TDC "D" and "S" question

[redrider90]

I've adjusted my valves many a time on my Mille. I always used the method to find TDC on the compression stroke to adjust to the valves.
In the books TDC is described as a timing mark D for the right side and S for the left side. So are D and S on TDC on the compression stroke or is it a timing only mark not to be used for valve adjustments? I have 2 manuals including Guzziology neither of which makes it clear if you can use D and S for both valve and timing.

[LowRyter]

if you can't get the feeler gauge in, you need to rotate another 360 revolution until the D/S comes up again to compression stroke.  Then adjust valves.

[redrider90]

if you can't get the feeler gauge in, you need to rotate another 360 revolution until the D/S comes up again to compression stroke.  Then adjust valves.

That's only 1/2 the answer. So you are saying that when D or S appears it is on compression every 720 degrees.

[nc43bsa]

D stands for "destra", "right" in Italian; S stands for "sinistra", "left" in Italian

When D is at the pointer mark, the right piston is at TDC; when S is at the mark, the left piston is at TDC.  Whether the piston in question is on compression or not must be determined by some other method.

[John Ulrich]

For laypeople:
D = driveshaft
S = side stand

[Wayne Orwig]

D = Dis side
S = Snot dis side

[ITSec]

From my years of Latin (which of course I have used extensively since leaving school!)...

D = Dextra (right)
S = Sinistra (left)

You'd also have seen this if you studied heraldry, and you'd see signs of it in most of the Latin languages (though most of them seem to have replaced the term for left with other words).

[sib]

From my years of Latin (which of course I have used extensively since leaving school!)...

D = Dextra (right)
S = Sinistra (left)

You'd also have seen this if you studied heraldry, and you'd see signs of it in most of the Latin languages (though most of them seem to have replaced the term for left with other words).

If you're left handed, as I am, you are sinister.  If you're right handed, you are dextrous.  Simple as that.

[Kev m]

Maybe everyone discussing this is perfectly aware of this, but I don't want to leave it unsaid.

D and S are timing marks that, when present, represent a flywheel/crankshaft position - i.e. that the PISTON on the respective side is at or near the top of its travel (whatever is specified for the model in question).

HOWEVER, that DOES NOT MEAN when you see them that the flywheel/crankshaft is necessarily on the COMPRESSION STROKE.

Half the time you see those timing marks the piston is at the top of the EXHAUST STROKE.

You need to watch the valves to determine which one it is.

Ok, I feel better now in case someone reading this didn't know that. Carry on.

[redrider90]

Maybe everyone discussing this is perfectly aware of this, but I don't want to leave it unsaid.

D and S are timing marks that represent a flywheel/crankshaft position - i.e. that the PISTON on the respective side is at or near the top of its travel (whatever the specified ignition timing is before TDC for the model in question).

HOWEVER, that DOES NOT MEAN when you see them that the flywheel/crankshaft is necessarily on the COMPRESSION STROKE.

Half the time you see those timing marks the piston is at the top of the EXHAUST STROKE.

You need to watch the valves to determine which one it is.

Ok, I feel better now in case someone reading this didn't know that. Carry on.

Kev,
Thanks
It needed to be said and that is the correct answer it. No one before you posted it made it clear the difference between to 2 different TDCs.
They were too busy trying to impress with their knowledge of Latin. 
 By the way for you Latin experts I found in Google under translate "right in Latin" quite a few words translate right into and including "rectum". 

[Sheepdog]

"D" and "S"...sorta like a secret handshake.

[Two Checks]

S=Sa lefta side.

D=Datsa da right side.

[handyandy]

Thumb over sparkplug hole tells if your on compression stroke.

[Kev m]

Thumb over sparkplug hole tells if your on compression stroke.

Personally I've always hated that method.

If you're adjusting a valve, then you can SEE the valve train, so it's a lot easier and more precise (especially if you are spinning the crankshaft slowly by hand) to just WATCH when the intake valve closes and the piston is going up - then you know damn well you're on the compression stroke.

