I have the rich pleasure of being the repairer of my student daughters Mk 3 Cavalier, which has for its sins blown a head gasket. I have owned and maintained the car for most of its life so am aware of its service history and likely residual life. With this in mind I set too last week to remove the cyl head and review what next, mentally absorbing the financial pain whilst on the job so to speak.
The late 1989 hatch' has done close to 200K but most of this has been motorway driving, oil being changed regularly at 5-6K. Other fluids have also been regularly changed against a maintenance log. The old dog still has some life I would like to think.
Two items have emerged so far to cause me frustration (hopefully shot term)
1
An inboard cyl head bolt, initially loosened, but tightened up and eventually sheared. I have managed since to remove the remainder of the bolt, but the thread in the block is distorted and stretched.
2
Replacement cylinder head bolts are Torx headed (E12) with a 3/8 drive socket.
I plan to Helicoil (with help from a retired specialist) the damaged thread. does anyone have experience using high torques on helicoil inserts?
I also have zero experience with high torques on Torx head bolts (they are male) The tightening instructions in Haynes are for an initial torque of 25 Nm followed by 180 degrees in three stages and a further 45 degrees after warm up. The 3/8 drive socket also seems rather puny (ex Halfords best)
Any feed back would be most gratefully recieved.
Kind regards,
Julian Lindley
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Julian,
Vauxhalls typically used internal tri-square (12-point) bolts that people often mistook for internal Torx with dire results when they tried to undo them. I hadn't realised that the Cavalier used external Torx, however you have made it clear that this is so.
I shouldn't worry too much about the socket being 'puny', though obviously the quality is important. Torx heads are quite kind to sockets as they don't provide much of a bursting force, this being one of their main benefits and is the reason that the heads can be comparatively small.
I assume that the cylinder block is cast iron, in which case I wouldn't have expected there to be much stretching of the threads. Stretching usually occurs only with male threads because it is the bolt that stretches. Internal threads usually shear rather than stretch. Have you considered running a tap down the threads to clean them up? I appreciate that these threads are under a high load from the bolts but it is surprising the loads that threads reclaimed in this way can take.
Helicoils can withstand high torques since they are made of a high-grade steel and are under compression in use, however there are a couple of things to watch out for:
1. Tapping out for the helicoil will eat into the parent metal around the tapping. I suspect that there will still be sufficient but you need to be sure of this.
2. The co-efficient of friction provided by the helicoil is likely to be very much different to that of the original thread and so the tightening torque that needs to be applied to the bolt is likely to be very different also. I think that there is a strong likelihood of taking the bolt beyond its yield limit and into breakage if you follow the specified tightening procedure (i.e. back to square one!). You might need to establish the grade of the bolt (perhaps 10,9 or 12,9) and to also establish the standard tightening torque for that grade and size of bolt under low friction conditions.
As you can see, using helicoils in this situation is quite tricky and rather risky so I would try to avoid this if at all possible. I would try running a tap down and then tighten the Torx bolt in the specified manner -- there's little to lose and I am quietly confident that this will answer your problem (though obviously I can't be sure without seeing the situation for myself).
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Julian, a further thought ...
Before resorting to running a tap down the thread, you could try cutting a slot with a hacksaw blade longitudinally along the thread of a bolt and screwing this in to clean up the tapping in the block. I once did this to clean up a damaged spark plug tapping and it worked well. It has a slight cutting action but also cold-forms the correct thread profile as it goes in. Best used with a good drop of oil.
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Julian
Excellent info from Dizzy. As you probably realise the tightening sequence indicates these are stretch bolts, which are tightened into the plastic region. Because of this these bolts should not be reused, as the correct gasket pressure will not be achieved. Regretably, a set of replacement head bolts is another cost you'll have to bear.
Regards
John S
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John,
I agree that there is a strong possibility of needing new bolts but I believe that this can sometimes be avoided. The manufacturers sometimes quote a check length for the bolt which, if met, shows that they have retained their elasticity and may be re-used if undamaged. The Haynes manual should show this if it applies to the Cavalier engine.
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Haynes manual for Cavalier:- they advise using new bolts.
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Julian,
I have just realised that if you need to use helicoils (which I still think are best avoided in this situation) the hazards are not as bad as I described.
You would need to be quite careful during the first stage, when tightening to the specified torque of 25 Nm, because this is when the difference in co-efficient of friction could let the bolt turn too far or too little. So long as you watched that the 'dodgy' bolt turned by about the same amount as the others during this stage there should be no problem. After all, this stage is only to settle the assembly down and the task will be shared by all of the bolts.
The rest of the tightening procedure relies on 'angle of turn' rather than 'resistance to turning' so friction should not enter into the equation here. You may find the 'rogue' bolt to be easier or harder to turn than the others but this shouldn't matter so long as the angle it is turned through is correct.
I'm sorry I didn't think of this before -- and a little surprised that our more expert friends didn't spot it! I think I've got it right now but would welcome comments from the real experts.
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After all, this stage is only to settle the assembly down and the task will be shared by all of the bolts.
In hindsight, that sentence looks a bit flippant. The 25 Nm stage is, of course, very critical in that it sets the start point for the angle tightening procedure!
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Dizzy, John,
Thank you for your substantial efforts on my behalf, they are much appreciated.
Only four of your messages had arrived before I closed down my PC. I retired reflecting on coefficient of friction bolt tightening v's angular bolt loading, and my wife asking me why I was so preoccupied! By dawn, I had formed the initial view that the influence of thread friction re the "Haynes" tightening procedure prescribed was minimal. Thankyou for confirming this in your later posts.
My original post gave no elaboration on my reasoning for the use of a Helicoil. The cylinder head bolt that sheared was adjacent the pot where the gasket failed, so I suspect that thread corrosion is implicated. Testing this thread with a new bolt, I found that it was surprisingly easy to screw to its full depth. Unfortunately, I was able to lower and raise it, perhaps by as much as 0.5 mm, and I noticed that on its last turn the float disappeared. Almost certainly, the original bolt did not engage this last turn of the thread. Unwilling to dismantle the engine twice, and again replace the cyl bolts, (The final bolt tightening is carried out with the engine warm) I am reluctant to experiment with the original thread, and my preference is thus for a Helicoil.
I am surprised with what seems like a stretched female thread in cast iron. Any views?
I am also a happier man to know that the E12 Torx socket for the cyl head bolts is likely to be up to the task. I asked for the best quality Halfords could provide!
Many thanks again for your help and viewpoint
Kind regards,
Julan Lindley
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Dizzy
You beat me to it by minutes! It had also occured to me that being stretch bolts, the usual inacuracies due to varying thread friction inherent in torque tightening are avoided. The angle of rotation ensures the correct stretch of the bolt, which is why these 'stretch' bolts are used in prefernce to 'normal' bolting techniques. I'd have no concerns about a heliccoil being used.
To be pedantic, these bolts lose their elasticity in use. I believe the longer studs used in K series engines can be used more than once because they have a longer section which distorts plastically. The shorter bolts have only a small stretch section which is why they should be replaced. Reuse will put the bolt too far along the load/extension curve and the correct gasket load will not be achieved.
Regards
John S
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I think all manufacturers recommend using new bolts when removing , replacing the head. I think to do otherwise would be a false economy.
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