I investigated further into how a camshaft bearing cap might break when tightening the bolts. I found that the bearing caps must be square with the head mating surface or the caps may bind. If the cap bolts are tightened with the cap off-square the cap may break. This failure can occur any time that the other three caps are removed or loose and a valve spring is compressed placing pressure on the camshaft. The valve spring pressure attempts to lift the camshaft but if the timing chain tensioner is in place one end of the camshaft is held down by the chain. The result is that the camshaft axis is at an angle to the head instead of parallel to the head. When a camshaft cap is tightened it attempts to seat on the journal which is at an angle to its seat on the block. As the cap is tightened the cap can bind and break from the stress.
The situation is most likely to occur with the right camshaft when it is in the correct position for timing. At this position the #2 cylinder valve springs are both significantly compressed placing a lot of pressure on the camshaft.
In most situations it is good practice to torque something long uniformly and in steps. In the case of the right camshaft I believe the risk of distorting the cam due to non-uniform torquing is much less than the risk of breaking a cap. I suggest that hand pressure be applied to the front end of the cam to compress the cylinder #2 springs as much as possible using the leverage of the camshaft length. Then the second from the front cap should be positioned with the other hand and its bolts carefully tightened. Repeat the hand pressure process for the third from the front cap and then return to the second from the front cap. The bolts on a particular cap should be tightened progressively so that the cap gaps stay comparable.
I'm pretty sure this is what happened in my case.
When I
loosened the Caps I did so in the sequence of front to back, rather then the normal torque sequence of middle on out. This may of caused the Cam to lift so that it ended up not parallel to the head but tipped higher in the front then the back due to the fact that the chain and sprocket is holding the back down. However I'm not sure this made a differance.
I was
VERY careful putting the caps back on, trying to keep them square to the head. I also finger tightened all of them before starting the torque sequence. Then, I torqued them in the correct sequence of middle on out, in steps!
Before the cap cracked, all the bolts were
NOT fully torqued, however, the value was close to 25 In/lb but, I believe less then 35 in/lb.
When I left the back cap to torque the front cap, I thought it was strange that the bolt felt to be bottomed out, like they were completely torqued, yet I wasn't finished with the others! When I turned just a little more, this is when I heard the click and saw the crack in the cap!
It is my opinion, that the cam did not start square to head and during torquing put uneven upward pressure on each cap, specially the front. So I probably over torqued the front cap accidentally with my beam style torque wrench. I have a 1/4" drive click style from Harbor fright on order.
One other thing.....I still think this vehicle is jinxed,...... and I still don't know how the cam jumped timing!
Cliff