According to the service CD, if the HLC is replaced, you need to check the basic position with the guage and install the correct shim. The basic position must be checked and corrected, if the rocker or cam is changed also. All 4: the valve, rocker, cam and HLC; work together to achieve proper opening and closing of the valves.

And yes, you can do just one bank of cyclinders. In fact, you could just replace the failing HLCs. The only reason I can think of that you would replace the entire side is to avoid going at it, a little later if another one fails. It is not a big job replace one. The overhead is quite light, just opening the valve cover, plus maybe a new valve cover gasket will be required if the existing is older than 1 year and the valve cover bolts were not over torqued.

Keep the old HLCs, since some them sound like they are still good.

Also, when you get a valve job, it is normal practise to test the HLCs, before replacing them. I presume they were removed from the head prior to the head going to the machine shop?

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1989 420 SEL