Determining top dead center

{{youcanedit}}1938 ford flathead

VERIFYING TOP DEAD CENTER AT THE DAMPER/POINTER WITH THE MOTOR ASSEMBLED.

First off, the large round hub on the front of the crankshaft is called a harmonic damper by some and a harmonic balancer by others. If it is bolted to the crank of a 350 Chevy that is internally balanced, then it serves only the function of being a harmonic damper, dampening vibrations set up in the crankshaft as a result of the rod journal springing back and forth from the forces applied to it during operation. Unless the motor has been unaltered and is "as built" by the manufacturer, you have no idea if the TDC notch on the ring matches the timing pointer attached to the block or front cover, even if the outer ring has not slipped at all.  

There are a multitude of different dampers and three different timing pointer locations on a Chevy. I don't know about other brands. Chevies use 12:00 noon, 2:00 O'Clock and 2:30 O'Clock and if anyone has had the motor apart, you don't know if they used the original damper and timing pointer or other swap meet parts when they went back together with the motor. The whole reason for doing this operation in the first place is to be able to time the motor with a timing light and know, absolutely, that the timing is correct. The elastomeric material that connects the outer ring of the harmonic damper to the inner hub which presses onto the snout of the crankshaft begins to break down over a long time due to ozone in the atmosphere and oil and fuel which may find its way onto the material. When this happens, the outer ring may slip in relation to the inner hub, rendering any attempt to time the motor with a timing light futile. Even though this operation you are about to do will bring the timing marks back to correct for the time being, there is no guarantee that the ring will not slip further after a while. If you want to bulletproof the operation, then start with a new or rebuilt damper and use the correct timing pointer for that damper.  

In this exercise, I suggest that you install a piston stop tool into #1 spark plug hole (driver's side front on a Chevy) with the Factory TDC notch on the damper ring to the right of, or past the timing pointer so that you can turn the crank clockwise through the whole operation. If you install the stop before the damper ring notch is at TDC (with the damper ring notch to the left of the pointer) and then turn the crank backwards (counterclockwise) until the piston comes up against the stop again, you risk loosening the bolt in the crank that retains the harmonic damper. Then you have to get your buddy to crawl under and hold a large flat-blade screwdriver into the flexplate/flywheel ring gear teeth to keep the motor from turning while you tighten the bolt back. Far better to just turn the crank clockwise only (keeping the bolt tight) and sidestep a problem before it occurs. Been there, done that.

Before starting this operation, measure the outside diameter of the damper and buy a timing tape which matches the diameter of the damper. Also buy a piston stop tool. Purchasing a tool that has a hole drilled through the center of the probe will allow pressure or vacuum to escape through the hole with the piston moving up and down in the bore with the rocker arms disabled (valves on their seats) and will make the job easier.  

http://store.summitracing.com/egnsearch.asp?Ntk=KeywordSearch&DDS=1&searchinresults=false&y=5&N=+115&Ntt=timing+tape&x=22

http://store.summitracing.com/egnsearch.asp?Ntt=piston+stop+tool&x=30&y=5&searchinresults=false&Ntk=KeywordSearch&DDS=1&N=700+115

You'll need to get everything out of the way so you can work on the harmonic damper ring. The perimeter of the ring has a notch cut in it which indicates top dead center when aligned with the pointer on the front cover or block of the motor. Remove the fan, belts, shroud and water pump pulley. You may have to remove the water pump to gain full access to the damper ring and do the measuring needed for this operation. This would be a good time to replace the pump anyway. Pumps and gaskets are not that pricey. Whether or not you replace it, removing it and installing a Flow Kooler impeller plate might be a good move. Summit lists them only for a Mopar, but they wouldn't be that hard to fabricate from sheet metal and install with rivets on a stamped steel impeller.

Space the pump backing plate out with another gasket or two to prevent contact at the rivet heads.  

