Broken piston ring? (longish)

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

 SFH3L wrote:

Ian,
When you fitted the pistons, did they have circlips to retain the gudgeon pins?  I rebuilt my M30 2800cs engine years ago with a set of Hepolite pistons (I was only 16!) and all ran well for a while. I ran the engine in well, but was having a high speed blast, cruising between 110 and flat out for about half an hour with an XR4i (in period) and had a minor disaster.  Light thrashing noise from the engine, which made me back off and stop as soon as I could.  From then on the engine had the same ticking of which you speak.  Most noticeablein the 1500 - 3000 rev range.  I can't speak for higher speeds, as I stayed slow till I took the head off.
My car looked OK on a dry test, but show problems (also pot 6) on a wet test.  Turned out one of the circlips had broken, and the head with the eyelet had found its way down the bore, past the rings.  Rings were not actually broken, but had a nick out of them, and the bore was scored.
I think for peace of mind you are best pulling the head again.  
Do you notice appreciable blow back when you take the oil filler cap off with the motor ticking over?  Mine was quite bad, and the tickover was a bit lumpy.
Hopefully yours isn't such a big deal, but on such a nicely done motor, I'd park it up and not drive till sorted.  
On the plug tracking - wait till evening, when the light is fading.  Pull the bonnet and spay the head gently witha little hand sprayer (like a windolene bottle).  It's just enough moisture to show anything wong really well.  Start her up, and if there is a tracking problem, you'll see a lightning storm till the heat clears it.  
Good luck......

Sam, thats great info.  Noise is actually more prominant in the 1500-3000 rpm range!  Pistons gudgeon pins had clips replaced with new items when the JE pistons were installed.  I do remember though that these were a pain to fit.  Maybe I damaged/weakened one when installing them.
I am beginning to think that it may well be little ends/ rings that have let go.  I don't think it has gone in a big way otherwise I would be picking up signs on the compression test.  I also did a vacuum test with a carbtune vacuum tester (for a 4 pot).  This showed up absolutely nothing no matter what I did with the revs!  Similar to the compression test really.
I'll try the trick with the HT leads but I am convinced this isn't the problem as the noise is metallic and is being amplified in the exhaust manifold for cylinders 4,5 and 6.
The car looks likely to be laid up for winter now.
Removing the head and piston on number 6 seems the sensible way forward.
Time is now to be taken up with another addition to the collection.  An original 62k mile Triumph Dolomite Sprint with a fantastic body but in need of some mechanical modification! (e30 m3 engine would be nice.........)
Thanks for the input.
Ian 