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How accurate is the crank-pulley TDC mark?

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Dave at OldSchool Avatar
Dave at OldSchool Dave Perry
Battleground, AL, USA   USA
@John Mechanical standards say that the actual TDC is half way between the number of degrees the piston is at the top.

a stop does not give you the " middle" , it only gives you the initial top. you can find the opposite side of the top by turning the crank counter clockwise, but due to clearances, reversing the rotation is not as accurate .

I have been doing this since the mid-1950's, and doing this correctly with a dial indicator is NOT subjective. The results can be repeated, can be done by anyone, and they can be repeated at a later time.

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Dave at OldSchool Avatar
Dave at OldSchool Dave Perry
Battleground, AL, USA   USA
.
@Lizzard, you apparaently don't understand what a piston actually does at TDC during a crank rotation.

Lizzard d id
san jose, CA, USA   USA
In reply to # 1595842 by Dave at OldSchool .
@Lizzard, you apparaently don't understand what a piston actually does at TDC during a crank rotation.

Yeah that must be the problem .

Please do tell me the details of "what a piston actually does at TDC during a crank rotation" . Show me what it is . Fill in the gaps of my knowledge .



Edited 1 time(s). Last edit at 2019-02-11 12:50 PM by Lizzard.

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sdebaker Silver Member Paul Rollins
Vancouver, WA, USA   USA
I made a fixture and checked TDC against the scale on the timing-chain cover. It is off 2 degrees. True TDC is at 2 degrees ATDC on the scale.

Lizzard d id
san jose, CA, USA   USA
In reply to # 1596501 by sdebaker I made a fixture and checked TDC against the scale on the timing-chain cover. It is off 2 degrees. True TDC is at 2 degrees ATDC on the scale.
Cool , I'm glad you put in the time and effort .

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1595840 by Dave at OldSchool @John Mechanical standards say that the actual TDC is half way between the number of degrees the piston is at the top.

a stop does not give you the " middle" , it only gives you the initial top. you can find the opposite side of the top by turning the crank counter clockwise, but due to clearances, reversing the rotation is not as accurate .

I have been doing this since the mid-1950's, and doing this correctly with a dial indicator is NOT subjective. The results can be repeated, can be done by anyone, and they can be repeated at a later time.

That said, directly finding TDC with just a DI is proven to be less accurate than using a piston stop.
That's not opinion, it's science and geometry.
When at TDC, for a Spitfire motor having stroke = 2.992", the difference in piston travel that represents plus/minus 1 degree is about 0.0002278".
That's FAR LESS than the mechanical clearances of the various parts involved:
crankshaft mains run around 0.002"
crankshaft rods run 0.0015"
piston pin to rod runs around 0.0005"
piston pin to piston runs around 0.0005"

The piston stop method ensures that all of those tolerances are taken up consistently when the piston hits the stop on either side of TDC, and cancel out.
It's important to apply firm but consistent hand force, too much can cause the stop to deform the top of the piston.

With a DI, each time you rotate around, you'll get a different result, maybe close, but the tolerance buildup means you never really know.

Measurement error from reading the degree wheel is always an issue, larger diameter is better, with a 6" wheel, 1 degree is about 0.052" wide,
but a 12" wheel, 1 degree is 0.104" wide.



to piston runs around 0.0005"

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Thank you Carter.

I too was wondering wondering what all the clearances might add up to.

I know that when using a DI even finger pressure on the piston crown will cause some movement of the needle.

The differances are very little, but if you are going to measure something you may as well do it as accurately as possible.
Little things add up.

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Hi,
Another point not mentioned. When using a dial indicator to find TDC rotating the crank (without reversing) in the normal clockwise direction, as the piston approaches TDC all of the clearances Carter has documented are taken up in the compressed direction. Once the piston passes TDC and begins to descend the clearances are all taken up in the extended direction. This means the piston will be delayed starting down as the clearances are changed from compressed to extended. The changeover is not symmetrical around TDC, it is all while the crank is on the descending side.
The piston stop method doesn't suffer from this inaccuracy.
All the best,
Paul

Falkon Avatar
Falkon Al Martin
Appleton, WI, USA   USA
This thread reminds me of an "expert" from long past.

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1596585 by spitfire50 Hi,
Another point not mentioned. When using a dial indicator to find TDC rotating the crank (without reversing) in the normal clockwise direction, as the piston approaches TDC all of the clearances Carter has documented are taken up in the compressed direction. Once the piston passes TDC and begins to descend the clearances are all taken up in the extended direction. This means the piston will be delayed starting down as the clearances are changed from compressed to extended. The changeover is not symmetrical around TDC, it is all while the crank is on the descending side.
The piston stop method doesn't suffer from this inaccuracy.
All the best,
Paul

Thanks for bringing that up Paul, the 'stiction' created by the rings against the bore require that all the clearances be 'taken up' in the opposite direction
before the piston can begin to be pulled back down from TDC by crank and rod and descend.

Were TDC truly critical, laser metrology gaging (as used by 3D scanners) could be employed against the crankshaft itself, although the clearance of the main
journals within the bearing bores would still present an issue, even though detectable by such instruments.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
In reply to # 1596595 by Falkon This thread reminds me of an "expert" from long past.

.......heh.........

Yeah, we crossed swords.

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