Finding TDC using a soda straw
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Road Warrior
Joined: Apr 2012
Posts: 1,755
Likes: 253
From: Northern Los Angeles area.
Finding TDC using a soda straw
Hi all,
I watched some youtube clips and a couple of them mentioned finding TDC using a drinking straw but they never really demonstrated it or explained it clearly.
So here is what I think:
I watched some youtube clips and a couple of them mentioned finding TDC using a drinking straw but they never really demonstrated it or explained it clearly.
So here is what I think:
- Strap bike down, level it, jack up rear wheel
- Remove spark plug
- Insert clean dry straw in spark plug hole
- Turn rear wheel slowly
- When straw is most exposed above hole, that's TDC for that piston
Road Master
Joined: Aug 2006
Posts: 1,148
Likes: 157
From: Banning, CA
The trouble is that it will be at TDC for some time - not just an up and down action.
Mark the straw about an inch before it reaches TDC, continue rotating on past TDC
until the straw goes back down to your mark on the straw. Half way between those
two points will be TDC.
Mark the straw about an inch before it reaches TDC, continue rotating on past TDC
until the straw goes back down to your mark on the straw. Half way between those
two points will be TDC.
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Road Warrior
Joined: Apr 2012
Posts: 1,755
Likes: 253
From: Northern Los Angeles area.
Ermmm... English please? Total newb here 
That one too, please explain? What will the rocker arms be doing, to indicate it's TDC?
Now that I understood! 
Do you mean one for each piston?
That one too, please explain? What will the rocker arms be doing, to indicate it's TDC?
Do you mean one for each piston?
Last edited by lewk; Jun 29, 2017 at 05:51 PM.
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Road Warrior
Joined: Apr 2012
Posts: 1,755
Likes: 253
From: Northern Los Angeles area.
Which rocker arms are the intake side? The ones closest to the air cleaner inlet?
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Road Master
Joined: Aug 2006
Posts: 1,148
Likes: 157
From: Banning, CA
Tdc ???
the engine until it falls back down to the mark the TDC measurement will be more accurate. This is because at half stroke rod angularity
is the highest along with associated piston speed per degree of crankshaft rotation.
On a four cycle/stroke engine TDC happens on the compression / firing power cycles and again on the exhaust / intake cycles. On V-twin
engines TDC is at an entirely different crankshaft rotation place for each cylinder.
Determining if you're on a compression or exhaust stroke can easily be done by just putting your finger over the spark plug hole while
rotating the engine - you'll only feel pressure on the power stroke.
Road Warrior
Joined: Jun 2012
Posts: 1,832
Likes: 67
From: Phoenix
So in a single given cylinder (our bikes have two), you have two upstrokes and two downstrokes. You want the piston to be at top dead center of the compression stroke (the first upstroke) because that means both valves are closed, so there's no more pressure on the valvetrain than there has to be. If the piston is at top dead center of the exhaust stroke (known as "overlap"), the exhaust valve still hasn't closed and the intake valve has already opened. There's a lot of pressure on the valvetrain, which can cause damage when you start unbolting things.
The way most of us know the four strokes is "suck, squeeze, bang, blow."