Valve clearances on modified camshafts

 In Engine - Modified, MGA-MGB, Midget

The page on valve clearances in the tuning section refers BMC cams. My comments above also assumed normal roadwork, not competition.

Other profiles have the ramps in different positions at different rates, so in the absence of any other data, when setting tappet clearances you have to be guided by what the camshaft manufacturer specifies. Generally, there is a small performance gain by setting the clearance just off the end of the ramp – witness BMC’s spec of 15 thou for A series competition work compared with 12 thou for road work. On the rolling road there have been useful gains by going 1-3 thou further than that.

However, the further it’s opened the faster the wear rate. It’s the old story with competition engines – a bit more go at the expense of a bit more stress and shorter component life. In the case of valve clearances, if you know the clearance puts take-up just off the ramp then you need to check the adjustment more frequently so it doesn’t get too agressive after wear. At 20 thou clearance the take up speed has doubled, and is still accelerating hard.

The biggest risk with higher wear rates is the wear on the cam followers. This has two effects: one is that it changes the follower profile to concave rather than slightly (.002″) convex which has a major (bad) effect on clearance take-up speed (a noisy tappet when set to the correct clearance is a sign this is happening); the other effect is that once through the hardening the pitted surface of a worn follower grinds away at the cam lobe, eventually destroying it. Not a happy job in a 1275! Other engines are not so bad as the followers are more easily removed for checking and replacement if necessary before it gets that far.

For myself, I keep it on the ramp for road work and then open up to just off the ramp for competition. This is less work and expense than replacing followers and cams!

It’s easy to find where the ramps are using a dial indicator and degree wheel, and it’s the first thing I do after fitting a different profile. They usually extend over about 20 degrees or so, so are easy to find:

(I usually do this by sitting the dial indicator probe in the top of the pushrod before fitting the rocker gear and reading cam lift from there. However, if the engine is already assembled it can be done by removing setting negative tappet clearance (ie valve held open a little) and reading off the valve spring cap, remembering that the motion is different from the pushrod end by the amount of the rocker ratio.)

  • Zero the dial indicator with the valve gear on the back of the cam (usual rule of nine – opposite valve fully open)
  • Rotate the engine until the indicator starts to move.
  • Continue to turn the engine 10 degrees at a time, noting the lift from the DTI every 10 degrees until reaching a cam lift at least 50 thou.
  • Calculate the lift in each 10 degree interval. (ie lift at the end of the interval minus lift at the beginning of the interval)
  • Plot these “lift per 10 degrees” figures on a graph (makes them easier to interpret)
  • Typically, the graph will show a small rise, then go flat (this is the ramp – some profiles climb slightly rather than go flat.) then rise rapidly as the opening acceleration kicks in.
  • The turning point between the flat bit and the climb is the end of the ramp. Note the crank degrees reading at this point of the graph then return the crank to that position. The dial indicator reading is the lift at the end of the ramp. (Alternatively you can read this off the lift figures you initially noted)

BMC opening ramps lift just over 3 thou per 10 degree interval at the cam, ( = 4 thou per 10 degrees at the valve for std A series rockers, 4.5 thou at the valve for std B & C series rockers.)
Performance opening ramps can be up to 5 thou per 10 degrees (at the cam).
After the end of the ramp the lift speed increases rapidly over the next 30-35 degrees to a figure a bit over 30 thou per 10 degrees – a 10-fold increase. For standard cams the figure is about 13 thou per 10 deg, for performance cams it’s about 33 thou per 10 deg. The max lift speed is dictated by the follower diameter, too much and the lobe is lifting on the corner rather than the face. At 33 the contact is nearly at the edge, at 36 it is on the corner.

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