TDC is TDC, period. - Wrist Twisters
 
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post #1 of 17 Old 09-22-2008, 04:14 AM Thread Starter
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TDC is TDC, period.

So the simplest of mechanical application seems to have eluded me for some time now. But since I'm rebuilding my first engine it has become apparent so I'll share my newfound knowledge, which might be longtime knowledge for you.

Top Dead Center is Top Dead Center, period.
Once the camshaft has been eliminated from the question the crankshaft has no clue if it's on the compression or exhaust stroke, it doesn't care. All it knows is that it goes up and down, the valves determine what's compression and what's exhaust.

Bring the piston to TDC, lay in the cam with the timing indicators in the correct spot, lace up the timing chain and viola, you're in business.

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post #2 of 17 Old 09-22-2008, 05:43 AM
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This sounds correct. Keep in mind that some counter balancers have to be in proper time as well. Rob T. ?

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post #3 of 17 Old 09-22-2008, 09:38 AM
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I never thought about it in a philosophical way but yeah TDC now is the same as TDC 360 degrees from now.

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post #4 of 17 Old 09-22-2008, 10:14 AM
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Quote:
Originally Posted by CMulford View Post
I never thought about it in a philosophical way but yeah TDC now is the same as TDC 360 degrees from now.
If two parallel TDC's continue on long enough, they will eventually cross.

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post #5 of 17 Old 09-22-2008, 10:38 AM
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It only took you once to figure out what most can only CLAIM to know about, let alone have first hand experience with! Good Job! To many try to complicate the process when it truly is so simple.


So you've got the 525 up and running I take it? How's she feel?

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post #6 of 17 Old 09-22-2008, 10:54 AM Thread Starter
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nope, it's not up and running yet. Waiting on 2 more parts to come in, hope to hear the rumble tomorrow night.

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post #7 of 17 Old 09-22-2008, 03:23 PM
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Years ago, I was rebuilding the top-end of my '89 Isuzu Trooper W/2.6L I4 engine. When I went to bolt the head back on, I went through the ritual of checking the cam timing against the crank and came to the same conclusion. Afterword, the engine ran for 100K more miles before my son "gave" it away. I believe it's still running....

I plan to die young, as late as possible.
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post #8 of 17 Old 09-23-2008, 09:12 AM
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Absolutely correct! The crankshaft doesn't care which of the two TDC's corresponds to the power stroke. There is a complication, however, when trying to be as accurate as possible, such as when blueprinting an engine: how to precisely determine TDC? The timing marks provided by the factory are not necessarily accurate enough for my delicate sensibilities, and trying to use a dial indicator to find it will only get you within a couple degrees due to the bearing clearances having to be taken up and the extremely small amount of movement at TDC. As an illustration use the calculator in the computer to take the cosine of 0 degrees, and 2 degrees -- the difference is less than 1%. There is a method of determining it with very good accuracy that I learned 30 years ago while visiting a well respected builder of Chevy small block racing motors, and have used since.

It involves the use of a large degree wheel (I use a drafting protractor about 10 inches in diameter with 1/2 degree increments) and a solid stop that can be threaded into a spark plug hole. Set up the degree wheel loosely on the crank and a pointer as close to it as possible, roll the crank down from TDC and thread the stop in. Now roll the crank slowly up until the piston contacts the stop and apply enough force to take up all bearing clearances, turn the degree wheel so the pointer is at zero degrees and tighten down the wheel, then turn the crank back away from TDC until it contacts the stop again and read the angle from zero. Half of the difference in angle is TDC within 1/2 degree. On all engines I've used this procedure on not one has had the factory TDC marks within 3 degrees of correct. Why be so picky? An error of 1 degree can produce as much as a 5% reduction in power -- significant in practically any racing.

As to counterbalancers: in most cases they are driven from the crankshaft and can only balance out the primary and secondary imbalances from the crank train -- second order imbalances from power impulses are partially balanced out by the speed fluxuations they produce in the crankshaft, so again it makes no difference where they are set as regards the power strokes. One case where it may be necessary is the Aprilia twins where the counterbalancers are mounted in the heads and driven from the cam sprockets, but I don't have enough information to know if the setting procedure is different.

