Combustion chamber sparkplug length question?

[AzBob]

I received the NOS aftermarket cast iron heads to replace the corroded aluminum heads on my '34. The 18MM Autolite 386 sparkplugs come up approx. 3 threads short compared to the aluminum heads. Is this something to be concerned about and should I run a longer reach sparkplug?

 

[19Fordy]

Here's info. that will help. Of course, you don't want spark plug to hit piston.

how long should the reach of a flathead ford spark plug be - Infospace Yahoo Search Results

search.yahoo.com

and https://www.jalopyjournal.com/forum/threads/ford-flathead-spark-plug-issues.1268926/

 

[AzBob]

19Fordy thanks for the valuable info. Most of those recommendations point to a 14mm 3/4" spark plug thread as being ideal. Not as many choices for 18MM plugs.

My existing 18mm Autolite 386's thread length are 1/2" long. Certainly don't want one long enough to interfere with pistons, valves or threads sticking out to collect carbon or create hot spots.

 

[hueyhoolihan]

i ran short plugs in an engine designed for longer ones for six months once. and found no discernable difference, not that i recommend it. the only downside was becoming a pariah on a forum focusing on those cars. .

 

[Ronnieroadster]

Bob You will be fine with the slight amount the plug is in the hole. Overall there would be no noticeable engine performance issues.
Ronnieroadster