Spark Plug Gap - Why the Change?

BigSky

As I was reading some information the other day I noticed that the 1937 212ci engine calls for a spark plug gap of .025”.  However the 1938 212ci engine calls for a spark plug gap of .032”.   The greater gap would likely require a bit more voltage to jump the gap & could tax the coil but it can also help to more completely burn the mixture. 

I cannot spot anything that would be a reason for the increase in gap?  Perhaps the continual improvement in gas allowed for this change?  Maybe a coil improvement?  Perhaps it was merely that they discovered the benefit to the increased plug gap and the system could handle it in 1938?  

If anyone has knowledge as to why this change took place and if it can be applied to my 1937, please let me know.  

Geoff

Whatever works!

BigSky

Hahaha.... agreed!  

However, has no one noticed that change before or brought this question up before?  I know most like to follow the ”book” on things but it makes one wonder what was going on then for that change?  Likewise, considering the changes since 1938 in gasoline alone it makes one wonder what gap one should be using today. 

I ran across a Chevy forum yesterday and the guys with late 1920’s Chevy 4 & 6 cylinder engines.  Across the board they found impressive improvements in idle, performance & less fowling by doing two things to their spark plugs; 1) use a longer reach plug to unshroud the spark plug tip & 2) open up the plug gap to 0.040”.  Along with that they would make sure their coils & condenser were newer, like from a 1954 6 volt vehicle.  

Terraplane8 had been the first I had seen who pioneered the use of a longer reach plug on the forum back in 2010-2011.  I also stumbled upon the shrouded plug problem when I had my head off my 212ci engine a year ago.  Some may recall I ended up using the NGK B6HS 1/2” reach plug vs stock 3/8” reach plug like Terraplane8 had done.  In my case (& I believe Terraplane8’s as well) I was using the high compression head off a 175 engine so I couldn’t go with the Longer reach of the protruding tip NGK BP6HS plug.  Additionally, I further unshrouded the plug tip by doing a little cone cut where the plug enters the chamber.  

Hopefully someone has experimented with the plug gap as well & can chime in here with their experience.  I would play around testing on my 212 but yesterday she got stuck in 2nd gear (electric hand) & is now layed up until I can figure out how to fix that problem!  

Photos from my evaluation a round a year ago of plugs in the head for my 212 engine.  Below is the original 3/8” reach plug.  You can barely see the top of the black tips edge reflecting light at the bottom right of the hole.


Here is the protruding tip & 1/2” reach plug.  If I was using my stock 212ci head I would have used this plug as it’s placement is ideal & wouldn’t have interfered with the valves. 


Here is the plug I chose with the high compression head I ended up using.  I also did a little cone cut to unshroud the plug tip even further. 

Old Fogey UK

You can't use the longer reach plugs in the 34 and 35 engines as they will bash against the pistons at top stroke.

terraplane8

Old Fogey UK said:

You can't use the longer reach plugs in the 34 and 35 engines as they will bash against the pistons at top stroke.

But the plugs are not located above the pistons.

50C8DAN

Perhaps there was a coil, plug wires or points change that made a wider gap possible?

Geoff

I think Old Fogey meant the valves would hit the plugs.   This happens on Step-down heads if you shave too much off. 

terraplane8

Geoff said:

I think Old Fogey meant the valves would hit the plugs.   This happens on Step-down heads if you shave too much off. 

The chamber was pretty deep on those '34-'35 standard heads though.

Old Fogey UK

Geoff said:

I think Old Fogey meant the valves would hit the plugs.   This happens on Step-down heads if you shave too much off. 

Sorry. In the heat of the moment, I "misspoke ". 

BigSky

I think I would have blamed it on the 4 pints I’d already had!  Hahaha!

Old Fogey UK

BigSky said:

I think I would have blamed it on the 4 pints I’d already had!  Hahaha!

I'm afraid that excuse wasn't available to me at the time !