A Real Puzzler

[IslipMinor]

The picture below shows a small piece of metal on top of No.3 piston - that is clear. What is not clear is how it got there, as it is part of the spark plug earth electrode from No. 4 cylinder!

Last Friday we had a head gasket go in the wilds of Lincolnshire, but it would keep going (ish), so we got ourselves back on to a main road, where the misfire was slightly worse and the water level was down, so we waited for the recovery truck organised by Heritage Insurance, which took us all the way back to Oxfordshire and helped get the car back into the garage up our rather steep driveway.

So far so good.

On removing the head, we could clearly see that the gasket had blown between 3 and 4, but on the edge of No.4 cylinder bore there seems to be a small split. The block has never been linered, so what is the split/crack? Our local engine repairers are completely stumped as well.

Fortunately I have a spare 'standard' bore block, so that is being converted into a 1380, with offset boring etc. The rather burnt Omega No.3 piston is also being replaced - the rest are fine.

So, why did part of the No. 4 plug electrode fail (it is not bent, but has snapped off) and then get into No.3 cylinder? The gap between the head and the block is much too small for it to pass through, which leaves the siamesed inlet ports as the only possible route, but is it really possible?

Need to get it all back together quickly as we are taking part in the MSA Spring Classic, starting near Chester a week on Friday - just 10 days away, including Easter. If I can get the block sorted and all the bits in time, we will be OK to go, but part of the run includes some time on the Anglesey circuit, which does not really suit a newly running-in engine!
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[philthehill]

Richard
Looks as if you used the same plugs as I did on my Suzuki Wagon R in which the earth electrode came away from the plug body, it just dropped off due to faulty welding. The earth electrode was embedded in the top of the piston in the same bore. Strange how it got in the adjacent bore of your engine but these things do happen
As regards your cracking around the top of the bore. Are you absolutely sure that it has not been re-linered at some stage because the crack in your photo looks decidedly like the edge of a liner and you indicate that the block has been off-set bored at some stage but that moves the centre of the bore rearwards. A good indicator of whether it has been re-linered is to look at the bottom of the bore from the underside and in 99% of cases you can see the liner protruding from the bore.
In the back of my mind I seem to recollect that in the later days of BMC/BL sometimes even new blocks had liners fitted but there again I may be mistaken; perhaps someone on here may be able to enlighten me as to whether that is true or not.
If it is not a liner it must be a casting fault. Luckily you have a spare block to play with .

[IslipMinor]

Phil,

I agree, it does look like a very thin liner top, so I looked from the bottom of the casting and no sign of a liner in any of the bores. The engine specialists that I took the spare block to also looked and cannot understand it at all. Apparently sometimes liners stop part way down the bore, well below where the rings might contact, leaving a lower part of the bore unlinered to provide a positive step to locate the liner vertically - no sign of that either.

The block is due back today and other parts are arriving so that I hope to get it rebuilt by mid next week, in time to do an initial 100 mile trip and head retorque, before completing the running-in on the MSA Spring Classic staring near Chester a week tomorrow and finishing at Gaydon, about 30 miles from home!

[bmcecosse]

Richard - are you sure that block has been offset bored? The centre pair look VERY close together. I can't say I can see any evidence of liners... BMC went through a period of poor castings when virtually ALL blocks had liners fitted. This is good of course - because the liner is far better wearing than the bare block! Is it possible that electrode is from an earlier spark plug..and just coincidence that another electrode has dropped off in the other bore? NGK plugs by any chance?? You say the piston and chamber are clean due to water - but how did it get water if the gasket only let go between 3 & 4? That piston appears (to me) to have suffered damage from pinking/detonation - and that could also explain the electrode coming off the sparky. Good luck with the new engine !

[IslipMinor]

Roy,

The gasket failed between 3 & 4 and extended up to the nearest waterway as well - the one just above the junction of 3 & 4 in the picture. There is definitely only one plug problem and that is with No. 4 and the burnt piston is in No.3, along with part of the No. 4 earth electrode as seen in the picture - all very bizarre!

[bmcecosse]

Ahh right - yes I see the mark to the waterway. That must have been quite a 'blow' though ? I don't imagine it happened suddenly. Well -the electrode will just remain a mystery then! But I still suspect detonation has been at the root of it all. What sort of CR does it run? Has the head escaped without damage - are you having it faced up?

