6 replies to this topic

[Jellevandok]

Hi there,

I'm installing a 123 ignition to my 1380 track day car. Since it was a new motor from Smart performance, there was no old distibutor as refrence point. Can anyone tell me how I can easily find the TDC (top dead center)? Which markers can I use?

Thanks,
Jelle

[grahama]

I think easiest way if you can get to the crank nut is to take a spark plug out, drop in a screwdriver onto the piston and turn the crank bolt. When the screwdriver comes up to it's highest point it's TDC. On piston 1 !!

(pretty sure this is one method).

[lrostoke]

The piston will be at the top twice though during the cycle.

As above but watch the valve rockers, when the piston is at the top, wiggle the rockers on No 1 cylinder (nearest radiator) If they both move this is TDC compression stroke and your rotor arm on the dizzy should point to roughly a 2 oclock position.

[Jellevandok]

Ok, thanks. That's what thought, but what are the markers on the crank pully for then?

[lrostoke]

They are also timing marks
I'm only familiar with the 998 set up which is one notch in the pulley , and a set of metal pointers next to the pulley.

When the notch in the pulley lines up with the larger pointer, thats TDC but to confirm its compression stroke you need to look at the valves like mentioned above.

[dklawson]

Be aware that there are reports of the drive dogs on 123 dizzys being 180 degrees out. So... don't be shocked or upset if the rotor points to ... 8:00 o'clock instead of 2:00 o'clock. What is important is that when you find TDC on the compression stroke for cylinder #1, you put the plug wire for #1 on the dizzy cap where the rotor is pointing. Then continue counterclockwise around the cap in the order 1-3-4-2.

I agree with Steve,it is best to bring the piston up to the top of the bore with the rocker cover off so you can "feel" that the rocker arms for cylinder #1 are loose when the piston is at the top. As he explained, pistons 1 & 4 are at TDC at the same time however, one will be at the top on its compression stroke (ready to fire) and the other will be at the top on the end of its exhaust stroke. Checking the rocker arms is an easy way to confirm you have identified the correct TDC... firing stroke for #1. When you reach that position it will be fairly obvious which timing marks line up to indicate TDC.

[mk=john]

Hi
I had to find TDC when I replaced the flywheel and distributor driveshaft after my engine rebuild. The way I did it was as follows:

• Remove rocker cover
• Remove spark plugs
• Turn the engine by hand in its normal direction until number one cylinder is at the top of its stroke. You may need to use a flashlight for this (NOT a screwdriver)
• Continue to turn and observe the piston moving downward.
• If both inlet and exhaust valves stay closed during main movement of the piston going down, then this is the power stroke. This then means the piston position in step 3 was at TDC, aproximately the start of the power stroke, and is therefore when the spark should occur.
• However, if the inlet valve was opening, then this is the induction (intake stroke). You will then need to continue turning the engine until you see what is happennig in step 3.

Again, once this is achieved, you will know that the TDC you just had is the start of the power stroke.

This is where all ignition timing is based, and the orientation of the flywheel is such that the timing marks are on top. When mounting the distributor driveshaft and dizzy itself, the rotor should be poining closest to the number one lead.

Hope this helps

John