Top Dead Center

[sdcbowler]

Hey Fellow Members,

Would like to ask a question a two part question concerning TDC. However, to keep things from getting too confusing, I will ask the first part now, and the second part after receiving an answer from the first part. It is just a procedure question, so don't want to get 'All in the weeds'.

Part One: (Stock RB Engine Rebuild) - Regardless of recommended ignition timing, and not being concerned about cam degree, or exhaust/compression stroke, would the subject engine be assembled by vertically lining up the two dimples on the cam and crankshaft sprockets, with the #1 piston at its highest point in the cylinder? No heads installed at this point.

Thanks

 

[pschlosser]

Part One: (Stock RB Engine Rebuild) - Regardless of recommended ignition timing, and NOT being concerned about cam degree, or [which] exhaust/compression stroke [its in at top-most position], would the subject engine be assembled by vertically lining up the two dimples on the cam and crankshaft sprockets, with the #1 piston at its highest point in the cylinder?

Yes, this sounds correct. Line up the timing gears so the the dots are aligned, timing chain installed, #1 cylinder at TDC (on compression stroke)

 

[sdcbowler]

Yes, this sounds correct. Line up the timing gears so the the dots are aligned, timing chain installed, #1 cylinder at TDC (on compression stroke)

Isn't it true that you can't determine the compression stroke until the heads are installed?

 

[pschlosser]

Isn't it true that you can't determine the compression stroke until the heads are installed?

Not really. Reminder: its the cam that controls if the stroke is on compression or not.

From memory:

the upper timing gear is keyed to the camshaft and only fits on one way. And when the "dot" on the gear is in the six-o-clock position, the camshaft, is at TDC on the compression stroke of cylinder #1.

rotate the crankshaft so cylinder #1 is at (more or less) TDC. (to match)

install the lower timing gear with the dot at the twelve-o-clock position, which usually lines up with the slotted woodruff key on the crankshaft.

Pull and reinstall the gears (in the same position) as needed to get the timing chain on there, and you are good to go.

To summarize:

you first put the camshaft at TDC on compression stroke #1, then you rotate the crankshaft moving the #1 piston into the TDC position. Lock it in with the chain.

 

[1972 AAR Cuda]

Isn't it true that you can't determine the compression stroke until the heads are installed?

The heads do not determine compression stroke. The compression stroke is determined from cam to crank relationship. The crank "could fire" once in each 720 degrees of rotation (2 full crank rotations), the cam fires once every 360 degrees (1 full cam rotation). The cam and crank are tied together (rotation/compression stroke) via the timing chain.
The cam determines when the valves are open and closed (specifically during each function - intake, compression, power, exhaust)
The actual firing of the spark plug is a function of the distributor via distributor/cam gear relationship. (This of course only applies to older engines using distributors - not distributorless, computer controlled engines)

 

[pschlosser]

I may be wrong about the lower gear dot lining up on the woodruff key slot.

Note in this image key is at 1:30 position. I just grabbed this image off the net:

 

[Chryco Psycho]

The woodriff key is NOT TDC
the crank does 2 revolutions for each revolution of the cam gear so the crank needs to be at 12 O Clock the cam can be at 12 or 6 O Clock when lined up correctly .

 

[Challenger RTA]

In theory yes. The one thing to understand is the piston is at top dead center for a few degrees. (Dwell) meaning it's stopped there while the crank and cam are still moving across top dead center. The are sprocket and cam key to advance and retard the cam. sometimes changes are ground into the cam.

 

[Chryco Psycho]

It should be degreed when installed to verify the cm position .

 

[pschlosser]

The one thing to understand is the piston is at top dead center for a few degrees.

While this is true, each tooth on the lower gear makes up about 15-degrees of rotation. Finding the right tooth at #1 TDC is pretty easy, even if you don't get it exactly at TDC, get it pretty close, and things line up nicely.

 

[moparleo]

A quick way to determine compression stork with no heads. Install 2 lifters on # 1 ( driver side) cylinder cam lifter bores.
Rotate the cam until the exhaust side lifter goes down.
The next rotation is the intake stroke, then compression,next power, next exhaust.
4 cycles.

4 stroke cylinder cycles, Norton Safe Search

 

[pschlosser]

A quick way to determine compression stork with no heads.

