I know there are no dumb questions - but let me stress that theory

At any rate I know there are two times cylinder 1 reaches TDC, one during a compression stroke, one not. Related to this, is that when cylinder 1 is at TDC the harmonic balancer is at 0 degrees.

My question is this - is the harmonic balancer at 0 degrees ONLY on the compression stroke or is it at 0 degrees at both cylinder 1 TDC

In the end - with the engine all buttoned up, how do you know if you are at cylinder 1 TDC on the compression stroke?

 

The mark on the damper will align with the "0" mark on the timing chain cover index mark twice; once at #1 TDC and again at #6 TDC. One will be at TDC compression while the other is at TDC Exhaust stroke.

You'll have to remove the #1 spark plug and place your finger over the hole while someone turns the crank. You'll feel air pressure on your finger when the piston is approaching TDC compression. At this time the index marks will be approaching each other. Bump the wrench on the damper bolt until the marks align. That's #1 TDC compression.

 

I was about to reply with the finger method described above!
Another option, if you have not removed your distributor, is to remove the cap and check if the rotor is pointing to the cylinder 1 post, that will be on compression stroke, if facing 180 degrees rotated, then it is on exhaust stroke

 

OP has a 2001 XJ.

 

My bad, so the finger method is the option

 

Only if distributors in correctly.

 

Thanks guys - the reason I needed to know is so that I can install the oil pump drive and CPS. (yes, everything has been moved - this is post rebuild)

Not following that it is at TDC once for #1 and then again for #6. I thought #1 and #6 reached TDC at the exact same time.

I suppose I can take off the valve cover and look at when both valves are relaxed on cylinder #1 but was wondering if there were another way.

 

Piston 1 and 6 should move exactly at the same, when piston 1 is up in compresion stroke, the piston 6 is up on exhaust stroke.

Harmonic balancer is attached directly to the crankshaft, so on the harmonic balancer you will see 0 degrees in both pistons TDC.

Being in cyl 1 TDC is a combination of the piston being up all the way and the intake and exhaust valves being closed for that cylinder (therefore compresion)

Hope I was clear enough (basic english here)

 

That is what I was thinking - so only way to really check is to remove valve cover it would seem?

 

On a couple of my cars I have a lead going to the starter solenoid, that is fastened up on the firewall where it's handy to use a push button switch to jog the starter. Of course the button switch has a clip lead going to the battery positive to jog the solenoid. Just a handy tool sometimes...

 

Not really, by using the finger method (removing the #1 plug and trying to seal the orifice with your finger) you will feel pressure when the piston 1 comes up in compression stroke, and you will not feel much pressure or any at all if it is on the exhaust stroke.

Trust me on this one, it will be a world difference from one stroke to the other, after you do it the first time, it will be cristal clear for you if the #1 is on compression stroke (closed valves) or exhaust (open exhaust valve)

 

Thank you for repeating this! I was SO missing it.

I was thinking only in terms of using it for TDC, totally bypassing the pressure related to both valves closed rather than the exhaust valve being open

 

Glad all this has been explained now. LOL

 

Setting Your 4.0 at #1 TDC


With #1 spark plug removed, turn the engine over clockwise using the 3/4" front crankshaft bolt. You will see the degree marks on your front cover. Mark the 0 with chalk or white-out. You will see the timing mark on the balancer. Mark it with chalk or white-out. While turning the engine over, put your finger/thumb over #1 spark plug hole. As the mark on the balancer approaches the marks on the front cover, if you are coming up on #1 TDC, pressure in the cylinder will push on your finger. If there's no pressure, you're at #6 TDC and need to crank the engine over until you see the marks coming together and you get pressure. Set the mark on the balancer to the 0 mark on the front cover.

 

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HOw can you use the finger method when the a/c compressor is in the way and it's hard enough getting the spark plug out?