The Long Story:
According to the Toyota Factory Service Manual (FSM) the cylinder head bolts on a 2F should be re-tightened every 15,000 miles for the life of the engine as the first step of a valve clearance check/adjustment. This is done while the engine is hot. As mentioned by others, these bolts are reusable and can be re-torqued.
The 15,000 mile valve adjustment schedule is not all about the valves. It is about the head bolts too. Your valves may not need adjusting every 15,000 miles, but your head bolts should be re-tourqued every 15,000 miles regardless of which brand of head gasket you use. The reason for this is explained at the end of this post.
For a 2F, the recommended procedure to adjust the valves (and re-tighten the head bolts) is to:
1. Warm up the engine to normal operating temperature
2. Stop the engine and re-tighten ALL the cylinder head bolts while the engine is still hot. This necessitates removing the rocker assembly to reach the pesky center head bolt, as that is the first bolt to be re-tightened. Re-tighten does not mean the same thing as "tighten". Re-tighten means loosening the bolt slightly and… well, re-tightening it... to spec.
3. Re-install the rocker assembly, then start the engine back up (warm it back up) and check the valve clearances while the engine is idling.
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Whether anyone actually follows these recommendations is another issue, but those are the words from our Land Cruiser God....Toyota
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Installing Cylinder Head Bolts: The First Time.
When you mount the 2F cylinder head to the block, the FSM recommends applying a LIGHT coat of motor oil to the threads and undersides of all the head bolt heads. Heed this advice. A light coat of oil is not a bolt dripping in oil. If a head bolt is coated with oil so that it is all nice and wet, and then inserted into the head/block assembly, the excessive oil will contaminate the head gasket around each bolt hole with seeping/puddling oil before the bolt is finally torqued down into the block. It doesn't take much oil on the bolt to cause a big puddle of oil to form around it on the block as the bolt is screwed down into the threads.. expelling excess oil. Also, excessive oil will burn off into carbon over time inside the block head-bolt threads.. gumming up the whole works and messing with your future torque values.
You can observe the oil spewing phenomena for yourself when the head is off. Grab a head bolt. Dip the threads in oil, screw the bolt into a block hole, then watch in fascination (horror?) as all the oil on the threads oozes up out of the threaded block hole and spreads out on to the deck of the block. Revise your bolt oiling technique and try it again so that no oil seeps out.
Can oil on the block next to the head bolts cause a problem? Especially for the center HB? Perhaps. One can only wonder. The Toyota head gasket is supposed to go on dry… and no HG manufacturer that I know of recommends oiling any part of the gasket prior to installation.
Remove a Head Bolt Prior to Re-torque?
Since Toyota recommends re-tightening the head bolts of the 2F when the engine is hot, that means that the cooling system will be under pressure when you are fiddling with a bolt… and since that is the case, it seems less than prudent to fully loosen a head bolt, no less remove it, clean it, oil it, and reinstall it, due to the potential (slight?) risk of allowing pressurized coolant to possibly seep underneath the HG when clamping force is fully released at a head-bolt hole juncture. If coolant actually does/can weep, or if that would be significant is unknown. The FSM does not recommend removing a head bolt prior to re-torquing it, either to clean it or re-oil it after first installation… and apparently there is a reason for this. Also, re-oiling just puts more crap down into the bolt hole that will burn off into carbon.
Maintaining Your Head Bolts:
Each time a head bolt is cycled through a re-torque, by backing it off a bit, then re-torquing it every 15,000 miles, it cleans the bolt by burnishing (polishing) the threads of both the bolt and the flange and the block threads. Re-tightening the bolt breaks free any rust/crap/stiction developing at the contact surfaces and can prevent future microscopic galling.
When the head bolts are regularly "exercised" like this every 15,000 miles, the sliding friction on the threads and under the bolt head is then maintained at a level that has been accounted for in the recommended torque values in the FSM.
The problem arises when we get lazy and stop re-torquing the head after we have owned the car for a few years… since all seems to be running fine.
When we finally do get around to re-torquing the head bolts or removing them to service the head… we need a 5 foot cheater bar slid over the breaker bar to break the head bolts free. If you've heard the loud crack of a "gunshot" when you finally "busted loose" your head bolt to loosen it in this manner, you know what I am talking about. Doing this places extreme stress on both the bolt and block threads and is unquestionably hard on everything. It is an invitation to fastener failure… and well, it's just not right.
Re-torquing the head bolts at the recommended intervals does more than just re-clamp the head to the HG and block. It keeps the head bolt clamping surfaces (ie threads and hex flange) clean, and sliding, which ensures accurate bolt torquing and ease of removal in the future.
Why not follow the recommendation from the FSM? (except for the fact that it is unquestionably a royal PITA)
