|The MGA With An Attitude
VALVE HEIGHT in Cylinder Head - CH-113
On 5/29/2009, someone (don't know who) wrote:
"Took the 53's head to the machinist to get the valve work done. He asked, "What's the valve stem height"? It seems that when you install new valve seats you need to know just where the tip of the valve stem is supposed to wind up when finished. Any tips"?
This is sort of a screwy question, for a few reasons. First I suppose a competent engine machinist should know such things well enough to not need to ask an inexperienced customer how to do the job. Second, I have never seen this dimension specified in any workshop manual. By comparing dimensions of parts you could calculate or measure to figure it out, but it is not supposed to be a normal set up dimension.
Third is, I have no idea what that dimension might be, not even for my MGA. This is why we have adjuster screws on the rocker arms, so it can tolerate a fairly wide range of height for the valves as it goes through cycles of wear and repair over many years of normal use. I would have to pull the valve cover to measure things to find out this dimension on my MGA. Reason I don't know is because it has always been NOT under my control, and generally irrelevant to form, fit and function of the engine. Even now I don't suppose I ever want to know that dimension.
As the valve heads and seats wear with time the valves moves higher in the head, and we progressively back off adjustment of the rocker arm to maintain correct valve lash. If the valve or seat become excessively worn it might run out of travel for the adjuster screw. On one or two occasions I have used a hand grinder to take a smidge off of the tip of a valve stem to regain enough valve clearance. That's about the time it may be due for machining to install valve seat inserts and new valves to restore original dimensions and original valve height).
In general, when the seats and valves are renewed the valve head should stand slightly proud of the surrounding surface of the combustion chamber. (maybe 0.030" high) This allows for some normal wear of the seat and valve head, leading to the valve head going down about flush with the surrounding surface of the chamber. If/when the seat might wear enough to make a new valve head sit below flush, then it's time to install seat inserts to replace the worn away material.
Meanwhile, as the valve changes height over the years it is supposed to be important to keep the spring height fairly constant in order to have correct spring force on all of the valves. Factory specifications in the workshop manual usually do give the correct dimension for spring height in assembly. If the valve seat becomes excessively worn the valve moves higher, and the spring gets longer. To restore proper spring height and spring force it is common to install a shim under the valve spring to restore correct compression height on the spring. When the valve seat might eventually be restored the spring shims can be removed. Spring shims are also sometimes used to compensate for slightly tired springs that may have lost a bit of their original height to age settling.
I don't like to be mean or derogatory to anyone, but if a machine shop would ask me for the valve height dimension, I think I might be looking for a different machinist. It has been my experience that a machinist does not like the customer telling him how to do his job, and will usually not be asking the customer for advice. If it is an odd old engine, and the shop may not have the shop manual or data for that engine, then they might ask for bit of data, or they might ask the customer to supply or lend them a copy of the workshop manual. It's just that this particular question about valve height strikes me as a very strange question to be coming from a professional engine machinist. The more pertinent questions would be valve head position in the combustion chamber, valve spring height, and valve spring force.