Coming soon: camshaft lobe centers and the effect they have on power characteristics. It does relate to accurate determination of TDC, and can have a significant effect with twin cam motors.

Rob

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post #9 of 17 Old 09-23-2008, 09:24 AM Thread Starter
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Rob, would you like to move to Florida and buy the house next door to me?

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post #10 of 17 Old 09-23-2008, 10:19 AM
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Hell, Rob needs to write books! I'd be first in line to buy one.




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post #11 of 17 Old 09-23-2008, 10:21 AM
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Quote:
Rob, would you like to move to Florida and buy the house next door to me?

Only if there can be a twisty road 2 1/2 inches from the end of the driveway.

Rob

If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------
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post #12 of 17 Old 09-23-2008, 10:44 AM Thread Starter
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Quote:
Originally Posted by robtharalson View Post
Only if there can be a twisty road 2 1/2 inches from the end of the driveway.

Rob
I could set up some cones.


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post #13 of 17 Old 09-23-2008, 11:04 AM
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I could set up some cones.

If you ever saw Rob ride mountain roads in person, then you'ed know that cones would just never do!

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post #14 of 17 Old 09-23-2008, 12:50 PM
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Not to derail, but, Aprillia put counterbalancers in the heads ? I have to hear the story of why. Please Mr Haralson, do go on.

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post #15 of 17 Old 09-23-2008, 05:25 PM
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Quote:
Originally Posted by Lemonhead View Post
Not to derail, but, Aprillia put counterbalancers in the heads ? I have to hear the story of why. Please Mr Tharalson, do go on.
Note correction.
It's not a derail unless you ask how much a baseball weighs (5 to 5 1/4 ounces) or some other irrelevancy.

Why? Because they felt like it. Okay, it goes beyond that, but given the almost complete lack of specifics I'm forced to speculate. First, The engine is a 60 degree twin, not the 90 degree of Ducati, Moto Guzzi, Honda RC51 and Superhawk, Suzuki SV series, and others I'm not aware of. The best part of a 90 degree is theoretical perfect primary and secondary balance, but there is a fly in the ointment in the offset connecting rod big ends on the single crankpin resulting in the mass of the con rods and milk bucket pistons being out of line with each other and the vertical centerline of the motor resulting in a "rocking couple" or longitudinal shake of the engine. Some of this can be compensated for with slightly skewed flyweights on the crankshaft, but I'm not aware of this being done. The Aprilia's 60 degree cylinder arrangement would produce significant primary shakes (in fact, does) that without the counterbalancers would be very hand numbing in short order. As to the placement of them, technically a counterbalancer can be placed anywhere on an engine as long as it operates on the same plane as the crank, so the placement of one in the rear cylinder head (the other is mounted in front of the crank and geared to it) is to keep the engine cases as short as possible to promote mass centralization and facilitate moving the engine as far forward as possible -- the reason for the 60 degree cylinder angle as well. Two shafts are required to cancel the longitudinal and primary imbalances due to the off plane nature of the vibrations, and the one in the head being further away from the crank doesn't need to be as heavy due to its longer moment arm of inertia and therefore requires less power to drive.

I found a parts diagram here: http://www.scootopiaonline.com/eshop...090.Engine.htm But it took a lot of searching.

Anyway, that's my take on it, and if anybody else has more specific information I'd appreciate a heads up.

Rob

If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------
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post #16 of 17 Old 09-23-2008, 06:05 PM
 
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Rob, did you search for Rotax v-twins?
Rotax is owned by Bombardier; they use Rotax motors in all their Sea-Doo watercraft. The IL2 motor in my Sea-Doo uses a gear driven countershaft. FWIW

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post #17 of 17 Old 09-23-2008, 07:59 PM
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My apologies Mr. Tharalson. I've spelled it correctly before, I, I, I, don't know what happened that time.

Um, Part number twenty on the rear timing schematic ? It looks like a counterbalancer, but it's so small.

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