Determining top dead center

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by: 41willys, Alittle1, Cobalt327, Crosley, Jon, Pepi, Techinspector1
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[edit] Before you begin

One of the first things that you will need is a degree wheel, this will tell you EXACTLY where you are at when you turn the engine over. You can take this picture of the degree wheel and have it blown up to a reasonable working size by your local photo copy center. A couple of ways to use it is to have it laminated or glue it to a piece of sheet metal or aluminum.


Degree wheel download

Another option is to make a timing tape as described below.

[edit] Verifying top dead center at the damper line/pointer with the engine assembled

[edit] The damper

First off, the large round hub attached to the front of the crankshaft is called a harmonic damper by some and a harmonic balancer or simply "balancer" or "damper" by others.

If it is bolted to the crankshaft of an engine 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.

If it is bolted to the crankshaft of an engine that is externally balanced, then it serves the dual purpose of damper and balancer.

Unless the engine 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 timing pointer locations on a small block Chevy. Refer to Timing Tabs and Damper TDC Lines SBC for more on them. Other engines can refer to a service manual.

The whole reason for doing this operation in the first place is to be able to time the engine with a timing light and know absolutely that the timing is correct. The elastomeric material that connects the outer inertia ring of the harmonic damper/balancer to the inner hub of the damper/balancer which presses onto the snout of the crankshaft begins to break down over time due to ozone in the atmosphere and oil and fuel or other foreign materials which may find their way onto the material. When this happens, the outer ring may slip circumferentially in relation to the inner hub, rendering any attempt to time the engine 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. You also have no idea if the timing pointer matched the inertia ring in the first place if the engine has been disassembled and reassembled by someone else in its lifetime. If you want to bulletproof the operation, then start with a new or rebuilt damper and use the correct timing pointer for that damper.

[edit] Checking the outer damper ring for movement

Damper line.jpg
Draw a sharpie line as shown in the image below. When the timing light is pointed at the TDC line, the sharpie line will also be seen. By revving the engine and running it at different speeds, if the outer ring is loose, the line on the outer ring will be seen to move independently of the line on the inner hub.

[edit] Rebuilt dampers

One noted place to buy a rebuilt damper/balancer is Damper Doctor. They disassemble stock, OEM production dampers, clock the hub to the inertia ring and reassemble the unit with new elastomeric material pressed together under tremendous hydraulic pressure. An 8" damper for a 350 Chevy can be had for a mere $32.95 (ca. 2012), deal of the century!!!!

The option is a used damper/balancer that may be clocked worse than the one you have or an aftermarket damper/balancer that will cost more money and may not have been correctly machined on the inner hub diameter. Some of these offshore (Chinese) dampers being sold are bored either oversize or undersize for the production crank snout diameter. The damper/balancer hub MUST BE A SNUG PRESS-FIT on the crank in order to properly transfer harmonics from the crankshaft to the damper/balancer hub and on to the inertia ring, where harmonics are dissipated.

On a street engine or a drag race engine down to 11.00 E.T. in the quarter mile, an OEM-type damper/balancer may be used legally. At 10.99 E.T. and quicker, an aftermarket SFI-18.1 damper/balancer is required. Blower motors normally do not use a balancer/damper, but instead, use an aluminum toothed hub on the crank snout to drive the blower. On these blown motors, the large Gilmer drive belt functions as a dampener to dissipate crankshaft harmonics.

[edit] Damper suppliers

[edit] Damper fasteners

[edit] Piston stop

If the engine is a short block on the stand, you can determine TDC with a simple homemade piston stop made from a strap of metal bolted across two head bolt holes, with the strap drilled and tapped for an adjustable bolt/nut assembly. A dial indicator can also be used on a fixture that bridges the bore or on a magnetic base. This would be an ideal time to note the piston-to-deck clearance for use in computing the static compression ratio and quench distance.

If the engine has the heads on, use a spark plug-type piston stop tool. If there is a timing tab present, use it to mark the position with. If no tab, use a length of stiff wire that's attached to the engine to use to show the positions. This may be made easier by using a degree wheel or a timing tape on the outer ring of the damper.

  1. Set the plug hole-mounted piston stop to contact the piston close to TDC
  2. Rotate engine until the stop just contacts the piston- mark the location
  3. Then rotate in the opposite direction until the piston is stopped
  4. Half way between the two marks is approximately TDC
Strap-type and spark plug-type piston stop tools

[edit] A word on using the damper retaining bolt to turn the engine over

If the damper/balancer retaining bolt is used to turn the crank over, the bolt can loosen if turned CCW. If this happens the bolt will need to be retorqued (60 ft/lbs on a SBC, 85 ft/lbs on a BBC).

Another risk is in stripping the threads or breaking the bolt off if you use the damper bolt to turn the engine over. If you feel resistance, or if the bolt tightens more as you attempt to turn the crankshaft- STOP! You will need to use a remote starter switch, or turn the damper by hand (plugs out), or turn the flexplate/flywheel from beneath the vehicle.

The safest/easiest way to turn the crank is to use a tool that allows a large ratchet or breaker bar to be used.

[edit] Getting started

Attach the degree wheel or refer to How To Make A Timing Tape or buy a timing tape that matches the diameter of the damper. Also buy or make a piston stop tool. Using a tool that has a hole drilled through the center of the probe will allow pressure or vacuum to escape through the hole from the piston moving up and down in the bore with the rocker arms disabled (valves on their seats). This makes turning the engine over w/the piston stop installed easier.

