Tech tip - checking timing top dead center (TDC)

[lumpy]

Sent: 7/23/2008 6:55 AM

I have been chasing a bad running 440 and when I checked the timing it was set at 23 deg. when it should be at 8 deg.
I suspected that the distributor was off a tooth so i set out the check top dead center. It was not as suspected and it right where it should be.

Following I will describe how to check it without sticking a foreign object in the spark plug hole.

get an old plug and break off the insulator on the bottom of the plug also score and snap off the insulator cap where the spark plug boot contacts the spark plug. now drive out the insulator from the bottom side and remove.
you should now have a plug with a hole through the insulator that is left.
get a piece of long 2-3 foot clear tubing that will fit tightly over the top of the insulator
remove the plug from the #1 cylinder and screw in "test" plug.
get a small glass jar and fill it 1/2 full of cooking oil and place the open end of the tube in the oil.
Get a breaker bar and the correct socket (1 1/4 in my case) rotate the crank clockwise slowly to get to T.D.C. you should be able to see either bubbles or the tube pushing down the oil in the tube cavity, keep rotating until the oil in the tube begins to be sucked up not down. check the timing chain tab and note where the timing mark is. now rotate the crank counterclockwise and the oil should be pushing back down, rotate until you see it begin to be sucked up. look again at the timing mark in relationship to the tab and top dead center should be the difference of the two reference points.

You may need to do this a few times to get the feel of it , but this method will get you close without sticking anything down the spark plug hole and will verify if your chain or distributor has skipped a tooth.

[denisondc]

Sent: 7/24/2008 4:33 AM

Ill add my usual comment to a good tech tip. Be sure to lubricate the centrifugal advance mechanism with a drop of oil each 10,000 miles - reached by pulling the rotor off the top end of the distributor shaft. Be sure the centrifugal advance is 'free' by twisting the rotor with your fingers. It should turn easily a few degrees (counter-clockwise in 440's and 413's, and clockwise on 318's) and letting it go. It should spring fully back. It will probably only turn 7° to 8°, as this amounts to 14° to 16° degrees of advance seen by the the crankshaft.

[bluebird]

Sent: 7/25/2008 8:58 PM

The only correct way to find tdc is with a positive piston stop. It can also be made with an old spark plug. Drill out the hole you made , so you can get atleast a 3/8 bolt through it. Weld a nut on it where you broke off the insulator, then screw a long bolt with jam nut on it. Screw it in a couple of inchs. Remove all plugs so the engine may be rotated easier. Rotate engine clockwise till it stops, put a mark on the balancer. Rotate the engine counter clockwise till it stops, put a mark on the balancer. True tdc is 1/2 way between the marks.

[brians1969]

Sent: 7/26/2008 5:08 AM

With the bolt method you just mentioned:
1. You have no way of being sure you are on the end of the compression stroke (which is where you have to be for tdc).

2. "Screwing the bolt in a couple inches" means the piston may hit the bolt somewhat before the top of the stroke. You may say that it doesn't matter because you are reversing and coming around to the other end of the stroke....but I wouldn't you have a big enough gap between the 2 lines you made (1/2 the pulley) to make knowing exact (timing mark) tdc rather difficult?

brian

[bluebird]

Sent: 7/26/2008 8:24 PM

TDC is TDC no matter what. TDC on #1 cyclinder is where engines are timed at no mater what manufacture we're talking about. Using a positive stop is the only way to find true TDC if you are tring to see if a balancer ring has slipped or if the chain has jumped on an engine. There is a lot more to it if your are suspecting that the chain has jumped. You then have to check intake valve center line. That's how cam shafts are installed.

Those marks made will only be less than 2" apart. The timing mark should be half way between the marks you made. If not the balancer ring has slipped and the balancer will need to be replaced. If the ring slips and is not corrected the snout of the crank shaft will break off.
Chuck

[MotorPro]

If you have that big a space back off the bolt a little and do it again.   The oil method is ok to tell if you are within a tooth but only a positive will do to degree a cam. or remark a damper.