[philthehill]

Richard
Your engine specialist is right in that for a positive location of the liner a step is left at the bottom of the bore to stop the liner moving down. The ID of step should be slightly less than the OD of the liner so you should be able to see the bottom of a liner if fitted. Had to have liners fitted to my 998cc race engine as they had over bored it and a step was left at the bottom to secure the liner.
bmc
Thanks for the heads up about BMC/BL fitting liners - nice to know I was not imaging BMC/BL fitting liners to new blocks
As you are most likely aware when offset boring the centre of cylinders 1 & 2 are moved forward by 0.015" and sideways 0.010 towards the cam shaft side of the engine; the centre of cylinders 3 & 4 are moved rearward 0.015" and sideways again 0.010" towards the cam shaft side of the engine.
The land between cylinders No 2 & No 3 is reduced considerably when off set bored to 1380cc. It is even worse on an 'A' series off set bored to 1400cc - 2.5mm max. One of my spare 1275cc Midget engine blocks is off set bored to 1400cc and I have just checked the width of the land between cylinders No 2 & No 3 to confirm that dimension.
The Marina A + engine block currently fitted to my Minor is off set bored to 1380cc so still have the opportunity to over bore to 1400cc if required.
I also have a later (12V) thick sump flange 1275cc Midget block with as new standard bores. When I get the opportunity I will measure the land between No 2 & No 3 cylinders.
.

[IslipMinor]

Roy,

The CR is around 10.4, probably a bit more now after facing the head as a precaution, although it looked undamaged, and the top of the 'new' block as well.

There was no sign of any problem during the last lap of the track day, until a few hundred yards from the pits on the 'cool down' lap, when it started to misfire. After that we drove about 20 miles, before losing water (heater started to blow cool air!) but not actually overheating, so stopped and called in the recovery people.

As for the sequence of events, I really don't know at all. If the block started to 'open up' as in the picture, that would have created a nice hot spot and started to damage the gasket. The gasket damage then extended out to the waterway, accompanied by a mist of water into the cylinder. Assuming that the No. 4 plug was intact at this point, I very much doubt it would have liked alternating hot combustion and then much colder water spray, which could have embrittled the earth electrode, which snapped off part way along its length. There is evidence of light marking on the top of No. 4 piston, so looks like the electrode had a few trips up and down until somehow escaping and reappearing in No.3. cylinder, and becoming trapped between No. 3 piston and the head as shown in the picture. There is a corresponding mark on the head, which will have gone after being skimmed.

On the other hand it could have been an entirely different sequence of events!

Newly bored block, with all new cam bearings, core plugs and washed is on the way back from the engine specialist at this moment, some parts received, the rest due on Saturday morning after the courier failed today. Engine paint due on Tuesday, the first business day after Easter. Oh, and I have various business commitments on Tuesday, Wednesday and Thursday, so no pressure to get it all ready for leaving on Friday lunchtime then?

[bmcecosse]

Just paint it with Dulux.......or Plasticote from local DIY place.. Gaskets don't last for ever - it may be a plan since you use your engine quite 'hard' to change the gasket regularly... against that of course is the philosophy to 'leave well alone'....... Carry a spare gasket at all times!
I do winder how they keep the pistons and gaskets together on these new F1 turbo engines! They must have a hard life!

[IslipMinor]

Well it's all back together, put in the car last night, new cam run-in for 30 minutes this morning and head re-torque + tappets, then a run of ~50 miles and a final head/tappet check.

We are about to leave for Chester and the MSA Spring Classic over the weekend - wish us luck! At least with a 5th gear and 3.9 final drive, 'running-in' is done at 60/65 mph.

[bmcecosse]

Good luck with it!

[IslipMinor]

All went well, thank goodness!

Set off for Chester early Friday afternoon to get there in time for supper and the start of the MSA Spring Classic on Saturday morning. There was another Minor there - a 64 or 66 Traveller with a Marina/Ital 1275 engine and the same Toyota 5-spd gearbox.

Saturday lunch time was at the Anglesey circuit, so with ~300 miles on the new engine, kept the revs down a bit and then by the time we got back home last night had around 550 miles, so due for the oil/filter change and ready to go again.

One visible change is the oil pressure, which at 2,500+ is around 65/70 lb/sq in as before, but at idle now sits around 50/55 lb/sq in at 90°C oil temp and 45 lb/sq in at 115°C (after Anglesey track). Not complaining, but apart from a new oil pump, crank bearing shells, oil pressure relief plunger and spring, all else is the same as from a previous rebuild some years ago when similar parts were replaced, and the idle was about 10lb lower. The 'running in' oil is Millers 20W/50, which seems quite 'gloopy' to pour and the 'normal' oil is Castrol 10W/60 synthetic.