MD11C overhaul

VicS

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Problem was lack of compression.

Pistons seen well within limits. Bores likewise with no significant wear ridge.

Can anybody offer any guidance on ring gaps.
We know the spec'd gaps when new but do not know how much wear can be tolerated.
 

VicS

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It not mine but I have suggested checking the valves and seats, although it did have two complete new heads not so long ago. Also asked if the head gaskets were Ok

The 'shop manual gives figures for max wear allowable for pistons and bores. It gives ranges for the ring gaps when new but not a max permissible figure. Hence the question.
 

vyv_cox

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End clearance figures for the Yanmar 1GM are .008 - .016" on all rings, if that's any help. Bore is 2.95 ins.

Same end clearance figures for the 2 cylinder engines but bore is 2.83 ins

For 3 cylinders the top ring clearance is .010 - .018" bores are either 2.83 or 2.95 ins
 
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VicS

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The figures in the manual are

Top ring: 0.40 - 0.55 mm

2nd and 3rd rings 0.30 - 0.45 mm

oil ring 0.25 - 0.40 mm

But they are minimum figures when fitting new rings

"We" have gaps up to 0.78 mm in one cyl and 0.58 mm in the other. I am assuming thats the top rings.

Question is are they they large enough to affect the compression so drastically.

As it is it will eventually start, given a good and well charged battery, but for a while runs on one cylinder, the other joining in later.
 

Baggywrinkle

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As a diesel engine relies on compression plus temperature (hence glowplugs) to ignite the fuel/air mixture, the symptoms would seem to verify what you have measured.
i.e. running on one cylinder (the one with the tighter clearances) until the engine heats up a bit and then the other kicks in.

A well charged battery will spin the engine faster and also therefore increase compression, increasing the likelyhood of ignition.

If you removed the oil filler cap while running was an oily mist blowing out continuously? This is a sign of poorly sealed pistons - place your hand over the hole and see how quickly the pressure builds up (but not after it's been running for a few hours ;) ).

Another way to look at it, the tolerances are 0.4 - 0.55. With 0.78 you are almost double the lower tolerance and almost 50% over the upper - and only on one cylinder - which I bet is the one that doesn't fire 'till later.

A petrol engine will be different as it doesn't rely on compression for ignition, a petrol engine with worn rings or bores will blow out the filler cap and suffer reduced power, but probably no misfiring.
 
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VicS

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As a diesel engine relies on compression plus temperature (hence glowplugs) to ignite the fuel/air mixture, the symptoms would seem to verify what you have measured.
i.e. running on one cylinder (the one with the tighter clearances) until the engine heats up a bit and then the other kicks in.

A well charged battery will spin the engine faster and also therefore increase compression, increasing the likelyhood of ignition.

If you removed the oil filler cap while running was an oily mist blowing out continuously? This is a sign of poorly sealed pistons - place your hand over the hole and see how quickly the pressure builds up (but not after it's been running for a few hours ;) ).

Another way to look at it, the tolerances are 0.4 - 0.55. With 0.78 you are almost double the lower tolerance and almost 50% over the upper - and only on one cylinder - which I bet is the one that doesn't fire 'till later.

A petrol engine will be different as it doesn't rely on compression for ignition, a petrol engine with worn rings or bores will blow out the filler cap and suffer reduced power, but probably no misfiring.

It's an MD11 so no glowplugs

Unfortunately now in bits so a bit late to investigate gases/oil mist being blown from the oil filler.
As you will know, however, the crankcase breather vents to the aft air filter "frying pan". I'd have expected much oil mist blown out to have resulted in oil dripping from there into the bilge. Maybe there was but just gone un-noticed.

To put the enlarged ring gap into perspective. Obviously it's measured in an unworn part of the bore and the worst is possibly 0.23 mm over the max figure in the spec for new rings.
The permitted wear in the bores is 0.25 mm. That would result in an ring gap increasing by about 0.79 mm in the worn part. That makes the above 0.23 mm look small. Esp as the measured bore wear is only 0.05mm

We thought the compressions were so bad that the cause would be obvious and were expecting to find that maybe both pistons c/w rings would need replacing perhaps also the liners.

At over €100 per set for piston rings, over €500 for a piston and rings and over €600 for a liner piston and rings we obviously don't want to replace bits that need not be replaced.
 

Jcorstorphine

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It's an MD11 so no glowplugs

At over €100 per set for piston rings, over €500 for a piston and rings and over €600 for a liner piston and rings we obviously don't want to replace bits that need not be replaced.

I had a similar dilemma with rebuilding my Ford XLD 1.6. The machine shop said that bores were slightly oval and that if I just fitted new rings, they may not bed in all that well and that I could spend a lot of time and effort and not get any benefit. In my case I had one cylinder down greatly on compression and the rest not that great.

My initial thoughts that I could get away with just honing the bores, a new set of rings at about £80 (for 4 sets) new cylinder bolts (£20) set of upper gaskets (£30) were ditched.

I ended up spending about £1200 on a complete engine rebuild which included a complete new Cylinder head (I found a crack in the existing one) a re-bore, new pistons and rings, new main and big end bearings, new water pump etc.

My logic was that by spending £1200 just now (as opposed to £5000 for a new engine) I could future proof my engine as parts were beginning to get difficult to get. Hopefully, the rebuild will last longer than I will.

In the case of the MD11C I think you will be in a similar position (only difference is that I could get a complete set of pistons and rings for my Ford for £280 as opposed to £500 each from Volvo) By spending money on the engine just now, you may greatly extend its life while parts are available.

If you do a partial rebuild, you may spend quite a bit of money as even the gasket set per cylinder is not cheap from Volvo and at the end of the day, not see much difference.

If there is a chance that the partial rebuild does not work, what you also need to take into account is what a new engine will cost plus all the hassle of modified bearers, new prop, new control lines, rewiing and so on and so on.

The other really important area you need to look at is corrosion of the block and the existing liners. I did a partial rebuild on a Volvo MD2B and the water jackets were blocked solid with rust. In your case, it will be possible to remove the liner and clean out the rust.

Very best of luck in your rebuild.

PS. Knowing what I know now, would I have totally rebuilt my engine with all the problems of getting some of the spares and the worry if it would work,

NOT A B?**&Y CHANCE!!!!!!!!!!!!
 

GrahamM376

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At over €100 per set for piston rings, over €500 for a piston and rings and over €600 for a liner piston and rings we obviously don't want to replace bits that need not be replaced.

Around 15 years ago, I had to rebuild ours and at that time the factors found equivalent pistons in their catalogues at IIRC about £150 each incl rings. Unfortunately, can't remember what they were out of but, many Volvo parts were standard off the shelf bits, re-badged.
 
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