Hi guys, I’m doing a cam swap. I brought the engine up to top dead center before taking it apart. When I pulled the timing chain cover off, the dot on the cam sprocket was at the top. I thought it should be at the bottom, dot to dot with the crank sprocket. The car ran ok, am I missing something? Thanks.

 

Ths piston is at TDC on both the compression stroke and the exhaust stroke.
At compression TDC, the cam dot is at 12 o'clock.
At exhaust TDC, the cam dot is at 6 o'clock.

The dot-to-dot method (12 and 6) is used as it is easier to ensure the dots are aligned in that position as opposed to both being at 12 o'clock.

 

Thanks. Just wanted to make sure I do the install properly.

 

A four stroke engine reaches TDC twice for every one rotation of the cam. The crank turns at 2x cam speed. This is why the gears are two different sizes.

 

Keep in mind a piston can be at TDC on either the exhaust or compression stroke. Timing chain install TDC references cylinder #1 TDC on the compression stroke. If a cylinder is on TDC compression stroke both of it's rocker arms will be on the base circle of the camshaft. On the opposing cylinder in this case #6, the piston will be at TDC exhaust stroke and the rocker arms will be "teetering" between opening and closing the valves i.e., if the engine is rotated a few degrees either way the rocker arms on #6 will move, the rocker arms on #1 TDC compression stroke will not move. This is how/why distributors often get installed 180° out and the engines cough through the carburetor when cranking.

Take a picture of the gear and chain before installing the front cover so if you second guess yourself you can refer back.

Good luck!!!

 

Just to be clear, I can install cam gear at six and crank gear at twelve, then bring number one piston up to TDC on compression stroke which would put both dots at twelve, then install distributor with rotor pointing to number plug? Thanks for helping this old man!

 

TJ,
Do yourself a favor. Put the engine on TDC on the compression stroke for #1 Then take the distributor cap off and mark exactly where the rotor is pointing. At the same time take a punch or something and mark exactly where the distributor housing is aligned to the block. When you put it back together line up all of your marks and it should be right on the money as far as timing goes.

 

^^^

 

Got it! Thanks.