[LowRyter]

That's only 1/2 the answer. So you are saying that when D or S appears it is on compression every 720 degrees.

yes. 

[johnr]

So, are the 'D' and 'S' marks timing marks or TDC marks?

[redrider90]

So, are the 'D' and 'S' marks timing marks or TDC marks?

The are TDC timing marks. But if you want to use them for valve adjustments you need to understand they only align on TDC compression once every 720 degrees the flywheel turns. So one time the S or D mark comes around on TDC it is on a exhaust stroke and then the next time it comes around it will be TDC on a compression stroke. That is why when checking  valves you have to watch you intake and exhaust valves to make sure the TDC you are using is a compression stroke. Otherwise you could end up setting the valves on the exhaust stroke which will result in valves adjusting incorrectly. 

[Zoom Zoom]

So, does that mean your question, (the second part), has been answered?

As an aside FWIW, I found S and D the easiest to remember as "starter" and "distributor" as the distributor was always located opposite the starter, until they went away............... . But still, you get the idea.

John Henry

[redrider90]

So, does that mean your question, (the second part), has been answered?

As an aside FWIW, I found S and D the easiest to remember as "starter" and "distributor" as the distributor was always located opposite the starter, until they went away............... . But still, you get the idea.

John Henry

yes it has been answered.

[Gootsz]

D = drive side
S = starter side

micky

[ritratto]

D... destra  (right)

S... sinistra  (left)

[Dukedesmo]

At the correct (compression) TDC, be it 'S' or 'D' then both/all valves on said cylinder will be closed and unloaded, otherwise one or other of the valves will be 'loaded'. Of course you'll need the rocker covers off to see this but you'll need them off to adjust the valves anyway.


And yes Destra = right and Sinistra = left.

[Tom]

Spin the crank.  Check to see if both valves are loose.  If not spin it again till both are loose.  Adjust.  If both valves are loose on both cylinders then you have to use other methods to check valve clearances.

[johnr]

The are TDC timing marks.

So, to be absolutely clear, the 'S' and 'D' are marks for TDC, NOT spark timing. Correct?

[wirespokes]

So, to be absolutely clear, the 'S' and 'D' are marks for TDC, NOT spark timing. Correct?

 CORRECT!

They only denote that the specified piston is at the top of its stroke.

The ignition timing marks are above (in the window) the TDC marks.

[redrider90]

So, to be absolutely clear, the 'S' and 'D' are marks for TDC, NOT spark timing. Correct?

The S and D marks with the advanced and retard marks are for spark timing. The S and D just tell you which cylinder you are looking at. Then your use the marks with that cylinder to time it. That is the whole purpose to is provide the correct marks for timing. For static timing you use the retarded mark as the place when the point just begin to open. To check advance you use the advanced mark with a strobe light. These are timing marks. 

This is a photo out of Haynes. Note in this drawing of [/img] it shows the fully advanced mark appearing in the window that reveals the flywheel. All 3 marks do not appear in the window at the same time. The advanced mark is further up the flywheel and cannot be view statically when either the S or D mark is in the window. Note the other 2 lines that represent 1. the retarded mark 2. the TDC mark
The only use of the fully advanced mark is when you put a strobe light and run the engine to the recommended specs and then align the fully advanced line with the line that is cut into the flywheel case. On my Mille I time the fully advanced with a strobe at 4500 RPMS.

[LowRyter]

I never noticed the two other marks.  But it's all I can do to see the letters with a flashlight.  In fact, I make a red mark on the alternator so as I spin it, so I know when to look for the mark.

[Tom]

Like finding the happy medium of timing two single cylinder engines.

[rodekyll]

Would someone please shoot this thread in the brain?

Thanks in advance.

[pete roper]

Would someone please shoot this thread in the brain?

Thanks in advance.