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31 August 2000 |
Equipment required - one 0.500”
minimum capacity DTI with magnetic stand, 360-degree protractor and a piece of reasonably
thick gauge wire to use as a pointer.
Terminology –
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DTI |
Dial Test Indicator/dial gauge |
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Protractor |
Degree wheel |
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TDC |
Top Dead Centre, when piston is at highest point in
bore |
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BTDC |
Before Top Dead Centre, before piston reaches
highest point in bore |
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ATDC |
After Top Dead Centre, after piston has reached
highest point in bore. |
Rotate crank so piston one is at the bore top (TDC). Fit
timing gears 'dot-to-dot'. Set up DTI with stem resting on number one piston
centre with a little preload. Rotate crank backwards slightly
(anti-clockwise looking at the front), then gently forwards observing the DTI,
watching where the needle stops before descending. This is TDC. Repeat
this procedure, stopping when the needle reaches its zenith. Zero the dial
scale, and repeat until satisfied the needle shows zero on the scale.
Fit the protractor to the crank using the pulley bolt.
Tighten so that you can just move the protractor. Fashion a pointer from the
wire, held to the block by a water pump bolt and pointing at the scale round
the protractor perimeter. Move the protractor to align pointer and zero, nip
the bolt up to hold firm. Go back and repeat the last paragraph to ensure
pointer is at zero on the protractor when the DTI indicates TDC.
Not essential, but to be super-accurate, reset the DTI
- piston at TDC - with around 0.300" pre-load showing. Rotate crank
backwards until needle on DTI is free of the piston then rotate forward until
needle shows 0.200" BTDC. Note the degree reading on the protractor then
continue rotating crank until DTI shows 0.200" ATDC. Note the degree
reading. They should be the same although obviously either side of zero (TDC).
The actual figure isn't important. If not, move the pointer in the relevant
direction to correct this. Re-check by doing the 0.200" BTDC and ATDC
again. Do it until the readings show exactly the same.
Pop a push rod down onto number one inlet lobe -
second push-rod hole along from front. On small-bore engines make sure you fit
the cam follower first! Reposition the DTI on top of the push rod, again with a
little pre-load. Rotate the crank forwards and observe the DTI. The needle will
swing round a couple of times, slow down, then reach
peak reading (maximum lift) before falling away again. Zero DTI at this peak
point. Rotate crank backwards and forwards a couple of times slowly until happy
that maximum lift is showing with the needle at zero.
Rotate crank backwards until the needle passes the
0.025” mark. Stop and rotate it forwards very slowly, stopping when the needle
reaches the 0.005" mark. Note the reading of degrees indicated by the
pointer on the protractor. Continue rotating the crank forwards until the
needle zeros and returns to the 0.005” mark again. Note the new reading from
the protractor. Add these two figures together, divide
by two, the result being where your cam is timed in at.
The idea is to get this to where manufacturers
recommend your particular cam be set to give optimum performance. If your
reading differs, corrective measures will have to be taken. How this is done
depends on your choice of cam drive.