spark plug

2manyprojects2manyprojects Posts: 803Expert Adviser
i have a 1938  eight that has  J12YC plugs my friend has a 1937 eight that has J8C PLUGS. WHAT IS THE DIFFERENCE IN THESE 2 PLUGS.  WOULD EITHER ONE BE FINE ? 


  • Ric West INRic West IN Posts: 518Senior Contributor
    Go with the J8C
  • BigSkyBigSky Posts: 1,091Senior Contributor
    Hudson buried the spark plugs in the hole & thus one doesn’t get good flame travel.  I’d look at the possibility of doing a deeper plug.  I haven’t worked on my 1937 8’s plug shrouding but most likely it’s just as terrible as the 1937 6’s.  Search my former threads & you will see a lot of photos of the problem & what I did to resolve it.   Don’t assume the Hudson engineers did the best thing possible for the engines because this is a prime example of missing the mark!
  • BigSkyBigSky Posts: 1,091Senior Contributor
    I took a look at my 49 Str8 engine since the head is off of it.  Plug a plug in it then took a couple pictures.  At least on this 1949 stock cast iron head you can see where the tip is in relationship to the chamber surface.  As well as how deeply recessed the plug is, just look at the threads showing in the photo, not good as it can lead to a hot spot & detonation. 

    If you could get a decent measurement of how much room there is between the exhaust valve & the end of the plug, you could then see if a deeper reaching plug is possible.  Like 1/2” reach vs 3/8” reach as I did on my 37’s 6 cylinder engine.  However my head was off at the time so I could get exact measurements on everything.  

  • terraplane8terraplane8 Posts: 604Senior Contributor
    I'd like to know what the actual reason was for Hudson to recess the short 3/8" plug so much, as it doesn't make sense on any level unless to keep the plug tip away from the exhaust valve due to overheating problems, or to reduce compression? Like you I had a good look at this and installed 1/2" reach plugs on my '36 212 with no issues.
  • bob wardbob ward Posts: 1,371Senior Contributor
    Put plasticine on top of the valve, put the head and gasket back on, rotate the engine, measure the dent in the plasticine.
    All Champion J series plugs have a 10mm (3/8") long thread, that's what the J stands for. Perhaps the next available thread length (1/2"? don't know, haven't looked) will fit the bill.
    Lockyer Valley, Queensland
  • GeoffGeoff Posts: 4,714Senior Contributor
    I would be very interested in what actual difference (if any) there is in using 1/2" reach plugs.  In theory, perhaps yes a little improvement, but in practice I'm willing to wager that you will notice no difference.  You are talking about 1/8" , so piddling an amount in such a small area. 
    If you're stuck in a hole, stop digging.
  • 7XPacemaker7XPacemaker Posts: 505Senior Contributor
    I agree with you Geoff to a certain extent. I don't believe that there would be much (if any) performance gain by extending the plugs. What I believe that you would see is better flame propagation in the combustion chamber, resulting in less carbon and a more complete burn. The amount of carbon that can build up in the spark plug area could be enough deposit to promote detonation, which is never good.
  • BigSkyBigSky Posts: 1,091Senior Contributor
    When I pulled my original head off a year ago I didn’t even know there were threads in the plug hole until I cleaned it all up & saw there was indeed threads under all the carbon.   So less carbon is good.  I doubt it’s gonna make 10 or 20hp but if it helps flame propagation (which is an issue with the Flathead design) & gives a little bump in compression it’s definitely an improvement.  

    If one looks at modern heads, manufacturers go to great pains to place the spark plug in the best location & to keep it from having any shrouding.  Shrouding of plugs just like valves isn’t a good thing so maybe try some playdoh in a ball at the end of the plug. It would likely come back out if wrapped around the plug tip then one can measure from a point on the plug to see how much space without removing the head. 

    IMHO; anything we can do to help improve our old Huddy’s is great for us & keeping our cars still ripping down the road!
  • HansHans Posts: 279Senior Contributor
    Just a thought:   if excess plug threads are removed it would not affect CR much but would reduce the tooth that carbon could cling to.

  • terraplane8terraplane8 Posts: 604Senior Contributor
    An extra step for detail freaks is to index the plugs by using different thickness washers, so that the open end of the earth electrode faces the chamber.
  • BigSkyBigSky Posts: 1,091Senior Contributor
    I am a detail freak but haven’t done that yet....

    I saw these new Edelbrock Flathead Ford heads & they have a pathway for the plug to be less shrouded & points toward the cylinder.  I’d say unshrouding the plug has benefits if they have gone to the trouble of casting that into their heads.

  • Smonica46Smonica46 Posts: 22Member
    Big Sky: Which Spark plug and depth into cyl and plug index made the best power for you on That 212 6cyl ? I really enjoyed all your Posts on this subject ! 
  • Old Fogey UKOld Fogey UK Posts: 1,036Expert Adviser
    I wondered why the electrodes on my longer reach plugs were always closing up until I discovered the head had been skimmed and the electrodes were being bashed by the valves. I swapped to short reach plugs and that solved the problem.
    It might be worth checking whether the thickness of the head has been reduced before deciding on which plugs to use.
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