finding TDC

[Gary S.]

I am almost ready to reinstall my distributor after a complete rebuild. I recall that the engine has to be at #1 TDC for the installation. I think I am there but I am getting myself confused. Both #1 intake and exhaust valves are closed and the rockers are loose. I thought that the #6 cylinder has to have both valves closed also. In my case, the #6 intake valve is closed (rocker loose) and the exhaust valve is open (rocker depressed). This is what I assume is the exhaust stroke on #6. I do remember reading that it is possible to have both #1 valves loose and be 180 degrees out.

I thought that I read that during the installation of the distributor, I should rotate the engine so that the timing mark is pointing at whatever your initial timing should be. I can't put my finger on that reference; I am just relying on memory. I also have been told to install the distributor after the timing mark has been set to 0. Confusion reigns.

Can anybody share light on my situation? Thank you.

Gary Schisler

[Clem Z.]

Re: finding TDC

turn the engine over with your finger over #1 spark plug hole and when you feel compression that this TDC #1.set the TDC mark on the balance inline with the 10/12 advance degree mark in the timing tab. mark the distributor housing where #1 plug wire goes into the cap,install the distributor so the rotor tip points to the mark you put on the distributor.

[Gary S.]

Re: finding TDC

You make it sound too simple, Clem. I think I will throw in a few extra steps to ensure that I get it complicated and screw it up. Thank you.
Gary

[Joe C.]

Re: finding TDC

Gary:

Let's see if I can confuse you. The timing mark @zero on the balancer will correspond to BOTH #1 TDC, and #6 TDC. As Clem says, when #1 is at the top of the compression stroke (rotor should be pointed at #1 plug.......facing approx the front of the engine)both valves are closed, so you will feel the compression. There is also a handy dandy whistle device sold for this purpose (might be marketted by Ronco, the seller of our good friend the "Popiel Pocket Fisherman", but not sure). When at #6 TDC, which is also zero on the balancer, then #6 will be at the top of the exhaust stroke, so you will not feel any compression.

You should also rotate the engine both ways, in order to perfectly locate #1 TDC. Right now, however, this is not necessary.

Joe

[Duke W.]

Re: finding TDC

These instructions refer to a C1 and C2 SMALL BLOCKS!!!

Not sure about C2 big blocks (see your CSM). C3s have different wire indexing and distributor clocking (see your CSM).

Prior to distributor installation verify that the dimple on the distributor gear points the same direction as the rotor tip.

At TDC (end of the compression stroke) #1 both valves are closed. Since #6 is 360 degrees away in the firing order it will be at TDC of the exhaust stroke. This is the "overlap period" where both valves should be slightly open. If you rotate the engine 360 degrees to TDC #6, both #6 valves will be closed and both #1 valves should be slightly open.

Set the balancer notch at the initial timing value or 10 degrees BTC if you don't have a definitive spec for initial timing.

Install the dist, so the rotor points about 20 degrees to the right of centerline when the distributor housing seats. IIRC you start with the rotor pointing approximately straight ahead, and the tip will rotate to the right as the bevel gears engage, so index the oil pump slot so it will engage the bottom of the drive gear with the rotor pointing straight ahead.

Once the housing is seated, rotate it until the points just open. You can perform this visually or with a VOM, which should be more accurate. With the distributor shaft indexed and housing rotated as above, the cap window should be approximately perpendicular to engine longitudinal axis, and the vacuum can should be about half way between the coil bracket and manifold interference points. If not, the gear engagement is off a tooth or more, or the distributor gear is not indexed properly.

If you are good at setting the housing so the points just begin to open, the initial timing should be within two degrees of where you set the balancer notch on the advance tab when you check it with a timing light.

Print these instructions out for future reference.

Duke

[Gary S.]

Re: finding TDC

Thank you, Joe.

Joe & Clem, why set the timing mark at the suggested initial timing (say 8-12 degrees BTDC) if you are going to time the engine anyhow? I remember hearing or reading about doing it this way, I just don't understand the theory behind it.

Once again, thank you.
Gary

[Gary S.]

Of course, I forgot

to mention year, engine, etc. All the things that are asked of folks posting here.

Duke et al,
This is for my 1972 LT1. I also have the Restorer article showing the orientation of the vacuum can and how it changes.

Duke, not only am I priniting this out, I am archiving it on my PC.

Thank you.

Gary

[Duke W.]

Re: Of course, I forgot

Yes, I didn't think you were talking about a C2. For a C3 SB the procedure is the same, but you will have to modify all the reference angles as necessary to account for the different C3 wire indexing and housing angle.

Duke

[Duke W.]

Re: finding TDC

You want the engine to start easily, right? The closer you can set the distributor to the correct range of initial timing the easier it will be to start.

If you set it to fire at TDC #1, but you were off five degrees in the direction of retard- which means the actual initial timing is 5 ATC - it won't start very easily.

If you set it at 10 BTC and are retarded by five degrees it will actually be at 5 BTC, and it will start much more easily at 5 BTC than 5 ATC.

I can visually achieve actual timing within 2 degrees of where I want it. Using a VOM I've been right on the money! When I do the job visually I watch to see where the points just open, and then tweak the housing back a hair.

Duke

[Clem Z.]

hey duke what's points????? *NM*

[Bruce Boatner]

Re: Of course, I forgot

Gary,

Unfortunately, I've become pretty good at installing distributors lately (another long story). A good point Duke raised is to be sure that the slot in the oil pump rod is aligned properly with the "tab" in the bottom of the drive gear. If the distributor base will not lay flush on the block and you are a quarter of an inch away, the pump rod is the culprit. Break out your longest flat bladed screwdriver and realign the rod slot to be in line with the FINAL orientation of the rotor.

In my case, the oil pump rod rotated slightly when I pulled the distributor even though I did not disturb the engine. After a few choice words I finally thought to check the pump rod alignment. After a few more choice words, I discovered that the pump rod slot needs to aligned with the FINAL orientation of the rotor (i.e., not the 20 degree offset orientation required when intially stabbing the distributor)

[Gary S.]

Re: Of course, I forgot

My buddy picked up a MOASD (mother of all screw drivers) for me at a local flea market. Three feet long and a blade tip to match. I always use this when turning the oil pump slot.

Gary

[Bruce Boatner]

Another Distributor InstallationTip

Gary

Ha! Ha! I'll have to remember MOASD--good one. This is a case where size does matter.

Here's another trick you can use to ensure the points are in the right position on a freshly stabbed distributor. After lining up the timing mark to the proper position, take the spark plug you removed from the number #1 cylinder to find TDC, and replace it in the plug wire boot. Turn the key to the "on" position. While holding the plug near a grounding source (I use the exhaust manifold) rotate the distributor back and forth noticing the point where you get a spark. Rotate the distributor slowly to this point and stop right where the plug sparks. This method ensures that the points are open right where you want them. Its also more fun than craning your back and neck looking at the points with the cap off. More exciting too as you never know when a mild electric shock may hit you!

Bruce

[Richard S.]

Re: finding TDC

Duke:
Thanks for the install information....I have printed for future reverence. As respects C2 Big Blocks can someone point out what install differences there might be compared to the Small Block installation. Thanks.

[John H.]

Re: finding TDC

Rick -

Same process - firing order is the same, and the distributors are identical.