I had to replace the seals on both cams due to water loss. The service told me that while undoing one of the screws, the nut in the block went bad and was destroyed.

I am kindly asking for an independent opinion here wether it can happen while unscrewing a difficult block screw or only when screwing and tightening back.

They tried putting a longer screw instead and it would tighten well but the moment the engine reved and ignit, the screw popped out. There seems to be a brain trust hanging over my bonnet in the garage over the past couple of days. What would be the solution in this case?

Hi John, if you mean that they stripped the thread in the block then the usual answer is to replace the damaged thread with a Helicoil insert. This involves using a special tap to cut a new, larger thread and then screwing the Helicoil into it. It's many years since I last used one but I seem to remember that when the Helicoil reaches the end the tang snaps off, leaving you with an as-good-as-new thread.

Derek

Thank you and excuse my inaccurate mechanical English. Would that involve taking the engine out of the bonnet, dissasembling the block and precise drilling?
Besides, I remember seeing a scraped XM block and the screw holes are so close to the edge, there's hardly a place for any larger diameter there.

Engine removal should not be needed because the drill will follow the existing hole.

I cannot advise about the available clearance, that is something for you to decide.

Derek

Problem solved using the helicoil. If that hadn't worked, we'd have to replace the block, said the guy. Anyway, the head is lower a couple of mm now, adding additional HP due to increased compression. I roared this evening on the highway on the way back home and it's swifter and faster than ever before!

Hi John,

Quote: "the head is lower a couple of mm now" does the engine not pink (pre ignition) like crazy ? I think a couple of mm is too much, I wonder how much clearance there is now between pistons and valves... I seriously think that unless you can retard the ignition in some way you're going to have melted pistons very soon.

Just my opinion of course and I would be delighted to be proven wrong and foolish on this

Good luck, Colin.P

No, it doesn't pink and maybe I got the wrong information from tech guys. By the way, your point is very interesting as I fail to see why would there be any pre-ignition you mention. With lower head, the piston travel remains the same, the timing is also the same, it is just the overhead volume which slightly decreases.

Hi John, problem is that you will have considerably raised the compression ratio, this causes pinking because of the extra heat generated (greater compression = higher temperature)- it gets so hot inside the combustion chamber that the petrol/air mix ignites far too early on the compression stroke, i.e. before the spark plug has fired to ignite the mix at just a few degrees before tdc. Instead of driving the piston back down, the piston receives a hefty smack while it is still rising, which, in conjunction with the extra heat starts to erode the crown of the piston and eventually blows through it.
I once proved this point while "home tuning" my old Honda motorbike (teenagers should not be allowed access to toolroom equipment )

Good luck, Colin.P

Right. I didn't think of the diesel engine effect of self-ignition. But I suppose that would happen only in extreme cases of head shaving! How close to the edge were you with Honda?

I can't imagine that any car mechanic or service/tech guy would shave a couple of mm off a cylinder head and get away with it. The tolerance is probably less than 0.5 mm. More is a sure way to quickly destroy the engine, unless the management system is completely re-tuned. Doubt they did that though.

I suspect the fitter skimmed a couple of thou (Thousandths of an inch) rather than a couple of mm.

John

If memory serves me correctly I milled away 0,8mm, a costly mistake - that much I do remember

Colin.P

If I may be allowed to make a slight correction here.

Pre-ignition occurs when the mixture ignites before the normal ignition point and is/was caused by an incandescent particle of carbon in the cylinder head. Once fairly common, it also caused the phenomenon known as "running on" where the engine would continue to run after the ignition was switched off. Improvements to fuel have virtually eliminated this now.

Pinking or detonation is caused by a too high compression ratio or overadvanced ignition. The spark initiates ignition at the correct point but the advancing flame front causes the unburnt mixture to ignite spontaneously. The effect is an audible "pink" noise when, as Colin wrote, the exploding charge strikes the piston before the increased pressure can be converted to work.

This would probably never happen now as the ECU would sense thr onset of pinking and retard the ignition.

Derek

Yes spot on, the ecu would alter the ignition timing to prevent pinking and knocking, generaly if the compression ratio is altered in the combustion chamber and piston crown can be mirror polished to improve heat disipation and help prevent carbon build up within the chambers which could otherwise cause run on etc etc as Derek has already said, but usualy you would change the pistons to increase compression, not hack the head away.
When the cylinderhead is skimmed though it is often considered essential to fit a thicker gasket to make up for the material lost from the head mating surface, the Garage in question may have done just that, but i cant see they would of removed 2mm.

D

Thanks Derek, I knew someone would be able to explain it more precisely and correctly than myself

Question: if the ecu could indeed retard the ignition enough to compensate for the raised compression problems, wouldn't this totally negate any gains in power or torque gained from raising the compression ? not to mention the increased stress to major parts of the engine


Colin.P