Finding TDC 1986 3 litre GMC

[jsurr]

I am reinstalling my cylinder head. The manual descibes how to set pistons at TDC with the points set to each cylinder and open. I have converted to electronic ignition therfore no points. What is the best way to ensure TDC of each cylinder to properly adjust valve lash?

Thanx
Jeff

 

[Fishermark]

Re: Finding TDC 1986 3 litre GMC

Have you removed the distributor? If not, you can turn the engine over with the piston at the top dead center of number 1. It will either be on the compression stroke (which you want) or not. 50-50 chance. To make sure, see if the rotor is pointed at the number one cylinder spark plug tower. With the cylinder at top dead center number 1 - and rotor pointed at number 1... then you are ready to begin adjusting the lash for number one. Once you finish number 1, then you can go from there as described in the manual.

 

[drrm123]

Re: Finding TDC 1986 3 litre GMC

the compression stroke will be when the exhaust valve has just closed and the intake if begining to open. to adjust valves if they are adjustable on this engine. have cam on base circle(opposite side of lobe) to adjust valves when you do one cylinder you can turn every 90 degrees and do the next in the firing order.

 

[jsurr]

Re: Finding TDC 1986 3 litre GMC

I guess that's where my confusion comes in. How do you see the position of the rotor with the distributor cap installed? Or, how do you ensure correct positioning of the rotor with the cap off? Also, how do I ensure the piston is at TDC without being able to see it? Do I assume that if the rotor is pointing to the firing post of the piston I'm working on that the piston is at TDC?

Jeff

 

[Fishermark]

Re: Finding TDC 1986 3 litre GMC

I guess that's where my confusion comes in. How do you see the position of the rotor with the distributor cap installed?

Take the cap off.

Or, how do you ensure correct positioning of the rotor with the cap off?

Taking the cap off doesn't affect the position of the rotor.

Also, how do I ensure the piston is at TDC without being able to see it?

When you said you were reinstalling your cylinder head, I had assumed that you did not have it back on yet. If you do have it on, you will need to align the timing marks to TDC. When you do that, you will still need to find out if you are on the compression stroke or not. That's where looking at the rotor comes into play.

Do I assume that if the rotor is pointing to the firing post of the piston I'm working on that the piston is at TDC?

Yep!

 

[Fishermark]

Re: Finding TDC 1986 3 litre GMC

All of the above information assumes (again, a dangerous thing perhaps ) that you have not messed with your distributor at all. In other words it hasn't been removed or changed in any way. I'm not talking about simply removing the cap, I am asking has the body of the distributor been removed or turned in this process?

 

[chiefalen]

Re: Finding TDC 1986 3 litre GMC

You just pulled the head?

Nothing else?

Whats that got to do with the timing?

You installed the head already?

What manual you using?

 

[Silvertip]

Re: Finding TDC 1986 3 litre GMC

You are making this way too difficult. Since you don't have the head on yet, turn the engine so the timing mark is at Zero. Look down the push rod holes in the block at the cam lobes for number 1 cylinder. If the cam lobes for that cylinder are pointing up, you are not at TDC. Rotate the engine one more rev and stop at TDC. Install the distributor, install the head, install the push rods and rockers. From this point on, pay attention to how you move the crank so you don't lose TDC. When you set the distributor rough adjustment, put a mark on the distributor housing where it matches with the number 1 plug wire on the distributor cap. Now pull the cap. Loosen the distributor clamp and turn it right or left until the rotor points to the mark you just made. That is close enough to get the engine started so you can set the timing with a light. This engine has hydraulic lifters so you are not setting valve lash -- you are actually adjusting the lifter.

 

[jsurr]

Re: Finding TDC 1986 3 litre GMC

Fishermark, I have not moved or loosened the distributor body only removed the cap---didn't want to open that can of worms. I understand that having the cap off does't effect the position of the rotor but then you have no reference to where the ignition post is. My idea is to mark the post position on the body with a marker then remove the cap to observe the rotor positon. Will that work? It still seems to be ballparking the position but will that be close enough.

Chiefalen, I only removed the head and the exhaust manifold. After removing the manifold and looking into the cylinder ports decided to remove and service. I am picking up today and reinstalling this weekend. Just looking for the expert's advice in advance. Nothing to do with timing. The SELOC manual is explaining the method of adjusting valve lash (see original post)

Thanx

 

[chiefalen]

Re: Finding TDC 1986 3 litre GMC

You using seloc manual: omc stern drives 1964-86. # 3400?

Page 3-19 middle of the page. Installing the head.

Page 3-20 top of the page for the lash adjustment.

Where does it mention anything about timing?

Oh 90-100 no more on the torque of the head bolts.

Use a torque wrench.

A drop of oil under the head of the bolt. For true torque.

 

[jsurr]

Re: Finding TDC 1986 3 litre GMC

Silvertip, your right about adjusting lifters. I guess it's a play on words. The SELOC manual task 6-10 describes it as "Valve Lash Adjustment" however the last sentence states "turn the nut one full turn down to place the lifter plunger in the center of its travel." I am only trying to ensure each piston is at TDC to make the adjustment. Except the manual describes using the points to ensure the piston is set correctly. I have electronic ignition, so I was looking for a alternate method to finding TDC.

 

[Bondo]

Re: Finding TDC 1986 3 litre GMC

so I was looking for a alternate method to finding TDC.

Ayuh,....

So use the Timng Mark on the front of the motor,....

 

[MikDee]

Re: Finding TDC 1986 3 litre GMC

Ayuh,....

So use the Timng Mark on the front of the motor,....

Thank you Bond-o, I was waiting for someone to tell him about the timing mark on the crank pulley (vibration damper), matching up with the mark on the engine index tab, as the rotor points to #1 on the Distributor. Remember, it takes 2 turns of the crank to make 1 revolution of the Dizzy. (a 4cycle motor). On TDC of the crank, you can adjust 1/2 the valves, then turn the crank over 1 full turn to TDC again, and adjust the other 1/2, but you need to know which ones, maybe the service book has it listed?

 

[Don S]

Re: Finding TDC 1986 3 litre GMC

To find #1 TDC on the Compression stroke (or firing position) what ever you want to call it.
Pull all the spark plugs. turn the engine by hand (use a ratchet and socket on the altenator) turn the engine in the normal direction of rotation while you have your thumb or finger in the #1 spark plug hole. Turn till you start feeling compression building. Now, with both hands continue turning till the timing mark on the harmonic balancer lines up with the TDC mark on the pointer.
Bingo. #1 TDC on the compression stroke.

Basically, just like it says in the OEM service manual.

 

[jsurr]

Re: Finding TDC 1986 3 litre GMC

Thanx Don S, I'll give that a shot. Once I have TDC for #1 cylinder can I adjust both the intake and exhaust for that cylider? Once I have adjusted those lifter, do I bring #3 (firing order 1,3,4,2) to TDC and adjust both of those lifters and so on? I am certain I can get TDC for #1 cylider. I guess the question is with the other three cylinders.
If it wasn't obvious, this is my first time digging this deep into an engine, so all of your expertise is greatly appreciated.

Jeff

 

[jsurr]

Re: Finding TDC 1986 3 litre GMC UPDATE

Re: Finding TDC 1986 3 litre GMC UPDATE

Well guys I got it all put back together and it's purring like a kitten and my original problem of overheating is resolved. Stays right at 160ish degrees. Thanks for your in put.

 

[MikDee]

Re: Finding TDC 1986 3 litre GMC

Finally, Good to hear!