How to find TDC

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Tinkertom2k8
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How to find TDC

Post by Tinkertom2k8 » Wed Nov 05, 2008 11:00 pm

I know to drop a screwdriver down the spark plug hole in an upright VW engine and watch it to find TDC on #1, but this does not work on a pancake engine. The piston clamps the foreign object like a vise against the head. Is there an equivilant way to test for TDC on a pancake? Thinking of closing one end of a clear plastic tube, filling with oil and crushing it while the oil rises/falls. Any better way?

wildthings
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Post by wildthings » Thu Nov 06, 2008 1:00 am

Are you missing your timing scale? There should be a metal or plastic timing scale bolted to the fan housing kind of the the top left of the fan. The fan itself should have a timing mark on it.

If you have the engine out of a T4 or Porsche you will have a black plastic plug in the top center of the fan shroud which must be removed to see the timing marks on those engines.

Tinkertom2k8
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Post by Tinkertom2k8 » Thu Nov 06, 2008 4:50 pm

I have a timing scale but don't know if it is trustworthy or if the fan is the right one for this engine. Suspect the distributor drive shaft is installed offset from normal because engine runs better at 27btdc than at normal but need to verify that TDC really is zero on the scale before anything else.

wildthings
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Post by wildthings » Thu Nov 06, 2008 5:29 pm

One fairly accurate way to check for TDC is to use a 1/2" wooden dowel in the spark plug hole. Turn the engine in one direction until it jambs against the dowel and make a mark on the fan and then turn the engine in the other direction until it jambs and make a mark. The true TDC mark will be half way between the two marks you made. You need to make sure the dowel isn't interfering with a valve and that it is in the exact same position both times it gets jambed.

Tinkertom2k8
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Post by Tinkertom2k8 » Thu Nov 06, 2008 10:15 pm

:wink: Right on, that is sort what I was thinking. Your method will at least verify #1 TDC is where it is supposed to be or not, close enough. I am going to try that tomorrow. If it looks good the fan is probably the original and the scale too, agreed? If so, I will pull the distributor and see if the slot is right. I think the slot should be on a line 12 degrees ccw from the case join.
There has to be a way to get this engine timed right and I WILL do it after replacing the tent and seat covers and all the other restoration we have done!
Thanks for the help
Tom

Tinkertom2k8
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Post by Tinkertom2k8 » Fri Nov 07, 2008 5:22 pm

Good news, used a small bore long screwdriver to block the piston and approached #1 TDC from left and then right side, marking the scale where the notch ended up. Wrapped a strip of paper over the fan and xfered the marks to it, folded the paper in half and the half way point between the marks is .....of course....zero degrees exactly, TDC. Need to clean top of the block before pulling distributor next.

QUESTION: This distributor swap raises questions. Since the distributor does not have a vac can on it, do I need to have the slot at 12 degrees "so the vac can won't hit the wall" as one book said, or should it be 90 degrees from the joining of the block like most others buses? The centrifigal Adv distributor does not need 10 ATDC, so would it be set 7 BTDC perhaps? Hmmmm.

wildthings
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Post by wildthings » Sat Nov 08, 2008 1:53 am

The distributor need to be clocked so that the oil hole in the distributor housing lines up with the oil hole in the block. I think with an 009 this causes the housing to be rotated 90° from the stock position but really don't know for sure. Position of the distributor drive to accomplish this.

So long as the rotor is pointing towards the number one plug wire when the engine is at TDC compression for #1 you should be able to time it correctly. You want your timing to be 28-30° BTDC at 3500 rpms and let the timing at idle fall where it may.

Tinkertom2k8
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Post by Tinkertom2k8 » Sat Nov 08, 2008 7:53 pm

I can do that.

Thanks for your help. Let you know.

Tom

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amskeptic
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Post by amskeptic » Sat Nov 22, 2008 12:07 am

Tinkertom2k8 wrote:I can do that.

Thanks for your help. Let you know.

Tom
For future reference, there is an easier way to find TDC without sticking crap in spark plug holes. Just take off the LEFT side valve cover and rotate engine until the front two rocker arms (#3 exhaust and intake) are moving in opposite directions > exhaust valve is almost closed and intake is just opening as you rotate crankshaft clockwise. Midway in the overlap is 4* ATDC for #1, but wait, there's more! at this point, look for a hole in the flywheel exactly in the middle of the big crankcase hole on the left case half. When that sucker is centered and #3 rockers are overlapping, you are dead-nuts TDC for #1.
Colin

Tinkertom2k8
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Post by Tinkertom2k8 » Thu Dec 04, 2008 8:20 pm

Amskeptic, I printed your explanation out for my notebook, that is new to me. Would this method not require the engine to be on the bench to see the flywheel hole? Thanks.

mentalQtip
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Re:

Post by mentalQtip » Thu May 01, 2014 11:25 pm

Hi Colin. I'm trying to get a '77 two liter bus engine going(pick and pull) and I'm confused about what I'm finding.
Using the below directions I seem to get the pulley notch at 20 degrees BTDC and the distributor drive is about 45 degrees away from parallel to the engine seam with the bigger portion to the right. The distributor is pointing at about 3:30 position.

I should also add that it has hydraulic valves.

I'm sorely confused.

Joseph

amskeptic wrote:
Tinkertom2k8 wrote:I can do that.

Thanks for your help. Let you know.

Tom
For future reference, there is an easier way to find TDC without sticking crap in spark plug holes. Just take off the LEFT side valve cover and rotate engine until the front two rocker arms (#3 exhaust and intake) are moving in opposite directions > exhaust valve is almost closed and intake is just opening as you rotate crankshaft clockwise. Midway in the overlap is 4* ATDC for #1, but wait, there's more! at this point, look for a hole in the flywheel exactly in the middle of the big crankcase hole on the left case half. When that sucker is centered and #3 rockers are overlapping, you are dead-nuts TDC for #1.
Colin

mentalQtip
Posts: 101
Joined: Tue Feb 05, 2008 11:08 am

Re: How to find TDC

Post by mentalQtip » Thu May 01, 2014 11:26 pm

Sorry about the age of the thread. Didn't notice til I replied. I'd be happy to have advice from anyone.

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