With the camshaft timing gear installed, and the dot in the six-o-clock position, the cam will be on the compression stroke of cylinder #1. That's why the dot is there. All (certainly most) camshafts are this way.

 

[sdcbowler]

Not really. Reminder: its the cam that controls if the stroke is on compression or not.

From memory:

the upper timing gear is keyed to the camshaft and only fits on one way. And when the "dot" on the gear is in the six-o-clock position, the camshaft, is at TDC on the compression stroke of cylinder #1.

rotate the crankshaft so cylinder #1 is at (more or less) TDC. (to match)

install the lower timing gear with the dot at the twelve-o-clock position, which usually lines up with the slotted woodruff key on the crankshaft.

Pull and reinstall the gears (in the same position) as needed to get the timing chain on there, and you are good to go.

To summarize:

you first put the camshaft at TDC on compression stroke #1, then you rotate the crankshaft moving the #1 piston into the TDC position. Lock it in with the chain.

OK,

I understand all that you guys have said. In my reply, I was alluding to the fact that installing the crank and camshafts alone does not indicate TDC on the compression stroke. One must use other information like referencing the lifters, installing the head and feeling air escape from the spark plug hole or watching the lifters rise and fall. Won't the dots would line up twice for each cylinder.....once on the exhaust stroke and once on the compression stroke? Also, thanks for mentioning the 'Dwell". I never knew what that was.

 

[Chryco Psycho]

Yes the dots do line up twice but the cam turns 1 rotation for 2 crank rotations so the cam gear ends up @ 6 oclock firing & 12 oclock exhaust stroke
Yes the piston will stop at bot BDC & TDC for a few * s

 

[sdcbowler]

Yes the dots do line up twice but the cam turns 1 rotation for 2 crank rotations so the cam gear ends up @ 6 oclock firing & 12 oclock exhaust stroke
Yes the piston will stop at bot BDC & TDC for a few * s

Cool. Got it!

 

[HooToTheCore]

No. You only need the cam sprocket dot at 6 and the crank sprocket at 12 and the chain installed to have the piston and valve correctly configured. Ignition timing is determined by the distributor, but TDC only involves the cam and crank positions. Notice that once the chain is installed, rotating the crank dot one rotation will position the cam dot at 12. So, when the dots point to each other you are at TDC (piston #1 compressed).

 

[Chryco Psycho]

Cam timing is NOT controlled by the dist , the cam needs to be degreed to insure the cam is in correct pahse with the crank & no lining up the dots doesn't insure the cam timing is correct .

 

[HooToTheCore]

Cam timing is NOT controlled by the dist , the cam needs to be degreed to insure the cam is in correct pahse with the crank & no lining up the dots doesn't insure the cam timing is correct .

Not sure if you are replying to me, but everything I said is correct for a single cam V8 if the cam sprocket is keyed to the camshaft (which is the case for stock engines).

 

[pschlosser]

Camshaft timing vs. ignition timing. It's easy to get the two confused.

The camshaft controls the timing of the valves, when they open, when they close, by how much they open, and for how long they stay open.

The distributor controls ignition timing, when the spark plugs fire in relation to the camshaft.

So yea, technically, the distributor does not alter camshaft timing, but instead alters ignition timing.

When the upper (CAMshaft) timing gear dot is positioned at the 6-o-clock position, the camshaft it at TDC on the compression stroke #1 cylinder.

When we rotate the lower (CRANKshaft, not the camshaft) to it's timing gear is at the 12-o-clock position, the piston on cylinder #1 should be at it's top-most position, TDC, to match with the camshaft.

The chain keeps them locked together.

Some suggest it's possible the #1 piston or the camshaft may be a hair off, but I'm pushing back on that a little bit.

If the (camshaft) timing IS off (in relation to the crankshaft position), then it has to be off by a whole tooth on the lower gear. That's actually quite a big amount of error, I believe close to plus/minus 15-degrees.

My pushback is a 15-degree error is easy to spot without the need for a precise measurement. If you have the gear to measure the position, by all means, please do and confirm.

But if you don't, and you can rotate things around with the chain installed and they look aligned at the TDC-dots-pointing-to-each-other-thing on cylinder #1, they probably are correctly aligned.

 

[moparleo]

Always use a degree wheel to determine exact TDC. Mark your harmonic balancer. Also good to use the proper sized balancer marking tape if TDC does not match your factory timing mark.