Leak around spark plug

[Gordon]

After getting frustrated on my Atkinson Differential build I decided to set it aside for a while and get back to the Howell V2 I finished about a year ago. It ran but not well. Problem as usual is compression. I found some leaks around the valve seats and some head gasket problems where the gasket material hardened and leaked after getting hot. I put a Teflon head gasket in and that seems to have solved that at least for now.

Now the problem is leakage around the spark plug. I am using a CM6 plug in an aluminum head. As soon as pressure is applied to the plug it makes a lot of soap bubbles. I have tried refacing the contact surface and making sure that the threads are good. I have tried a couple of different plugs but still have leaks. I even tried using a copper washer under the plug. So far the only thing which gets the leak small enough to run is to put anti seize on the threads and really torquing it down. I have used similar setups on other engines and have not had this problem. If I torque it down much more I am afraid that I will strip the aluminum.

Anything I am missing? Anyone else had this problem?

Gordon

 

[Brian Rupnow]

I had a similar problem one time with a cylinder head on which I had cut cooling fins. I thought I had a flat bottomed counterbore in the area where the plug seated, but a close inspection showed that the flat bottom counterbore wasn't quite deep enough to get past the bottom of the fin grooves. The plug was tightening up okay, but there was enough space between the bottom of the fin slots and the bottom of the counterbore that the plugs leaked badly. Deepening the counterbore by about 0.020" solved the problem.---Brian

 

[Gordon]

I had a similar problem one time with a cylinder head on which I had cut cooling fins. I thought I had a flat bottomed counterbore in the area where the plug seated, but a close inspection showed that the flat bottom counterbore wasn't quite deep enough to get past the bottom of the fin grooves. The plug was tightening up okay, but there was enough space between the bottom of the fin slots and the bottom of the counterbore that the plugs leaked badly. Deepening the counterbore by about 0.020" solved the problem.---Brian

That is the same as my situation. I made a special counterbore to make the recess. I have re-counterbored it so that it is well below the depth of the fins. For some reason the problem seems to be worse at some times and then at others. It is really baffling me. I have never had a situation where I had to tighten the plug to this extent. The plug is at a 30° angle to the head but even if the spark plug threads are not exactly perpendicular to the surface It would seem that it would have to be quite a bit to cause the problem. The only solution which I can think of is to make a steel bushing, retaping the head to something like 1/2-20, epoxy the bushing in place and then installing the plug in the steel bushing. That would make the plug electrodes at least 3/8" above the combustion chamber which may or may not be a problem.

 

[Longboy]

Putting an O-ring up the threads of the CM-6 spark plug will give you the seal. You won't need a wrench to tighten them down anymore.

 

[cheepo45]

I'm thinking that the sealing surface is not perpendicular to the threads.
This will cause a lot of problems with leakage.
Scott

 

[GailInNM]

If using a copper washer it should be at least 1.5mm and preferably close to 2mm thick. The washer supplies with NGK CM-6 plugs measures about1.9mm thick when new. If the plug has been tightened up a few times on the washer then the washer should be removed and annealed by heating to read heat and allowing to cool. Copper work hardens quickly. If making your own washers be sure to anneal them before installing it. If the threads in the head are not quite square to the sealing surface then two stacked washers may cure the problem.
Gail in NM

 

[GailInNM]

Also check the threaded hole in the head for any possible burr caused by the tapping operation. It's a good idea to countersink the thread SLIGHTLY to remmove any possibility of a burr being formed when tightening the plug.
Gail in NM

 

[Gordon]

OK Thank you. The problem is probably the tapped hole not being perfectly perpendicular to the sealing surface. I am not sure of how to actually check that. Perhaps put the tap in the hole and check it with an indicator. I tried an o ring but I think that I probably tighten it down too tight and scored the o ring. I will continue to play around with it.

 

[mayhugh1]

See posts starting with #220:

https://www.homemodelenginemachinist.com/threads/another-radial-this-time-18-cylinders.21601/page-11

Terry

 

[Gordon]

Thanks Terry. Sounds like my problem. I will pursue that.

 

[Gordon]

I finally got back on this engine. We had an ice storm with power going out and two trees down in our yard. Real life keeps interrupting the important stuff.

I made some SS washers and put them in the spark plug holes. It seems to be sealing without even using JB Weld. I will try it for a while without the JB Weld.