5.3 timing marks

[Ken early]

My 2006 Tahoe Z71 5.3 was running fine, but lost oil pressure. It’s been lower than my 02 Tahoe, but it suddenly dropped while idling. I opened it up to replace that stupid O ring at the top of the pickup tube and the oil pump, and decided the timing chain needed to be replaced as well.

I haven’t removed the chain yet, because when I set the #1 piston to TDC on the compression stroke, the timing marks were both at 12:00, not 6:00 on the cam and 12:00 on the crank (pointing at each other) as the manual says they should be. Since the engine was running fine, I’m assuming I should put it back that way, but I don’t know if there’s a reason I should move it 180° to point the marks at each other. Has anyone else found this to be the case?

I’ve only had this truck for about 20k miles and 6 months. My ‘02 is at 554k and needs a rebuild soon, so this is the replacement for now and I don’t have a lot of history on it. The guy I bought it from does specialty engine rebuilding and LS swaps for hot rods but said he had never done anything to this truck other than drain all the fluids as soon as he got it. I don’t have any history prior to that, but it’s in good shape with under 200k so I doubt the timing chain has been done before.

 

[Fless]

What do the marks look like if you're at TDC on the exhaust stroke?

 

[V30Crewcab]

turn the crank around one turn and it will be lined up right.

 

[Ken early]

If I rotate the crank 1 turn, the marks are then pointed at each other, but then the #1 piston is on the exhaust stroke. I understand the 2:1 ratio, and I rotated the crank all the way around, checking the piston to make sure I had it at TDC on compression, because that’s what the manual and pretty much every reference I’ve found say to do. Seeing my known good engine set up with the cam 180° off kind of blew my mind. I’ve seen some comments that it doesn’t matter if the cam is off 180° because the sensor controls spark and injection, but that just feels wrong.

 

[Ken early]

turn the crank around one turn and it will be lined up right.

If I rotate the crank 1 turn, the marks are then pointed at each other, but then the #1 piston is on the exhaust stroke. I understand the 2:1 ratio, and I rotated the crank all the way around, checking the piston to make sure I had it at TDC on compression, because that’s what the manual and pretty much every reference I’ve found say to do. Seeing my known good engine set up with the cam 180° off kind of blew my mind. I’ve seen some comments that it doesn’t matter if the cam is off 180° because the sensor controls spark and injection, but that just feels wrong.

 

[Ken early]

What do the marks look like if you're

What do the marks look like if you're at TDC on the exhaust stroke?

They’re pointed at each other at TDC on the exhaust stroke, but that’s exactly backwards from what the manual says to do.

 

[Donal]

The timming marks are illistrated being together for ease of positioning during assembly. Piston number one will be at top dead center once every revolution. Piston number six will be at top dead center once every revolution. These two pistions are 180 degrees of each other. When the marks are togther, number one, valves wil be timed for exhaust and number six, valves will be timed for compression, etc.
Correct timming is easier align corectly when marks are positioned closest together and have been illustrated in that relationship since the first manual issued in 1955. FYI I have been fornicating with Chevrolet since 1966 when I got my first C 2 roadster. During my first assembly, I also misunderstood and timmed the distributor 180 degrees off.

 

[Rocket Man]

I have followed these instructions on LS1howto. It doesn’t mention exhaust or compression stroke, just get the dots lined up. Works for me.

LS1Howto.com

 

[MassHoe04]

TDC is that very position, where the piston has ended exhaust stroke and has not yet begun the next intake stroke.

The index marks on the gears should line up as the FSM mentions. They main thing is that the cam shaft and the crankshaft remain in the exact same relative positions to each other and do not budge in the process.

 

[rockola1971]

LS block timing marks are just like Gen 1 and 2 SBC. They never changed the timing mark relationship on SBC v8's between generations. Just the firing order.
Cam gear mark should be at 6oclock and crank gear mark should be at 12oclock while #1 piston is at TDC.

You should feel compression while piston is on its way up to TDC because both exhaust and intake valves are closed. Depending on timing the spark would happen around 12 deg BEFORE TDC (BTDC).

Is OP sure he is actually physically checking the #1 and not #2? #1 is drivers side very front cylinder.