Spark Plug Leak

Fri Nov 19, 2004 9:28 am

Anyone have a quick solution for a leaky thread on spark plug #3 for my Cub? I used teflon pipe dope (yeah I know probably won't withstand the temp) which seemed to help control some of the blow through but I was wondering whether there was something industrial out there that is typically used?

Fri Nov 19, 2004 9:31 am

A new gasket?

Fri Nov 19, 2004 9:31 am

Maybe a copper washer would work ? Untill you can get the threads repaired...........

Re: Spark Plug Leak

Fri Nov 19, 2004 9:56 am

ScottyG wrote:Anyone have a quick solution for a leaky thread on spark plug #3 for my Cub? I used teflon pipe dope (yeah I know probably won't withstand the temp) which seemed to help control some of the blow through but I was wondering whether there was something industrial out there that is typically used?


Scotty,

Don't worry about the teflon holding up to the heat. A fellow model builder makes his own spark plugs. When I expressed surprise that he could make the insulator he said he uses teflon. When I questioned its' ability to withstand the temperature he said the plugs don't get hot enough to keep from fouling. The oil will burn off a plug a couple hundred degrees below the limit for teflon.

I agree that cleanliness and a good gasket should stop the leak, however.

Fri Nov 19, 2004 11:31 am

These are all great suggestions but let's talk about repairing the threads for a second. In order for me to do this, I'd think that I'd need to retap the head rather than to chase the existing thread. Now we're into a whole different world of which I know nothing about whatsoever. Size of the new plug vs. required plug rating, etc since the threaded end of the new plug would be physically bigger.

Fri Nov 19, 2004 11:38 am

I believe Heli-coil has a thread repair kit for sparkplugs.

Spark plug hole repair

Fri Nov 19, 2004 11:53 am

It's been quite some time. I purchased and used a repair kit for spark plug holes from the local auto parts store. It was designed for and used on an aluminum engine. Basically. It was a self tapping bolt to cut new threads and an insert installed with a socket.

Fri Nov 19, 2004 12:22 pm

How did we jump from a leaky thread that wouldn't hold pressure to one that wouldn't hold the spark plug in the hole? What is wrong with the threads in the head?

Get more specific and maybe we can give better suggestions.

Fri Nov 19, 2004 2:01 pm

Scotty, are you sure there isn't some dirt on the surface where th plug seats. The plugs on a cub have a metal gasket that seals them against a flat spot on the head, they don't use a pipe thread seal like Ford cars do. Also are you sure the leak is coming from where the plug screws in, and not between the metal and porcelain of the plug? I've seen that happen more than once.

Fri Nov 19, 2004 3:11 pm

Well I suppose it could be leaking between the plug gasket and the porcelin but this particular plug I've consistently had to play with in order for it to thread in correctly which usually means that someone cross threaded at one time or another. Basically, I just assumed that it was the threads in the cast causing the problem. When I refurbed the tractor, I was careful to insure that all dirt and paint were removed from the metal gasket in the head. I didn't know that the Cubs don't rely on pipe thread to seal so I've learned something there.

Once the plug is in, it does seat firmly. It's just that I've noticed some blow through next to the plug by a visible sooty mark nearby that develops right away. This will, as Jim pointed out, reduce pressure so I want to stop the problem. If I can replace the gasket under the plug, that's what I'd opt for. Later in the thread, I was just curious as to what people normally do to retap plug threads.

I think I'll try using a copper washer and see if that does the trick. Any other suggestions would be greatly appreciated.

Fri Nov 19, 2004 4:50 pm

Scotty. Spark plugs do not rely on the threads alone to seal. They usually come with a gasket (looks like a double washer) that seals the pressure. Some without washers have a tapor just above the threads to seal (like your valves). I never heard of a spark plug with NPT threads. Cub plugs are 18mm thread size. It is possible some one tried to clean out the threads with a standard tap or straight pipe threads. Regardless, you should check the flat surface where your plugs go in to see if it there is gouges or something else causing pressure to blow out. That is why both Jim & I suggested a gasket (new gasket or copper washer) and as GW suggests (good to see his posts again!!! ) make sure your clean around the plug area. Hope this helps..........

Sat Nov 20, 2004 10:39 am

Ron L wrote: I never heard of a spark plug with NPT threads. .
Thanks for catching my error. I intended to type that the tapered ones seal like pipe threads, but my typing and thinking sometimes don't follow each other. Guess I'm going to have to start proofreading closer.

Sat Nov 20, 2004 11:17 am

APO had installed tapered plugs in my Low Boy. The blow by had formed carbon in the threads. I had to use a 3 foot pipe on a 1/2 in breaker bar to get them out. I ground flutes similar to a tap in an old plug and used that to chase the threads. I grould away some of the metal on the plug as well so that I could clean deep in to the hole. That was the sunday evening shade tree mech idea that fixed my problem. With the added compression I thou ght that I had an engine overhaul.
Bill

Sat Nov 20, 2004 11:31 am

Ron L wrote:Scotty. Spark plugs do not rely on the threads alone to seal. They usually come with a gasket (looks like a double washer) that seals the pressure. Some without washers have a tapor just above the threads to seal (like your valves). I never heard of a spark plug with NPT threads. Cub plugs are 18mm thread size. It is possible some one tried to clean out the threads with a standard tap or straight pipe threads. Regardless, you should check the flat surface where your plugs go in to see if it there is gouges or something else causing pressure to blow out. That is why both Jim & I suggested a gasket (new gasket or copper washer) and as GW suggests (good to see his posts again!!! ) make sure your clean around the plug area. Hope this helps..........


Since the Cub plugs are 18MM straight thread this doesn't really apply, but there were a lot of plugs made with tapered pipe threads. The most notable I can think of was the model T Ford. Some early tractors and hit and miss engines also used them. I'm not certain, but I think our John Deere GP that I haven't seen since 1947 had them as well.

Sat Nov 20, 2004 11:53 am

Thanks for the insight, George. When I said "I never heard of NPT threaded spark plugs", I should have added "with my limited knowledge of the subject"! John, it was in reference to Scotty's post. NPT threads would surely hold plenty enough pressure, but gosh, what a bugger to get them out if left in for years like some cubs!! Probably why modern plugs go strictly with gaskets (as far as I know, with my limited knowledge!). This is just the MACHINIST in me talking...............