So I did the darn thing and replaced the head gasket on my '88. Fired her up and she sounds great and everything seemed to be working perfect until I noticed some oil burning off somewhere on the back of the head. Figured it could be some residue from the project but after further inspection I'm leaking oil via the head gasket in the back.

I was extremely thorough during the job, taking lots of time to meticulously clean and prep both surfaces so I'm not really sure what went wrong.

The only thing I can think of is that my machinist recommended I use a head shim as he took .004" off the head during the machining process. I was a bit hesitant about the shim but the machinist assured me they work just fine so I followed the instructions to the T using spray copper gasket sealer on the shim, putting the shim on the block, then head gasket & head as per usual. The Shim was specific to the 4Y which is why i was more comfortable using it, I know the failure could be due to several different reasons but I wanted to see what people here thought, if anyone has used a spacer shim before, and if I am to re-do the head gasket job again (which I'm willing to do if I get some confidence) what my best plan of action is to avoid the same problem.

Thanks in advance for your advice!