What is the procedure for taking tappet clearance after a major overhaul?

After major overhaul:

• After all the components are assembled and cylinder heads are mounted and tightened, place the rocker arms and tighten
• open the jacket cooling water system and check for leaks
• If no leaks, then activate the jacket water warming/heating system
• Fill the lube oil sump to required level and prime the system
• When the engine attains warm condition, tappet clearance can be carried out

Why JKT water is filled up first and then lub oil?

Answer:  JKT water is filled up first and then tested or checked for any leakage if there is no leakage only then lube oil is filled up because if there is any leakage and lube oil is filled up first then fresh lube oil will get contaminated.