[Tynan918]

First time swapping out a sbc timing chain set... '84 SBC 305...

I'm down to installing the new set, but due to timing setting inspection, and not realizing it, I turned over the crankshaft without the timing gears in and don't know if the crankshaft is on TDC anymore.

The camshaft is at TDC, was at #1 TDC (Both timing gear dots up at the noon position), rotor points at cylinder #1, and both valves are up (closed)...on compression stroke.

How can I realign the crankshaft back to TDC ?

 

[2001Blazer4x4]

Dot on the crank gear goes straight up at TDC for either cylinder 1 or cylinder 6. Which cylinder is at TDC on compression stroke depends on cam position. So dot on cam gear then goes either straight up or straight down, whichever position has both the #1 cylinder valves closed. Then distributor drops in pointing at #1 (or should be there if not removed). With timing chain installed turn engine 2 revs and make sure all dots are still lined up with a line going vertically through the crank and cam.

Sent from my moto g power using Tapatalk

 

[johnsongrass1]

Align the gears dot to dot and you'll be TDC on the firing stroke for number 6.
Align the gears with the dots at 12 oclock on both gears and you'll be at TDC firing stroke for number 1.

neither way is wrong, if you do dot to dot, turn the crank 2 full turns to get back to cylinder 1 TDC on the firing stroke.

 

[BogiesAnnex1]

The most reliable way is with the crank dot at noon and cam dot at 6. This has them pointing at each other which makes seeing any alignment error very clear to you. In this position the cam is timed such that number 6 would be firing (so the rotor would be pointing at number six’s cap terminal if the distributor was installed and aligned) and number one is on overlap both number six and one pistons are at TDC just doing different things six is at firing for the power stroke and one is on overlap preparing for the intake stroke.

From the alignment as stated above since the crank turns 2 revolutions for one of the cam. One more revolution of the crank returns it to the TDC marks again. With the timing case open you will see the crank gear dot return to the noon position with the cam gear dot also being at noon, this position now has number one ready to fire the power stroke while number 6 is now on overlap preparing for its intake stroke.

Number one and number 6 are exactly 180 degrees cam/distributor rotation apart out of 360 degrees of cam rotation in a complete cycle of 4 strokes per cylinder. 4 strokes of the cylinder/piston requires two revolutions of the crank shaft so there are 720 degrees of crank rotations to 360 degrees of cam rotation, 2 turns on the crank to 1 of the cam to do 4 strokes. Set up the way the factory assembles the distributor there are some subtle giveaways on the SBC. With the rotor pointing at the number one cap terminal, that terminal points at the number one cylinder, at the same time the hose nipple of the vacuum advance canister points at the number 6 cylinder. Those Chevy engineers were pretty clever at writing pirate treasure maps. If you follow the map the treasure is right in front of you. The other totally cool thing they did was to have the distributor shaft rotate clockwise as the crankshaft does, like the makeup commercials on TV say “it’s easy peasy”.

Bogie

 

[curtis73]

These guys are right. Without a cam, the crank doesn't care which TDC it is - exhaust or compression. The only thing that tells the engine which TDC you're on is the cam.

Do check your valves though. If they weren't moving but the pistons were, you may have caused some carnage.

 

[The Factory Man]

Align the gears dot to dot and you'll be TDC on the firing stroke for number 6.
Align the gears with the dots at 12 oclock on both gears and you'll be at TDC firing stroke for number 1.

neither way is wrong, if you do dot to dot, turn the crank 2 full turns to get back to cylinder 1 TDC on the firing stroke.

Turn the crank one turn.

 

[johnsongrass1]

you got me

 

[BogiesAnnex1]

you got me

Sounds like the opening line in Stevie Ray Vaughn’s song DFW.

Bogie