|The MGA With An Attitude
Cam Timing and VALVE HEAD CLEARANCE -- CM-201A
CM-201 is a more advanced course for the problems of cam timing and valve clearance from internal engine parts. This is important if you increase compression or use a high performance camshaft. Sorry about the glare on the dial indicator here. Maybe I'll get a better picture later.
Here the cylinder head is sitting temporarily on a new head gasket and secured (gently) with just two head studs at the front and rear of the head. At the side I have installed one coarse threaded bolt in place of the center head stud. This bolt has the head drilled and tapped with a 1/4-20 coarse thread to accept the vertical shaft of the dial indicator stand. By loosening just the one clamp on the left the indicator can be repositioned to reach any valve on the cylinder head. What appear to be valve springs here are in fact temporary soft springs with only a few pounds of force, allowing you to depress the valve with your finger to check the travel to any point of internal mechanical interference. The long travel dial indicator with fixtures, soft springs, a degree wheel for the crankshaft, and a piston stop for accurately finding top dead center are all parts of a Tune-A-Cam tool kit from Crane Cams, Inc.
The picture above shows the indicator set up on the #1 exhaust valve with the valve closed (up). Since the valve will be traveling downward during operation the indicator must be preloaded to at least half of its maximum of 1" total travel. The picture immediately below shows the same valve fully open (down). The difference between the before and after readings on the indicator is the total travel (lift) of the valve.
Now the picture below shows my fat finger depressing the rocker arm until the valve hits something to check the amount of clearance for the valve head. In this case the valve head will hit the top of the engine block just a short distance beyond where it attains full lift. For a stock MGA engine (not twin cam) with all original type parts this will be about 1/8", but this depends on the condition of the valves, the valve seats, thickness of the head gasket, and how much the head has been shaved over the years. If you shave the cylinder head, this clearance diminishes. If you install a high lift cam, this clearance diminishes. If you install high ratio rocker arms, this clearance diminishes. At some point you may (will) discover that you do not have sufficient clearance left for safe operation of the engine.
It is recommended that you have at least .080" clearance for the intake valve and .100" clearance for the exhaust valve between the valve head and the piston at the point of closest approach. For an MG engine with dished or flat top pistons this is no problem, as the clearance there is about .400" in stock form, which will accommodate lots of extra valve lift and head shaving. The problem with these engines is that the valves, and the exhaust valve in particular, overlap the side of the cylinder bore and will come down on top of the engine block if they open very much farther than the original amount. Here the minimum clearance from the block at full lift should be at least .060" for the intake valve and .080" for the exhaust valve.
You can see that it doesn't take a whole lot of modification to the head, rockers or camshaft to get there in a hurry from the original 1/8". If you just increase the cam lift by 1/16" at the valve you may already be in trouble. For the solution to this valve head clearance problem, just continue on a couple of pages to the review of adding "eyebrows" to the top of the engine block for added clearance for the valves.
The page immediately following here shows the installation of the degree wheel and pointer used for checking and adjusting the cam timing.