the "0" on the flywheel

Kneale Brownson kneale at
Thu Sep 14 12:07:47 EDT 2006

The problem with using the vibration damper as a marking point is that it's
subject to change.  The outer rim is separated from the part attached to
the crankshaft by the rubber.  So SJ's tricks only apply to one-time usage,
not as a reference for the future.

At 11:34 AM 9/14/2006 -0400, SJ wrote:
>Couple of tricks to finding accurately the "real" TDC
>1. Find compression stroke for cyl #1
>Find #1 plug wire on Dist Cap, take off cap
>Turn engine till rotor lines up with #1 plug wire position on Dist. I think
>dist has a mark for this.
>Back off engine, put finger over spark plug hole, turn engine in correct
>rotation, verify you have compression
>2. Back off engine
>- place measuring stick, dowel, whatever into sparkplug hole and let it rest
>on piston top
>- turn engine by hand, go past TDC(stick goes up and then down) and turn
>engine backwards  . .make sure measuring stick does not bind
>- Back off engine
>3. Find or make a reference point for the measuring stick
>- piece of sheet metal (tin can)bolted to valve cover stud will do the trick
>4. Locate visual TDC
>- rotate engine till measuring stick stops moving
>- Mark Vibration Damper Pulley so you can get back to this visual TDC
>reference later
>5. Back off the engine
>- rotate engine in correct rotation till the mark you placed on damper is
>about 1 or 2 inches before the reference.
>- Put a new mark on the vibration damper pulley
>- mark the measuring stick where it intersects the reference point (see 3
>6. Turn engine in same direction while watching measuring stick rise to TDC
>and then start to descend
>- turn engine very slowly till measuring stick aligns with the mark you put
>there at step #5
>- Put a new mark on vibration damper
>7. You have 3 marks now on vibration damper - BTDC, TDC, ATDC
>- measure distance between the two outside marks using a flexible or cloth
>- put new mark at the half-point of the measured distance
>    This Mark is your Actual TDC
>You must eliminate any slack(play) in the system to get accurate results.
>You always have to start at the "end point of slack".
>Always make all measurements by backing off the engine and then turning
>engine in proper direction. This puts all engine components at their one
>'end point of slack'.
>All measurements are taken by backing off the engine and then rotating
>engine in normal rotation  - this eliminates any error due to slack(play) in
>If you have too much time on your hands and dont know what to do, you can
>finesse this to get a more accurate reading of TDC.
>Once you have found TDC per above instructions, you can now use the
>hydraulic pump pulley for a more precise TDC location. The hyd pump pulley
>is smaller than vibration damper pulley so it turns maybe twice for every
>rotation of the damper pulley. In other words, the hyd pump pulley has more
>resolution than the damper pulley.
>1. Place damper pulley on TDC mark you found before
>2. Mark hyd pump pulley for visual TDC - make yourself a reference point
>that is over the hyd pulley and almost touching it
>3. Back off engine until the measuring stick in spark plug hole goes down
>below the BTDC mark on stick.
>4. Rotate engine in correct rotation till measuring stick BTDC mark lines up
>with reference.
>5. Mark hyd pump pulley where it intersects with hyd pump pulley reference
>6. Rotate engine in correct rotation till you go past TDC and the measuring
>stick mark again lines up with the reference.
>7. Mark hyd pump pulley
>8. Measure distance between hyd pulley outside marks
>- halve the distance
>- mark pulley
>You now have a more accurate, but temporary, TDC reference on the hyd pump
>You can now compare the three TDC marks(damper, flywheel, hyd pump pulley)
>against each other.
>85 Dodge PU, D-250, 318, auto
>85 Audi 4k - - sold but still on the road
>88 Audi 5kq
>90 Audi 100q
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