6

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.

  • 4
    FSM = Flying Spaghetti Monster? – DucatiKiller Oct 16 '16 at 17:43
  • 1
    :) In this context, Factory Service Manual. – Ryan V. Bissell Oct 16 '16 at 17:44
2

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.

  • That is a compelling answer, thanks. Which would you personally consider to be the conservative approach: don't touch the other bearing cap bolts, or retorque them all (at least, the intake ones)? – Ryan V. Bissell Oct 16 '16 at 23:31
  • 1
    @RyanV.Bissell re-torquing all bolts would be the conservative approach in this case. It's more work, but more peace of mind as well :) – Zaid Oct 17 '16 at 3:00
  • I had the pleasure this week of paying a mechanic $1300 to replace some IRS axles, so I took that opportunity to ask him for his opinion on this one. He basically said "just torque what you loosened... unless it's a Volkswagen." (I didn't follow up on that exception.) – Ryan V. Bissell Oct 20 '16 at 1:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.