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The crank shaft timing gear and the camshaft timing gears have marks on them. These marks are lined up so the crank gear is on top and the cam gear is on the bottom. when a straight edge when placed through the center line of the crankshaft and camshaft and lined up with the marks on the gears, the timing is correct. this is the procedure that I found in a old maintenance book for the GMC 305 V6 engine. I found that when you line up the timing marks this way that you are actually 180 degrees out ( number one is on the exhaust stroke) so your distributor will be pointing at cylinder #4. roll the engine around 180 degrees from this point( if looking at the timing chain the cam mark will be on top and the crank mark will be on top and they will both line up with a straight edge through the center-line of both shafts) and make sure that the distributor is actually pointing at number one. I do not know why the book does not have you line up the timing marks so you are on top dead center compression but it is easier to line them up the way the book says.

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13y ago
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Q: What is the Timing mark for a 305Cu 1969 GMC V6?
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