i hear alot of guys running the f plug but isnt the plug longer than the number 8 plug will it hit the piston

The "F" plug is for 4-stroke engines. It is longer than the 2-stroke plugs and is a hotter plug. Some people will use the longer plug in a 2-stroke engine for better performance with low nitro fuel, but I can't see the advantage. Just use a hotter 2-stroke plug. And yes, it may hit the piston.

You can check by pulling the head off and installing the plug. You will see if it protrudes beyond the head. I have one in an OS 55ax and clears fine.

Hi,

The main advantage with a four stroke plug is it provides a more reliable idle, (like when you are landing) in inverted two stroke motors. With motors mounted vertically or on their side there isn't any real advantage. A four stroke plug is designed and manufactured to keep glowing between ignition strokes which in a four stroke, (if my maths is correct) are exactly half those of a two stroke.

Cheers,

Colin

"F" plug works great in my .55AX...... stopped the unexpected flameouts that mine and many others have experienced. Do a search and you will find a lot on this.

I did some research and used my calculator... twice.. you are correct.

Thanks Mate,

It doesn't happen very often. Just ask my wife.

Cheers,

Colin

It's a mystery why OS plugs like the #8 have such short threads. There are ample threads in the head for a true long reach plug. In theory a hot plug like the F will advance the combustion timing slightly and/or require a slightly richer needle setting to offset that. If you are having flameouts though due to the plug getting wet with an inverted mounting position, the F plug makes more sense than using an idle bar plug which will cost you a few hundred rpm on the top end.

Ernie