a 'today' exact equivalent in Champion, Motorcraft or . . . AC-Delco? :shrug:

Also, does anyone know what thread this plug (or the FM head) has?


(I have every thought to buy an NGK and put it in all my FM's just to show up the Detroit mindset of "we don't have access to those records any longer. :D)



New member
When threading a 3095 into the head, with the head removed, the plug end does not reach all the way through the hole. Instead, its electrode is still up in the hole's threaded portion - which isn't correct or healthy. Is there a plug that has a longer reach to reach to the combustion chamber?

Ed Radtke

Active member
Is there a plug that has a longer reach to reach to the combustion chamber?[/QUOTE]

champion A25 is the more modern new plug.old champion X is the best or better old plug


I have found the old take apart plugs work the best. Whenever I find them cheap at auctions, swap meets etc. I try to buy them. I take them apart and clean them. They always seem to work and are period correct. Also I would not worry about the reach of the plug. In the engine you are running it does not matter.


New member
Hello: Been there done that. Sometimes hard to find intelligent life behind a parts counter. Your Fairbanks Morse used a 1/2" pipe thread plug. Back in the day that would probably have been a Champion 34 or 33 Gas Engine special two piece take apart plug. The Model T Ford used a Champion X that was a 1/2" pipe thread plug. Our Fairbanks Morse Z engines seem to run just fine on just about any 1/2" pipe thread plug. If your AC plug is in one piece clean it and use it. Find someone with a spark plug servicer like the aircraft shops use or the old time ones used in gas stations and give it a light brief bead blast.Cean out any remaininf abrasive with brake cleaner. One can also use a modern Autolite 3095. Contact us at oldcroak.com if any questions.

Andrew Mackey

A lot of the pipe thread plugs fell short of the combustion chamber. there was a reason for that! A lot of the old engines would oil foul the plugs, due to the nature of the engine design. A plug that did not extend into the combustion chamber tended to not foul, as the plug was somewhat sheilded from direct contact with the oil. Todays new plugs have a different problem. To deal with todays alcholized gas, the insulaters are not glazed on the inside. This can lead to fouling in a cool running engine. The older glazed plugs will clean up quickly with a bit of carb cleaner.