Anyway, back to the task at hand. First use solvent and then hot soapy water to thoroughly clean the harmonic damper and timing pointer. Remove the valve cover for #1 cylinder and back off the rocker arms for both valves for that cylinder. COUNTING THE NUMBER OF TURNS YOU LOOSEN THE ROCKER NUTS WILL MAKE IT A SNAP TO RE-ESTABLISH THE CORRECT LASH WHEN YOU TIGHTEN THEM BACK AFTER THIS OPERATION. Disabling the valves by backing off the rocker arms will circumvent any interference between the timing tool probe and the valves while turning the crank. Turning the motor over by hand will be easier if you remove ALL the spark plugs. With a socket on the damper retaining bolt and a long socket handle, rotate the crankshaft clockwise until the notch on the damper ring has gone past (to the right of) the timing pointer on the block or front cover by about 1 to 1 1/2 inches. At this point, we are not concerned with where the crank/piston is in the total 720 cycle of operation. In other words, we have the valves disabled, so it doesn't matter whether you are on the exhaust cycle or the compression cycle as the piston comes up to TDC. All we are working with at this point is the 360 degrees of the damper, regardless of the cam and valves. Install the piston stop tool housing into the #1 spark plug hole. Insert the probe of the tool into the housing and screw it in until you feel resistance of the tool probe against the piston crown. If the damper ring is twisted so far out of phase that the piston stop tool will not contact the piston at this position, then use a strong flashlight and peer into the spark plug hole. Have a buddy turn the motor over clockwise and watch for the piston coming up to top dead center. After the piston gets to top dead center and starts down the bore, stop and install the piston stop tool. Snug it down against the piston crown and start from there.  

Affix a 4 to 5 inch length of masking tape to the damper ring with the left end of the masking tape about 1 inch to the left of the timing pointer on the timing cover, positioning the masking tape toward the block-side edge of the damper ring, leaving room at the front edge of the ring to affix your timing tape later. With a ball-point pen, make a thin mark front to rear on the masking tape right at the point of the timing pointer on the timing cover. Rotate the crankshaft clockwise (here's where you'll be glad you purchased a piston stop tool with a hole drilled through the center of the tool probe) until the piston comes up against the timing tool probe again. Easy does it here, you don't want to bring the piston up against the tool so hard that it will dent the piston crown. Make another mark on the masking tape with your ball-point pen. With your 6 inch caliper, measure the distance between the two marks you made with the pen. Divide this distance in half. Move the jaws of the caliper to show this half distance. With one caliper jaw on one of the marks you made with the pen, the other jaw of the caliper will be at true top dead center. Make another mark on the masking tape at the caliper jaw to show this center (or middle) position on the masking tape and affix your degree tape onto the ring, aligning TDC on the timing tape with the center mark you made on the masking tape. Remove the piston stop tool housing and probe. Tighten the rocker arms back the same number of turns you used when you loosened them. With your buddy holding his thumb over the #1 spark plug hole, rotate the crankshaft until he feels compression against his thumb. Continue to rotate the crank slowly until the timing pointer aligns with about 10 degrees before top dead center on the timing tape. DO NOT ROTATE THE CRANKSHAFT ANY MORE. LEAVE IT ALONE FOR NOW. Replace spark plugs and wires. Remove the masking tape, reinstall water pump, pulley, fan, shroud and belts. Replace any coolant/water you may have lost in the operation. Remove the cap from the distributor and align the rotor with the cap terminal that coincides with #1 plug by rotating the distributor housing. On a Chevy, clockwise rotation of the housing retards the ignition timing, counter-clockwise advances it. From #1 plug position on the cap, the wires will be plugged in clockwise around the cap from there and will go in the firing order (Chevy 1,8,4,3,6,5,7,2). The rotor should be pointed to the 5:30 O'Clock position to fire #1 plug. This will give maximum clearance between the vacuum cannister and the runners of the intake manifold so that you have room to twist the distributor to fine-tune the ignition timing when you get the motor running.  

Set the valve cover on the head to keep oil from flying everywhere and fire the motor to let it warm up an little. Do final adjustment on the #1 cylinder rockers. (Chevy, back off the rocker 'til it audibly clatters, then tighten it down 3/4 to one full turn). If you want to go ahead and run the valves on the rest of that side of the block, now would be a good time. Replace the valve cover.

Hook up your timing light to #1. Remove the vacuum advance rubber hose from the vacuum canister at the distributor and plug the end of the hose with a golf tee or other suitable plug. Adjust initial timing at the crank to what you want by rotating the distributor housing. Some use the factory setting while others prefer to set it a little more advanced for good throttle response. If using a little more initial advance at the crank, make certain you don't have so much mechanical advance in the distributor that you exceed the total timing (initial and centrifugal) specified for the motor. Most small block chevies will run best with around 32-34 degrees (initial and centrifugal) with fast-burn heads and 35-36 with conventional heads. Ignition curve kits are available to customize the centrifugal advance curve in order to limit total advance when using more advance at the crank.  

There, you're done. You should feel much better now, knowing that when you time the motor with a light, it's dead nuts on the money.