Jim,
Valve clearance is quite specific to the cam profile and should be set
per the manufacturer's recommendation. Cams are generally ground with a
gentle ramp at the opening and closing ends of the lobe to take up the
clearance at a fairly low velocity then once the clearance is taken up
the velocity ramps up quickly. Setting the clearance too low causes the
valve to crack open way early and close late resulting in loss of low
end. Setting the clearance too high causes the rocker to hit the valve
stem at high velocity causing noise and potential wear.
If you don't have specs for your cam you may be able to get a pretty
good idea of the correct clearance by charting the lift at the beginning
of the lobe. Put a degree wheel on the crank and a dial indicator on a
push rod. Zero the indicator on the cam base circle then turn one
degree at a time and record the lift at each degree. You should see a
point at which the lift starts to change much more rapidly. It will
help the plot the lift on a chart. Excel works great. If you don't
want to go to all the work of a degree wheel and a chart, you should be
able to see the transition point just by watching the dial indicator
while you slowly and steadily turn the crank. If the indicator moves
slowly up to .010" then starts to move much more quickly then .010"
would be a good number for your cam clearance. Take that times your
rocker ratio and you have your valve clearance.
Valve clearance recommendations for a given cam are based on the
clearance desired at the cam times an assumed rocker ratio. Cam
manufacturers often use a nominal rocker ratio of 1.50:1 for the
calculation. If you are using a different rocker ratio then you should
change the valve clearance to maintain the correct clearance at the
cam. For instance, if a manufacturer recommends .015" at the valve
using a 1.50:1 rocker they are looking for .010" at the cam. If you are
running 1.65:1 rockers your clearance should be .0165" to maintain .010"
at the cam.
Richard Good
Good Parts
jim hearn wrote:
>It has been suggested to me that the valves on my '74 TR6 may need other
>than the stock settings since it has a hotter cam. I don't know what the
>cam is but it is definitely a hotter cam. I have only had the car a short
>time so my primary mission is to make sure there are no extraordinarily
>tight or loose valves. May be I could get a feel for the general settings
>and take any tight or loose ones to that settings.
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