Small Engine Repair: How to Determine Piston Position and Stroke on a 4 Stroke OHV Industrial Engine



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**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**

The 4 stokes of a 4 stroke or cycle engine are:

1) Intake: During the intake stroke the piston moves from the very top of its stroke (TDC-Top Dead Center) to the very bottom of its stroke (BDC-Bottom Dead Center), and the intake valve opens at the start of this stroke letting in a fresh air and fuel mixture, and closes near the bottom of this stroke.

2) Compression: During the compression stroke the piston moves from BDC to TDC compressing the fresh air and fuel mixture.

3) Power: The air and fuel mixture is ignited at the start of the power stroke. The heat from the air and fuel burning forces the piston to move from TDC to the BDC.

4) Exhaust: During the exhaust stroke the piston moves from BDC to TDC, and the exhaust valve opens at the start of this stroke to let out the burnt air and fuel mixture from the power stroke, and closes at the end of this stroke.

After the 4 strokes or cycles the process repeats in this order. If you know the 4 cycles of a 4 stroke engine you can determine what stroke the piston is in and roughly know where the piston is by viewing the intake and exhaust valves.

Thank you for Watching!

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33 thoughts on “Small Engine Repair: How to Determine Piston Position and Stroke on a 4 Stroke OHV Industrial Engine

  1. Really don't under stand why a screw driver is almost always shown to check for TDC , I have use a plastic straw for years guaranteed your not going to damage any thing in the combustion chamber.

  2. Very helpful and very explaining! tnx! Another question would be….i have problems starting a b&s motor, it gets so hard to pull. But when i remove the spark it just goes normal, i bet this is because of bad valve setup, what would you say? I did disasamble the engine and there arent any problems. 

  3. Need some help I opened my valve cover off and one of my piston rods were off, can I just put it back in and restart? ( B&S Generator )

  4. So would this be the same in an atv 4 stroke engine? i am new to all of this, i took a class on it in school but didn't get that hands on, so i am taking an old four wheeler of mine apart this winter to get it ready for spring, would these valves be somewhere and i could judge what stroke the engine is in the same way as in the video? So when i am done cleaning everything and fixed and put it all back together, i make sure the piston is at TDC?

  5. I have to tell you, this is best, the very best, explanation I've seen in this subject. I'm new to this kind of stuff and up to now, this was totally confusing to me. Not any more, thanks so much. Subscribing to your channel.

  6. Best explanation on the net. The valve clearance specs on my troy built tiller do not specify what stroke engine is on when piston is at TDC. Does it matter?

  7. Great Job! This was very helpful. Another way to confirm that you have TDC is to rotate the crankshaft slightly back and forth – there should be no rocker arm/push rod movement.

  8. Ok i understand about T.D.C. what a the measurement of the valves of a 6.5hp lawnmower what feeler gauge settings do i use to set the valves clearance also do i just turn the flywheel counterclockwise one time and set the valves clearance are do i turn again to set each valves clearance

  9. Question-I understand how a 4-stroke engine works. I also have the same engine in this video. Why after the air filter is there 2 places for air to intake. One that goes directly to the OHV and one that mixes with the fuel through the carborator? Do they both go to the same place? Is this to get more air in the cylinder? thanks

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