Hey I have a 2000 f150 with the 4.2l v6. Im trying to do a compression test on it and I cant get the compression test hose to thread into the 14mm spark plug holes. I can thread both the plug and the hose in a common non fouler, demonstrating they are both the same thread. I can start the spark plug easily in the engine block, but I cannot thread the tester hose. Whats up with that? How can they both thread into the same non fouler but only the plug will thread into the engine block?

 

Why don't you just thread non Fouler in the head then and then screw it into that....

Sometimes you just have to have things aligned perfectly and that's hard to do with the end of a hose down a hole... It always wants to curve.... Particularly if there's a little bit of thread damage where it doesn't want to catch and straighten itself out.

I used to have a oil drain plug on a vehicle I had to screw with for about 10 minutes to finally get it to go back in. Nothing wrong with the plug but a broken thread at the opening of the hole just made it not want to run unless it was just perfect. Sometimes chasing the hole with a tap can straighten out the entrance threads and make it easier

 

Sometimes you can bend the hose straight and it will stay that way just long enough to get it started.

If you do use an adapter or a non-fouler thingy, remember that they add volume to the effective combustion chamber. So the pressure numbers will be lower because you're compressing the cylinder volume to a larger volume. Basically lowering the compression ratio. Pressure checks are really only good for comparing cylinders, not absolute numbers.

 

So, I'll admit... I'm not familiar with the 4.2's setup. I have this for modular motors. I'm not sure if this will solve your problem or not... It works for 14mm or 18mm threads.