: Stumped!! - problem setting cam timing



brad3378
03-20-04, 03:38 PM
I replaced the front head gasket on my 98 Deville and
I'm having a problem with the cam timing. It appears that the pistons are hitting the valves. I only get about 90 degrees of travel in each direction.

I've verified that the Timing marks are set correctly according to my chilton manual (same as here: http://www.babcox.com/editorial/us/us100232.htm )

Also verified that there is no slack in the timing chain / adjuster,
and that the Chain sprockets are correctly placed in the intake/exhaust positions.

I'm stumped.
Anybody have any guesses what I'm doing wrong?
It must be something I did wrong, because this problem definitely didn't exist before I removed the cylinder head.

brad3378
03-20-04, 05:23 PM
Okay -
I figured it out - I'm an idiot :bonkers:

I took a shortcut and only lined up the intermediate shaft and the 2 cam sprockets. I made the assumption that the crankshaft mark would be aligned with the intermediate shaft automatically. On a conventional engine like a smallblock ford or chevy, the cam sprocket has exactly twice the number of teeth as the crank sprocket. - this is not the case with the Northstar Intermediate shaft sprocket & crank sprocket. I think the crankshaft mark only lines up with the intermediate shaft mark about every 5 full revolutions or so.


All 4 marks must be lined up, which means you can't take a shortcut and leave the oil pump in place. - I'm just glad I found out now and not after putting the whole thing back together.


If I had it to do all over again, I would have rotated the engine by hand to align the engine timing marks before taking off the cam sprockets. Then I could have even left the oil pump on.

BeelzeBob
03-21-04, 01:52 AM
Glad you found the problem.

Cam in block engines have a 2:1 drive ratio on the crank to cam sprockets by definition so that the cams turn at 1/2 engine speed.

The cam drive in the Northstar was engineered to provide very stable timing chain operation and to be a quite as possible...that is why there is an uneven drive ratio on the intermediate sprocket...not to make assembly or service difficult.

The timing marks on the intermediate sprockets and the crank shaft will only line up every 7 revolutions of the crank....after 7 revolutions the intermediate shaft marks will be lined up again but the marks on the cams will be off 180 degrees. It takes 7 more revolutions of the crank to get all the marks "back in time". Or, if the timing drive is in the as assembled position with all the marks lined up , it will take 14 turns of the crank to get all the marks back in line.

The uneven drive ratio between the crank and intermediate sprocket and between the intermediate sprocket set and the cams was done intentionally to eliminate any harmonic resonance in the timing drive and to minimize chain whip and timing drive noise and wear. It works pretty good....that is why the timing drive can run for hundreds of thousands of miles with no chain wear or problems...and that is why all the timing drives look brand new at 150K when you take them apart....!!!....LOL


If you touched pistons to the valves I would strongly encourge you to leak test the combustion chambers before you stick the engine back into the car....it is VERY easy to bend a valve as the valves are small and the stems are thin. With the heads on and the cams rotated so that the valves are closed put compressed air to the combustion chambers thru the spark plug port and listen at the intake port and the exhuast port for excessive leakage indicating a bent valve...easier to fix now than later.