XJ6 & XJ12 Series I, II & III 1968-1992

Yet another cylinder head question

 
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Old 09-21-2011, 04:32 PM
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Default Yet another cylinder head question

Okay, I'm having some confusion, most likely due to the fact that this is my first DOHC head.

My Haynes manual says quite clearly to no move the crankshaft once I have disconnected the camshafts.

It then goes on to say that I should move the pistons to TDC a number of times before reinstalling the head. I seem to recall that this requires turning the crankshaft.

Help?
 
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Old 09-21-2011, 04:49 PM
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Ben, I am assuming that you are attempting to either replace a head or install a new timing chain. This is the trick with the situation that you are in. If you are replacing a head, I would tell you to put the block (with the head removed) in the #1 cylinder TDC position. Then, with the head on a bench, set that to near where it should be for #1 cylinder at TDC. Now, install the head and do your final tweaks to get the match marks lined up to get the timing correct.

If you are doing the timing chain, I would have rotated the engine so that #1 cylinder is at TDC using a socket and breaker bar (make sure to rotate the engine in the correct direction) and then dismantle everything. Then slight movements of the camshaft is all that should be needed to restore the timing.

This is the trick with the engine. If the engine is an interference fit engine (ie, there is a spot inside the cylinder where the valves and the piston occupy a common space), then you want to make sure that the valves are not open when that cylinder is at TDC. If you do do this, damage to the valves will occur and you are pulling the head off to replace valves. So, knowing where the individual cylinders are is key. That way you don't have valves open when the piston for that cylinder comes to TDC.

I normally try to set the block up prior to removing the timing chain such that I know where the match marks are and that they can be looked at easily. On most motors, the match marks are normally set up where they all point straight up or at each other (depends on the engineers mind on that day). I don't know your block specifically, so, hopefully another member can chime in on that.

Once you get the crankshaft positioned, you can feel free to rotate the camshaft up to about 90 degrees in either direction form the point that the timing chain has tension on the camshaft. My experience is that when you release the timing chain, the camshafts are going to rotate slightly to go to some balance position. so, keep this in mind.

With the heads off, you can turn the crankshaft all you want. The only caution that I would tell you is to put a little bit of light machine oil in each cylinder to provide a little bit of lube to the rings so you don't damage them. After that, you can crank on the crankshaft all you want.
 
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Old 09-21-2011, 07:00 PM
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Thanks, Thermo. I'm replacing the head gasket, an possibly rebuilding the head, based on what I find in there.

Could I safely read this as "don't turn the crankshaft after disconnecting the sprokets *while the head is still in place*"?
 
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Old 09-21-2011, 08:44 PM
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Ben, yes, you can read it that way. But, at the same time, limiting the rotation of the cam shaft is key too. You can move it, just don't move it a lot. If you feel resistance, stop.
 
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Old 09-21-2011, 10:41 PM
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Thanks. I'll obviously do as little as possible, since I hate adjusting timing.

The other question I have is regarding gaskets. I've noticed a pretty big variation in prices. Is there a particular brand or whatever that I should lean toward?
 
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Old 09-22-2011, 06:05 AM
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Ben, i have had good luck using FelPro gaskets. Not saying that they are the best, but they have never failed on me either.
 
 
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