Smoking' MD2B / piston removal

[phammy]

HELLO all!

New to sailing, new to engine overhaul!!! and new to the forum - Please help...

We took our westerly centaur out for a spin at the weekend equipped with the MD2B and after about 10mins at 3-5kts quite a lot of smoke began steadily coming out of the filler cap - there's a short vent pipe there.... I'd previously run the engine up to temp on several occasions (but not under any load) for 40ish mins each time and didn't notice any smoke.

Long story short, I suspect blow-by of exhaust gasses past the pistons or a head gasket issues or valves in the cylinder head need re-grinding/seating

so I proceeded to take both cylinder heads off figuring I would install new head gaskets at the very least. I will re-grind the valves and valve seats actually. The waterjacket paths happen to be 75% filled with corrosion/minerals so will do my best to clean these throughout.

This is the part I need the most help with:

I'm thinking I may as well replace the piston rings with the engine half apart. Right?
• I see in the workshop manual that I simply lift the cylinder blocks upwards
• Then I have to guess (not mentioned in manual) the next step is to unbolt the con rods?
• Then I'm guessing I next re-fit the piston on the bench (using ring compression tool)
• Then bolt the con rod back on via the inspection window????

Otherwise i don't understand how i'd get the piston rings compressed if refitting was simply the reverse of removal step (lifting the crankcase straight up)

Thanks so much for any help!

[a64pilot]

There are piston ring compressors that are inexpensive.
I usually just use my finger tips and or a wood stick like one from a popsicle. That of course only gets one ring at a time and you do have to hold the piston down slightly or it may pop right back out.
A proper ring compressor allows one push to put a piston in, so instead of it taking a minute to do, it’s down to a couple of seconds.
I’ve had issues with ring compressors as if they are just a little bit too tight, they get stuck between the piston and cylinder and doing it by hand I’m certain I’m doing no damage.
But then I enjoy assembling engines too.
I have no experience with your engine, but doubt there are inspection windows that will allow the rods to be removed, usually an engine has to come out and then the oil pan removed, and if your doing all of that, then you may as well go all of the way and measure the bores etc and have them bored out if necessary, that means new pistons etc.
It’s most likely that excessive blow by is coming from the rings, and not the valves, only place it can come from in valves is valve guide wear and wear that bad is extremely unusual

[Pete7]

Sounds like you will 5 pairs of hands, but why take the conrod off?

Take a cylinder off upwards, stuff rag in around the crank shaft so nothing drops in there. Replace piston, remove rag, then using ring compressor lower the cylinder back down (lots of hands needed). Don't trap a compression or oil scraper ring, been there, got the t shirt.

Pete

[a64pilot]

So this motor is like a motorcycle engine where the cylinder can be removed?
If so then most have a chamfer on the bottom of the cylinder that makes rind installation much easier, the key is if suddenly it stops going down, don’t force it, it’s likely a ring caught

[phammy]

So can a piston ring compressor tool be used with the piston still connected to the crankshaft - in other words, from below then lowering the crankcase down? or does the engine always need to be removed to replace the piston rings?

[a64pilot]

This may help. It’s a motorcycle and it’s the way that I install pistons on an engine with removable cylinders whether it be a motorcycle or an aircraft engine, you’ll notice he uses an awl. Which is fine, I like a small flat bladed screwdriver or pop sickle stick, and my thumb nail
I’m sure there are video’s of ring compressors as well. What makes ring installation so much easier on a removable cylinder is that there is a chamfer that guides the ring in that isn’t present on an engine that the piston goes in through the top of the cylinder like normal
https://youtu.be/ju2RE6mTrbo

[a64pilot]

Originally Posted by phammy

So can a piston ring compressor tool be used with the piston still connected to the crankshaft - in other words, from below then lowering the crankcase down? or does the engine always need to be removed to replace the piston rings?

