Spark Plug Length

[Coadster32 793]

So I'm doing a "once over" on a tractor that I just recently got, 855. The PO had paid someone to go thru this tractor, and the more I do, the less I see he did, (ie: oil change, transaxle oil change, air filter, electrical connections broke, etc.) I had bought all the parts I needed from Pats (PSEP) a few weeks ago, and am now getting to doing the work. Normally never a problem getting the right stuff from there. I knew the carb needed a GOOD cleaning, as did the combustion chamber. Figure I'd do the carb and run seafoam thru once back together, would be my first try at using it. 

 

Anyhow, I pulled the shortie plug out, and realized the new plug was longer. Obviously one is wrong. Looks like the new plug would bottom out on the top of the block if put in all the way. It wouldn't screw in all the way anyhow, as the last few threads were messed up in the head. (I'm figuring due to possibly arking). Anyhow, decided to pull the head off, (good thing as tons of grass and crap would have overheated me eventually). I noticed the shortie plug didn't  protrude into the chamber, as the head is thicker than the plug. I never had really thought of this before, as usually when I take the head off, the spark plug is out of the head already. I repaired the threads, tested my theory (plug bottoms out before head touches block), and proceeded to think.    

 

 

Why could/should the spark be in the threaded hole of the head, instead of the chamber itself? Perhaps extra plug fouling of the exhaust side? I think not as in the new head designs the plug was moved closer to/over the exhaust opening. Perhaps the gas from the intake soaking the plug? Maybe. What I did was make a spacer to be able to use the new plugs I had just purchase. It puts the electrode just into the chamber.

 

 

It's an easy change if it doesn't work, but just wanted to throw this out there for some technical discussion. (Sorry the pictures kind of stink, and my camera battery crapped out.)

[shorts 182]

Take your engine make, model and serial number to the auto parts and help the counter person find the right factory recommended spark plug for your engine, 3 or 5 dollars is a small price to pay to not do something that may damage your engine seriously.  Just about any sparkplug that fits the hole can be made to work but their is a specific sparkplug that is right for your engine.

[Coadster32 793]

Thanks Shorts...certainly good advice.  I certainly agree that a couple bucks isn't worth ruining something. (honestly don't see that happening in this case though).

 

I do know that the shortie plug is the correct one,  (RJ8C,  in accordance to the Kohler manual), but just wanted to further discuss the theory behind it.

Edited December 11, 2013 by Coadster32

[Fordiesel69 271]

If you have a 855 model with a kohler 8HP K181, the correct plug would be a J8C or RJ8C.  Head castings will vary but plug will be the same.   

 

You really need to watch on the newer K series and Magnums, if you put too long of plug in, you will hit the ex valve and crack the cam.  You will see they moved the spark plug hole directly above the valve for emmisions and better resistance to fouling.

Edited December 11, 2013 by Fordiesel69

[Coadster32 793]

Kind of surprised nobody's really chiming in on this...especially the puller gents.  Anyhow, I have three hours on this 855 tractor with the longer plug. It starts up pretty easy, and runs really good. I pulled the plug out of my 854, and of course it was a shorty..(rj8c). Both tractors are running well, so I'm just going to keep things the way they are. Still wondering though.

 

 

 

The black lines represent the head width. The bottom plug is the stock plug recommended by Kohler. The depth in the head of this plug is shown in the first blurred picture in this thread. The top plug is the longer reach plug, with a washer I made up added to it. I switch the crush washer position with the washer I made so the crush washer goes against the head...(of course). Only the electrode sticks out of the head into the chamber. I know three hours isn't a lot of time, but thought it would have fouled out by now if this was the case...especially since I have been snow throwing with it. I had it idling for some time as well, just to see.

Edited December 22, 2013 by Coadster32

[specialwheelhorse 174]

Coadster I done the same thing with some Bosch Platinum 2s

I inherited for some reason, made some rings for them much

like you did just to see if they would work in my 857 mow

tractor, Started much better and had 1 in tat tractor for quit

some time and it worked just great,no fouling and after many

hours the plug looked much like the day I put it in. Now I don't

know if this was due to the plug in the chamber a little

farther or the bosch plug. Just don't know !!

The only reason I don't have it in the tractor now is because

It gave me a fit not running right later on and to eliminate all

possibilitys I went back to original plug but found my problem was

fuel related and just never changed back. But I would run these

plugs again any day.

[Coadster32 793]

Thanks for the reply. I figure someone has to have done it a little off the beaten path as well. Interesting to hear it ran great for so long. Gives me a better feeling already.