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MG MG Y Type - YA Road Trip - Melbourne to Brisbane

Well it seemed like a good idea - two and a half days for 1800 miles of motoring - but it didn't last.

110miles out of Melbourne we blew the head gasket - the previous mechanic had fitted the gasket back to front !!. So 6 hours later & a new head gasket we were under way again, cruising around 55MPH and the temp gauge now steady on 175F & 25MPG.

However, after another 220 odd miles, one of the valves in number one cylinder embedded itself in the piston at 4,000rpm & the fire went out for good.

So we flew home today and the car comes back on a truck next week.

I suspect we will never find out why the valve head fell off - any advice from those who might know would be appreciated.

The engine had had a full rebuild by the PO, but maybe not a "good" rebuild.
A L SLATTERY

Hi Tony

Bad luck- try again ... third time is a charm?

Suspect that maybe the collets were not properly fitted as to why the valve dropped maybe?

Paul
Paul Barrow

I have started the engine strip down.

Removing the rocker cover and rocker gear has identified it was the #1 exhaust valve that failed - the head of the valve has separated from the stem.

If I get the head off later today, I will post some pictures if anyone is interested. It's not the only project in the garage at the moment !.
A L SLATTERY

Well the head is off - no real surprises.

The head of the exhaust valve is embedded in what's left of the piston.


Looking at the failed valve stem, it's been modified to take the XPAG collets, so it was obviously not an XPAG valve that failed.
A L SLATTERY

To follow up on this saga, the valve remains have been examined by a few wise souls and it appears to have failed due to becoming very, very hot. This was a result of the head gasket being installed backwards, and of course the #1 exhaust valve suffered the worst.

There are signs of impending failure of the #2 exhaust valve & 3 and 4 have "heat" marks too.

Lucky for me the block had been sleeved back to standard and the is no damage to the bore or con rod - a new piston, four new exhaust valves and a gasket set should be all that is needed.

Back to the garage !

A L SLATTERY

Mystery is slowly being revealed Tony!

Paul
Paul Barrow

Update - the piston I had hoped to fit is an original 3 ring design, while the remaining three are "Specialoid" 4 ring jobbies.

Even if the 4th ring is removed from the Specialoids, I can only get the weight of the replacement piston down to 17grams (heavier), which is not good enough.

I machined 100thou from the skirt which accounted for 4 grams, but I don't think I can take another 400 thou (10mm) off the skirt. I has a 21gram overweight to start with, even after taking the 10gram lighter gudgeon pin from the holed piston.

This will be a daily drive car, not a revved engine, but reliability is everything to me so taking a total of 12.5mm off the piston skirt is going too far I think.

So I'm now looking for a full set of matched pistons or one S/H "Specialoid". Standard bore - no oversize.

Anyone got any better ideas ?.
A L SLATTERY

Further Update !!! - Success.

After a good sleep and dreaming about the problem, a solution was reached.

Yesterday I had one piston that was 17 grams heavier than the other 3. So unable to take all 17 grams from the heavy piston, I decided to re-fit the 4th ring to the 3 light pistons - 11 grams saved - only 6 more to find.

By using original piston pin clamp bolts instead of new cap screws in the 3 light pistons, and a cap screw in the heaviest piston - saved another 2 grams - 4 to find.

By grinding an internal chamfer in each end of the piston pin - 1 gram - 3 to go.

So taking another 30thou off the piston skirt (180 thou total - 4.5mm) - 3 grams saved - correct weight !.

All the rod & piston sets are now within one gram - (1027grams)- good enough for this little black duck !.

The pistons and rods are now back in the block, and I am starting the tedious process of aligning the castellated nuts on the main & con rod bearing caps, before lock wiring the lot.

Happy Days !.

PS - further investigation of the failed valve has shown it was actually a non-BMC inlet valve - not enough meat below the head to dissipate heat in an exhaust installation which contributed (along with the back to front head gasket fitting) to the head falling off & destroying the piston.

PPS - There was also one piston pin bolt working loose, which would have thrown a rod out the side of the block, AND the main bearing cap nuts were only split pinned and were winding the studs out of the block. Had the valve failure not stopped the engine, these last two would have soon after in a BIG WAY !.

So sometimes bad luck can save you from something even worse. Now if I can find the previous builder of this engine, I think we need to talk !!!!!!.
A L SLATTERY

This thread was discussed between 06/08/2013 and 19/11/2013

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