How to re-torque a partially loosened overhead camshaft?

Question

While doing a timing belt service on my 17 year-old, 57K-mile truck (DOHC), I was having a very hard time getting it to let go of the front oil seals on the intake camshafts.¹ I eventually realized that, since I was also needing to replace my leaking valve cover gaskets, I might as well just crack loose the front bearing caps on the intake cams to release the compression that was holding the oil seals tightly in place. (This idea seemed to be supported by the Factory Service Manual (FSM), which only ever discusses installation or removal of these oil seals during removal/installation of the camshafts.)

That worked as expected, but now I have a dilemma: what is the correct process to retorque these bolts?

At first I told myself "self, according to the FSM, this bearing cap you loosened was the last one in the torquing sequence for this shaft. All you did was rewind the sequence one step, so you can just retorque what you loosened and be done with it."

But I can think of another argument: there is now 57K of wear on those camshafts, which wasn't present when the factory worker torqued them down. That means that stresses could have built up in the intervening 17 years, and by loosening just one of them I have allowed those stresses to redistribute. If I just torque down what I loosened, I will be captuing those redistributed stresses where they currently are, and possibly causing a bearing to get bound up. (Those well-oiled bearing cap bolts also felt quite a bit tighter than the 12 ft-lbf that the FSM calls for. I suppose that could be stiction, but it could also be representative of the built-up stresses pulling on the bolts laterally.)

After considering that, I had another fear: this engine has two separate cams for each bank of cylinders: one for intake, and one for exhaust. They are kept in sync via gears that mesh between them. And according to the FSM, it is actually the exhaust camshaft that gets torqued down last, not the intake camshaft. So one could make the argument that I didn't loosen the last bearing cap in the sequence, but rather the one in the middle of the sequence. Depending on whether stresses can transfer laterally from intake cam to exhaust, I suppose the correct thing to do might be to loosen all bearing caps on all camshafts-- intake & exhaust-- and retorque them in the originally-spec'd order.

How should I proceed?

¹ Those original oil seals looked fine, and you could easily argue I should have left them alone, after just 57K original miles. My counter argument to myself was: (1) the old ones were 17 years old & took a lot of effort to get to, and (2) nobody makes that same if-it-ain't-broke-don't-fix-it argument with the water pump, which usually gets changed along with the timing belt. But truthfully, I only considered changing the seals in the first place because new ones just happened to come with my OEM timing belt service kit.

Answer 1

There is now 57K of wear on those camshafts, which wasn't present when the factory worker torqued them down. That means that stresses could have built up in the intervening 17 years

I don't see how stresses would build up anywhere in the top end. If anything, bolts would lose stress through thermal fatigue via thousands of heat cycles (but even that doesn't happen appreciably with steel bolts).

and by loosening just one of them I have allowed those stresses to redistribute

No, not when loosening.

I will be captuing those redistributed stresses where they currently are, and possibly causing a bearing to get bound up.

No, see above.

Those well-oiled bearing cap bolts also felt quite a bit tighter than the 12 ft-lbf that the FSM calls for. I suppose that could be stiction, but it could also be representative of the built-up stresses pulling on the bolts laterally

Almost certainly this is due to static friction.

according to the FSM, it is actually the exhaust camshaft that gets torqued down last, not the intake camshaft. So one could make the argument that I didn't loosen the last bearing cap in the sequence, but rather the one in the middle of the sequence.

The degree of interaction between the camshafts will depend on how much the camshafts deflect via the gears.

Depending on whether stresses can transfer laterally from intake cam to exhaust, I suppose the correct thing to do might be to loosen all bearing caps on all camshafts-- intake & exhaust-- and retorque them in the originally-spec'd order.

I would encourage you to err on the side of caution when in doubt.

Maybe some other person with intimate knowledge of the Toyota 5VZ-FE engine has a more "done-it-before" sort of answer, but in the absence of such the conservative approach should be your default.

Answer 2

edit: I'm sorry I thought you had removed the cam. It doesn't matter in what order you torque the bearing cap bolts. The purpose of the order is to ensure that the cam and bearing caps are not moved unevenly. If the bolts are already partially torqued, then it should be fine to re-torque them in any order. But I will keep what I wrote because you can damage the bearings by loosening too many bearing caps without following the correct order.

You must keep the bearing caps in order. They should be numbered. I believe the arrows on them point to the timing belt side. You're supposed to keep the camshaft level while installing it. You do this by having it push the valves on two cylinders that have one cylinder in between down equally. Then you tighten the bolts for the bearing cap that is in between them 1/4 turn at a time on each of the two bolts until it goes all the way down. This keeps the camshaft level as it is pushed down. Then put on all the other bearing caps. Do not tighten only one side of the bearing cap if it is under load from a valve spring! Get everything snug and then torque them to spec. This is how Toyota does it. Toyota are non interference. The pistons must be out of the way on an interference engine! You may have to turn the intake cam to get the exhaust cam to where it can push down two valves equally.

If you released the cam on one side while it was bolted down on the other side by just one bearing cap, in theory it could have damaged the bearing on that side.