Timing tabs and damper TDC lines SBC

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by: Cobalt327
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[edit] Overview

There are several different dampers and timing tabs. Both the diameter, thickness and the placement of the line on the outer ring of the damper can differ, as well as how the timing tab is positioned. So you need to know what damper you have, to know what tab to use.

[edit] The three most common locations for the timing mark on the damper

The exact years of the type of damper timing marks overlap one another, depending on the exact application.

  1. The pre-1969 damper has the TDC line on the outer ring at the 2:30 o’clock position- or 2º before the keyway centerline- i.e. the line is to the LEFT of the keyway, looking at the front of the damper or engine. The keyway is seen in the ID of the damper nose.
  2. The damper used from 1969 to about 1984 has the TDC line at the 2 o’clock position- or 10º before the keyway. You'll find that aftermarket dampers are the 10º type, as are the bolt-on tabs sold by the aftermarket, unless they're adjustable.
    Warning Note: This includes the SBC 400, although the 400 damper is counterweighted because the engine is externally balanced, and the 1985-present 4.3L V6 with its respective 6.75" damper (raised outer lip) that has both a 2 o'clock and a 4 o'clock position (for some vans eg. G-series, Express, and Astro/Safari). Do not mix and match internal and external balanced dampers!
  3. A third timing mark was used from (some) 1978 to about 1995, and nearly all 1984-1995 (exc for 1985-present 4.3L V6 and van applications). It is at the 12 o’clock position- or 40º before the keyway. This damper uses a timing cover that has the tab welded on at about the 12 o'clock position - the timing covers with this setup had either the 6.75" (305 and pre-1985 90 degree V6) or 8" (350/5.7 only). Professional Products lists the years for this type damper line as being 1984-1995 and is a 6-3/4” diameter damper.

[edit] Relocating the damper line

Damper with original "12 o'clock" TDC line re marked for a "2 o'clock" timing tab

In some cases it might be necessary to relocate the TDC damper line, like to use a different tab position so the tab can be seen easier due to the placement of the front accessories. This can also be done simply to use whatever timing cover/tab that is on hand.

The photo shows a damper that was originally for a 12 o'clock timing tab- notice where the original TDC line is located. The damper line was relocated by hacksawing a shallow groove into the damper at the correct location to indicate TDC using the new tab. The new line can be easily highlighted with white paint, etc. so it can be identified as the correct line to use when timing the engine.

[edit] Checking the timing tab/damper for accuracy

If you're unsure, or just want to check the accuracy of the tab and damper mark for TDC, DETERMINE TDC and MAKE A TIMING TAPE will help you to see for yourself.

[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. Over time the line may be seen to have separated as the outer ring has moved independently of the inner hub.


[edit] Vortec plastic timing cover vs. steel cover

Below is a side-by-side shot of a plastic cover from a 1997 Vortec engine and to the right of it is a '85-back style steel timing cover for comparison.

Vortec plastic timing cover, left; steel cover, right

[edit] Some differences are:

  • The plastic cover has more room around the crank gear. This cover was equipped with a crank sensor, so the additional room may be to make room for the crank position sensor rotor.
  • The steel cover is deeper, from inside the front to the sealing surface.
  • Generally, there seems to be more room around the inside of the steel cover, even though the photos don't look that way due to how the covers are shaped. This is except for the area around the crank gear.
  • The plastic cover has fewer bolt holes at the top- an older steel cover would have holes that are not used on the Vortec-type block. The Vortec-specific holes could be put into the older block as long as care was used to position them correctly using a transfer punch.
  • The plastic cover has male pins or dowels to locate the cover onto the face of the block; the steel cover has holes in the same position for the dowels that are used in the older blocks.
  • The plastic cover bolts are shouldered to prevent over-tightening the cover.


Vortec block, casting number 880

The older style block used dowel pins (male pins in block, holes in tin cover), while the Vortec plastic cover has the male pins (larger diameter than older blocks dowel pins) in the same location.

This 880 block has all the holes present and tapped; the two top holes are not correct for the tin cover, though. The dowels are absent, instead there are holes.

The dowel holes in the 880 block (used as locator holes for the "pins" on the plastic timing chain cover) are >15/64" and <1/4" (right for a press-fit 1/4" dowel pin). There is a chamfer at the mouth of the holes that made them look a bit larger.

The hole in a tin timing cover is ~17/64- so there's ~0.015" of play.

[edit] Part numbers

  • The p/n for plastic cover with sensor hole is: p/n10244600
  • The p/n for plastic without sensor hole is: p/n12552557

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