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How accurate is the crank-pulley TDC mark?

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sdebaker Silver Member Paul Rollins
Vancouver, WA, USA   USA
Generally, how accurate is the alignment of the crank-pulley TDC mark alignment with the "0 degree" mark on the front cover scale?

I know how to determine TDC, but don't see any reason to do that if there is little benefit to the time spent.

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Lizzard d id
san jose, CA, USA   USA
The benefit is the answer to the question of putting in the time effort once .
Or
How can someone tell if its accurate sans doing the test ?

If you do the test 4 times for 4 cylinders you can have 2 , 3 or 4 marks . Iykwim .

Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
Generally it is right on.
da



Dan Aycock
Walla Walla, Wa.
Yellowhawk Valley Spitfires
69, 69, 72, 75, 78, 79 Spitfires

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
After 40 + years, who knows?

The timing cover may have been changed, and in any case the front pully is small.

It is good enough to set ignition timing, which is kind of arbitrary anyway.

If timing a cam etc. You will use other means anyway.

SpiTazz72 Avatar
SpiTazz72 Bryan H
Magnolia, TX, USA   USA
For spark timing it's close enough but for checking valve timing It depends on your timing chain and would be safer to use another method.

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Paul,
Even if most of them are dead accurate you don't know the one on your engine is good. Carefully checking it is the only way to know what is TDC.
All the best,
Paul

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Test it with a dial indicator.

tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
Paul,

You don't say if you have a four or six-cylinder engine. This is an important factor, because of the difference between the front pullies on the two engines.

A four cylinder engine, Spitfire, Herald etc is a solid metal disc.
On a six, the disc has a separate ring on it, held in place by a rubber layer. This acts as a vibration damper, thought to be necessary for the longer cranked six, but not the four. The timing marks are engraved onto the outer surface of the outer ring.

Rubber loses its properties in the heat and hydrocarbon-filled environment of an engine bay, and sometimes it will lose adhesion, so that the outer ring moves on the inner hub, making the timing marks a complete fiction. It is difficult to tell, unless you find TDC by other means, as the outer surface of the rubber may become swollen, cracked and perished, but inside it still holds on. A failed damper pulley also fails in its damping task, and excess vibration and even crank failure can follow, due to resonance at certain speeds.

If you are concerned that your timing marks are incorrect through this mechanism, then find TDC by other means and compare. I'm researching crank damper pulley testing and have a prototype technique to do so, but as dampers are a heavy itme to post, I fear that it would cost you a lot to send me your damper for checking. Be glad to if you wish.

John

Dave at OldSchool Avatar
Dave at OldSchool Dave Perry
Battleground, AL, USA   USA
consider it a reference point ONLY.........

Tony has it right " It is good enough to set ignition timing, which is kind of arbitrary anyway. "


Dave Perry OldSchool Restorations of North Alabama USA

Dave at OldSchool Avatar
Dave at OldSchool Dave Perry
Battleground, AL, USA   USA
Paul, the ONLY way to determine actual TDC is with a dial indicator, observing both the start and the end degrees of TDC and dividing the result by two, and of course it requires removing the head to do that (unless you have the tool setup to do it through a spark plug hole)

ExPatBrit Avatar
ExPatBrit Mike W
Redmond, WA, USA   USA
The woodruff key that holds the pulley has been known to fracture, if you have timing shifting around it's worth a look.

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1595571 by Dave at OldSchool Paul, the ONLY way to determine actual TDC is with a dial indicator, observing both the start and the end degrees of TDC and dividing the result by two, and of course it requires removing the head to do that (unless you have the tool setup to do it through a spark plug hole)

No dial indicator or head removal needed to find TDC, but a degree wheel is required.
The 'through the spark plug hole' style piston stops are dead cheap on eBay, and used properly, work very well..
That said, timing a cam DOES require a dial indicator, adequate kits can be had for under $25 USD.

Dave at OldSchool Avatar
Dave at OldSchool Dave Perry
Battleground, AL, USA   USA
Carter, "well " may be 'good enough' , but a stop does not give accurate enough results to be actual

Lizzard d id
san jose, CA, USA   USA
In reply to # 1595804 by Dave at OldSchool Carter, "well " may be 'good enough' , but a stop does not give accurate enough results to be actual
I think you have that backwards . Nothing is as actual as a stop .

tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
Dave,
Trying to find TDC with a dial gauge. The piston slows down and slows down, and finally stops rising. As you continue to turn it starts to go down again, after a wait around TDC due to tolerances in the bearings.
Where does it stop and start? Somewhere on the degree wheel, could be anywhere within that degree mark. It's a subjective judgement.

In contrast, if you set the piston stop in the middle of the stroke, it is moving as fast as it can for each degree of movement, and there is absoluely no doubt about where it stops. The measurement is objective.
Then simple arithmetic, divide the angle by 2 and you KNOW where is TDC.

JOhn

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