View single post by Esprit2
 Posted: 09-11-2017 07:28 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 343
As Don mentioned, the stock cams are identical. Even the cam carriers can be interchanged between the intake and exhaust sides of the head (they bolt on in either position). HOWEVER, the cam pulleys are reversed (flipped around front to back) between intake and exhaust. If the cam carriers were swapped around without correcting the pulley orientations, then the resulting cam timing would be way off. Perhaps far enough to cause a lack of compression, or worst case, to bend valves.

Whenever you rotate the crank, first remove the spark plugs. It's more difficult to turn the crank with the plugs in, so you'll be putting more effort into it. 'IF' a piston hits valves, you'll probably just power right past them... bending them. With the plugs out, you can put minimal effort into turning the crank. Then any hard spot due to a piston/ valve collision will be more noticeable. Pay attention, and use a little mechanical empathy. If you feel the engine clunk to a stop, or hit a hard spot, that may well be a piston hitting valves. STOP immediately, and don't force your way past it.

Without knowing more about your engine, getting into specifics would simply be conjecture. With that in mind...

Start your diagnosis by confirming the cam timing with the crank at TDC. Once the timing is correct, it will be safe to crank the engine over on the starter... as in doing a Compression test. Follow up with a Leak Down test.

The first step in all of that is to make certain both cam pulleys are properly installed.

For each timing mark, it's IN/ EX twin is on the back side of the same tooth. Next to each mark there should be an IN (INtake) or EX (EXhaust) mark on the pulley's center web, near the rim. In early pulleys, it will be stamped. Often not very legibly. In later pulleys, it's cast in, and generally easier to read.

The timing mark next to the 'IN' should face forward on the intake pulley.
The timing mark next to the "EX' mark should face forward on the exhaust pulley.
If either cam pulley is reversed, that cam's timing is way off. Correct it before going any further.

The second step is to set the crank to TDC (spark plugs out as noted above), and observe the timing marks on the cam pulleys. If they are on far opposite sides of the pulleys, then the #1 piston is at BDC (Bottom Dead Center) instead of TDC. Turn the crank through one more clockwise rotation, and back to TDC. That will set #1 to TDC... ie, the only correct crank position for checking cam timing. The pulley timing marks should now align with one another 'ON' the imaginary centerline between the cams. If they align, but are a whole tooth PITCH high or low relative to the centerline, then they are still not properly timed. 'Alignment' is not everything. They must align 'ON' the centerline.

DEFINTION: One TOOTH PITCH is the total width of one tooth plus one gap. In other words, from any point on one tooth to exactly the same point on the adjacent tooth. Being off by only the actual width of one tooth is not being off "by one tooth" in cam timing jargon. Unless clearly stated otherwise, "by one tooth" should always be taken to mean by "one TOOTH PITCH".

If a pulley is 'reversed', then the amount of timing error that results will vary with each MOP target. If the MOP of the timing mark being used was the early J-H's 115 MOP, and the pulley was installed reversed, then the resulting timing will be off by 24.5 pulley degrees. All cam timing events are stated in crankshaft degrees. The cams run at half crank speed, so 24.5 x 2 = 49 degrees of cam timing error. That's a big error.

Similarly, if the target timing was 110 MOP, reversing the pulley will result in the timing being off by 29.5 'pulley degrees, which equates to 59 crankshaft degrees. That's huge.

For 100 MOP, the error is 42.5 pulley degrees, or 85 crankshaft degrees. Humongous... and the pistons are into the valves... not much doubt about that.

In any case, the intake will be retarded, and the exhaust will be advanced.

The pulleys have 40 teeth, so 360 degrees in a circle divided by 40 teeth equals 9 pulley degrees per tooth PITCH (a tooth plus a gap), or 18 crankshaft degrees per tooth. So...

115 MOP pulley backwards = 49 degree error divided by 18 deg/ tooth pitch = 2.7222 tooth pitches off.
110 MOP pulley backwards = 59 degree error divided by 18 deg/ tooth pitch = 3.2778 tooth pitches off.
100 MOP pulley backwards = 85 degree error divided by 18 deg/ tooth pitch = 4.7222 tooth pitches off.

At 3.0 teeth off, you may get lucky and not wipe out any valves... but you're scary close. At 4.7 teeth, keep diagnosing, but prepare yourself for the worst (bent valves)... you'll probably find it.

Good luck,
Tim Engel

Last edited on 10-08-2017 04:18 am by Esprit2