You'll need access to the harmonic damper outer ring. If necessary, 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.

If this is necessary (removing the water pump), this would be a good time to replace the pump if there is any question as to its condition. Pumps and gaskets are not that pricey. Whether or not you replace it, remove the pump and check for impeller slip on the pump driveshaft by holding the impeller securely with one hand and the drive hub of the pump with the other hand and twist in opposite directions. If there is any movement, replace the water pump before it fails completely. Even if the pump is good, you may want to replace it with a high-flow unit. FlowKooler, Stewart and Edelbrock are names that come to mind, there may be others who produce a quality high-flow pump. A good high-flow pump is nearly MANDATORY on a 400 SBC or any other engine which uses siamesed cylinders.

[edit] Water pump suppliers

[edit] The procedure

Thoroughly clean the harmonic damper and timing pointer.

[edit] Procedure done with valves closed

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 GET NEAR TO THE CORRECT LASH WHEN YOU TIGHTEN THEM BACK AFTER THIS OPERATION.

Disabling the valves by backing off the rocker arms will prevent interference between the timing tool probe and the valves while turning the crank. Turning the engine over by hand will be easier if you remove ALL the spark plugs.

With a socket on the damper retaining bolt or using the tool described and a long socket handle, rotate the crankshaft clockwise while your buddy uses a strong flashlight to look into the #1 spark plug hole.

Bring the piston up to top dead center, then continue rotating the crank very slowly so that the piston comes down in the bore slightly. You want the piston slightly past top dead center down in the bore, but not so far down in the bore that the probe of the top dead center tool will not contact the piston crown.

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.

[edit] Procedure without removing valve cover

If checking an assembled/long block engine, you will install a piston stop tool into #1 spark plug hole (domestic V8 firing orders). Screw the top dead center housing into the spark plug hole and snug it down. Insert the probe of the tool into the tool housing and screw it in until you feel resistance of the tool probe against the piston crown. Snug it down slightly against the piston crown and start from there.

Attach a degree wheel or 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. 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 piston stop 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 to coincide with the tip of the timing pointer. 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. If the timing tab and damper line agree with this, they're accurate. If not, the tab can be readjusted or another line made on the damper to coincide with true TDC.

Remove the piston stop tool probe and housing. If the rocker arms were loosened previously, 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. STOP. DO NOT ROTATE THE CRANKSHAFT ANY MORE.

Replace the spark plugs and wires. Remove the masking tape, reinstall water pump, pulley, fan, shroud and belts if they were removed previously. 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. Looking down on the motor from a birds-eye view, the rotor needs to be pointing toward 5:30 on an analog clock face (pointed toward #1 cylinder). The gears that drive the distributor off the camshaft are helical, so you need to drop the distributor into place so that it can spiral down and be in the correct position when the distributor housing is seated on the intake manifold. On a Chevy, clockwise rotation of the housing retards the ignition timing, counter-clockwise advances it.

On a Chevy V8, because of the pitch of the helical gears on the cam and distributor, the rotor will turn about 2 inches CW as the distributor is lowered into position. That means the rotor needs to start out about 2 inches CCW from where it needs to end up with the distributor fully seated.

From #1 plug position on the cap, the wires will be attached clockwise around the cap following the firing order (Chevy V8: 1-8-4-3-6-5-7-2). As was stated in the last paragraph, the rotor should be pointed to the 5:30 o'clock position to fire #1 plug. This will give maximum clearance between the vacuum canister and the runners of the intake manifold and firewall so that you have room to twist the distributor to fine tune the ignition timing when you get the engine running.

If the valve cover was removed earlier, set the valve cover on the head to keep oil from flying everywhere and start the engine and allow it to warm up. Do the final adjustment on the #1 cylinder rockers.

  • Chevy: back off the rocker until it audibly clatters, then tighten it down 1/2 to 3/4 turn past where it quiets down. If you want to adjust the valves on the rest of that side of the block, now would be a good time. Replace the valve cover.

[edit] Timing the engine

Hook up your timing light to #1 plug wire.

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 engine. Most small block Chevy's will run best with around 30-34 degrees (initial and centrifugal) with fast-burn heads and 35-36 degrees with conventional heads.

If using a camshaft with more duration, you may want to increase the ignition timing lead at the crank and limit the centrifugal advance in the distributor to achieve your total ignition timing. Some distributors, like those sold by MSD, use different bushings to control the amount of mechanical advance. The rate of mechanical advance is tailored by changing the springs and/or centrifugal weights. Usually just spring changes are all that is needed. If working on a GM HEI, the original weights are almost always a better choice than the weights sold in the various advance curve kits available from Mr. Gasket, Crane, Summit, Moroso, etc.

If you are using a radical cam and/or a converter that allows the engine RPM to come up past where you would normally limit centrifugal advance (about 2800 rpm), you may want to modify the distributor so that centrifugal advance is locked out and put your total amount of ignition advance in at the crank. Of course, the engine will not want to crank against this much ignition lead, so you will want to install a momentary switch in the wire going to the "+" terminal of the coil to disable the coil while you crank the engine. Once the engine is spinning, release the switch and the engine will fire normally.

[edit] Resources

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