# Piston not visible though plug hole



## Paul40 (May 29, 2011)

:tractorsmFinally got around to taking a look at a Gilson I picked up a while ago. Took out the plug to do a compression test, turned the engine by hand and noticed that I couldn't see the top of the piston through the plug hole. Took a closer look and noticed that what I do see inside is not the piston. Whatever it is, it doesn't move. 

I've had the enging running, and even though it took some coaxing to get it going, it did run.

The engine is a B&S 8 HP vertical shaft, Model 190707, Type 2134 02

Anyone have any idea what's with this engine? At this stage I don't want to take the head off until I have a new head gasket on hand. How do you find TDC?

Paul


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## Country Boy (Mar 18, 2010)

The spark plug hole on that head is probably located closer to the valves rather than over the piston. If you can look in at an angle you will probably see the piston moving up and down and the other way should be the valves opening and closing. What you are seeing is probably the top of the block between the cylinder and the valve train.


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## Paul40 (May 29, 2011)

Country Boy

I took another look, this time with the help of a flashlight, and it turns out you're right! I could see a valve move, and about 1/16" of the edge of the piston was juuuust visisble.

Maybe there is a reason for that design that might be obvious if I remove the head, but I would've thought that it would be more efficient to fire the spark directly into the main combustion area. 

Paul


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## Argee (Sep 17, 2003)

It really doesn't matter where the plug location is....its function is to supply a spark to ignite the highly flammable compressed gas.


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## Country Boy (Mar 18, 2010)

The plug can be anywhere for it to ignite the fuel, but there is a science for plug angle and location to get the most optimal burn pattern and flame dispersal. I wouldn't even pretend to know how they come to that conclusion, but I do know they are located where they are for a reason. I have noticed that the area directly under the plug stays a bit cleaner as far as carbon build up in an engine, so maybe they were trying to keep the valves cleaner or something.


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## phogbound (Jan 16, 2012)

The head is flat, or nearly flat, over the piston on an L-head engine. There is no room for the spark plug. The area over the valves is the central area of the air/fuel mixture when the piston is at TDC. That gives the most efficient combustion.


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