Flathead leaking spark plugs

[WPVT]

I recently replaced the thermostat on my 1954 C-1-B flathead 218. Afterwards, the neck gasket leaked coolant all over the engine, so I'll have to redo it with some silicone gasket.

Leaking coolant had collected in the spark lug wells, so I tried to get most of it out with paper towels. I didn't get all of it out because when I started the engine to verify the leak, I saw bubbles around all the plugs that had residual coolant surrounding them.

They're torqued right down, new AC plugs with, of course, new compression washers. I'd taken them out and replaced them a few times though, and I guess that was enough to render the washers useless. I'll get some new washers.

Makes me wonder how many flathead plugs would pass this "water test" ? Also makes me wonder what the effects of this small amount of leakage would be. I never saw any visual signs of leakage until I did the "water test". 

The plugs have really short threads, as does the head. Seems like maybe leakage might be more common than one would suppose.

Any thoughts or experiences ?

[JBNeal]

What you're seeing is a contributing factor to that engine's inefficiency:  what is the volume of charged air in the sealed combustion chamber vs the volume of charged air in the leaking combustion chamber...what is the hp generated in the sealed combustion chamber vs the hp generated in the leaking combustion chamber.  Ideally, the spark plug sealing washer is compressed once for maximum hp, but each time the sealing washer is deformed, hp production declines.  But the reduction in hp here is probably <5%, so it's not debilitating to the engine.  Flatheads aren't really known for high hp or efficiency anyway, so we can only hope to get close to maximizing hp during usage...it's part of the flathead fun factor

[kencombs]

The fix:  https://www.ebay.com/itm/363508639874?hash=item54a2cda882:g:6lgAAOSwV2JhGuaS

[WPVT]

25 minutes ago, kencombs said:

The fix:  https://www.ebay.com/itm/363508639874?hash=item54a2cda882:g:6lgAAOSwV2JhGuaS

Ken,

   Thanks. They're on their way. Copper's got to seal better than steel.

Edited August 22, 2021 by WPVT

[Plymouthy Adams]

I visualized a whole pile of spark plugs laying on the ground beneath the engine compartment........

[WPVT]

1 hour ago, JBNeal said:

What you're seeing is a contributing factor to that engine's inefficiency:  what is the volume of charged air in the sealed combustion chamber vs the volume of charged air in the leaking combustion chamber...what is the hp generated in the sealed combustion chamber vs the hp generated in the leaking combustion chamber.  Ideally, the spark plug sealing washer is compressed once for maximum hp, but each time the sealing washer is deformed, hp production declines.  But the reduction in hp here is probably <5%, so it's not debilitating to the engine.  Flatheads aren't really known for high hp or efficiency anyway, so we can only hope to get close to maximizing hp during usage...it's part of the flathead fun factor

For all I know, every engine I've ever owned may have leaked through the plugs. I've never checked using soapy water, the way you'd check for a gas leak. 

[Matt Wilson]

I suppose the next question coming to mind has to do with the opposite of air leaking out of the cylinder.  That is to say, if the seal at the spark plugs is compromised enough to let air escape, then is it enough to let water into the cylinders whenever water has puddled in the spark plug recesses?  Maybe not, as it seems like we would have heard about it being an issue and causing rusty cylinders by now.  But it's just a thought.... 

[Tooljunkie]

I gave mine a little extra snugging, leak stopped. 
 

my nissan i bought used had a bit of a rough idle, and could smell raw fuel on cold starts. 
decided to give it a tune up,all 4 plugs were loose. As in plug wells and coil boots were covered in unburned/evaporated gas. That was a new one to me. 

[bluebanshee]

I noticed the same thing with mine. I don't want to snug them too much with 64 year old threads. I used Teflon pipe dope to stop the leak.

[WPVT]

I bought some NOS copper crush washers sized for spark plugs. I'm confident they will seal better than the steel washers that came on the plugs. 

Eliminating that air leakage may not make a big difference, but it can't hurt.