Absolutely it can

[phammy]

Great! This really gives me the confidence to continue with the piston rings, especially as it’s the most likely reason for the smoke emerging oil side.

It’s been good to have the head off to decoke, clear water jackets, check rocker clearances etc, etc
I really got to know the engine yesterday which can only be a good thing.

I’ll order the parts and book a day off work.

Thanks for the pointers everyone!

[a64pilot]

If it were me, I’d take the head to a machine shop to have the valves ground and new valve seals fitted, just don’t let them shave the head, some do as a matter of course, and it can cause issues on a Diesel, so unless it’s warped, don’t have it trued, another name.
I’d also have them measure the cylinder(s)? To see if it needs to be bored, but if not it still needs to be honed lightly for the new rings, and they can do that.
If you don’t have them fit the rings, read up on that, sometimes rings have to be trimmed a little or they can break when run, simple step just needs to be checked is all.
Yes you can do this by yourself, but some things are I think best left to the experts, which brings up injectors, maybe since the heads off and they are coming out anyway, have them cleaned and tested.
If you do this, then you have done a “top end” overhaul and if you keep the oil changed to protect the bottom end, it may be a very long time before anything else is needed.

Now I know nothing at all about your engine, this is generic advice, for any Diesel from little one cylinder ones to big V 12’s.

Once you get it back together you may want to do an early oil change to get any wear in metal out, and don’t baby the engine, you want to run it pretty hard to seat the rings, if you baby it, it may burn some oil for the rest of its life.

[phammy]

Thanks - i've just looked up a local machine shop who will are able to help me with those bits.

Can i get away with running it in 'pretty hard' with the existing oil after assembly or do you mean Change the oil, run it in, then change the oil again?

[Milko Berben]

There are so many small things that can go wrong. I don't want to make you afraid but you should look first one time and have it explained to you if you have never done it before it is so easy to put the pistons in the wrong way around for instance

[phammy]

The marina is cool - loads of helpful folks around with different experience and happy to help, just happens i'm away from the marina at the moment with the cylinder head at home. I'll be sure to get help if i need it on the big day

[DougR]

When you pull the cylinders up off the engine block, you should take care to measure the thickness of the gasket "pack" between the cylinder and the block.These gaskets are available in different thicknesses, and the correct thickness must be reinstalled to get the piston top to cylinder head clearance correct. Record the thickness below each cylinder so that you can reinstall new gaskets of the same thickness. Also mark the cylinder to be sure that the correct cylinder goes back in the correct location.

Review this in the manual so you get it correct and the compression ratio is correct for each cylinder. If you already mixed things up, the manual gives a clear explaination of the measurement process, and how to check if after assembly.

DougR

[a64pilot]

Originally Posted by phammy

Can i get away with running it in 'pretty hard' with the existing oil after assembly or do you mean Change the oil, run it in, then change the oil again?

Your call. I feel you can’t change oil too often, but it s likely that I have wasted oil in my life too, but oil is cheap.
I’d run it in on new oil myself, and then change that at say 10 hours and then from then on, regular oil change intervals myself.

If the cylinders are bored or honed, I’d wash them out with hot soapy water too. A lot of that grey stuff that will come off on a rag is the stones that were used to hone it, and that stuff is of course very abrasive, that’s it use, best I think to remove it. I have tried everything to remove it from mineral spirits, safety clean solvent, automatic transmission fluid, whatever you use, you think it’s clean, but hot soapy water will get lots more out, so I have learned to just wash it with hot soapy water.
Dry it of course, as a cylinder is high carbon steel and rusts almost instantly, it’s not deep rust and won’t really harm it, but dry it out and a light coat of oil and there will be zero rust.

A lot of what you will be told by me and others is opinion and what we consider best practices, but the manual is your friend, what the manual says usually always trumps anything you read on the internet, unless of course your reading the manual on the internet

[Dougtiff]

Reminds me of rebuilding a MD 2 in a little back water village in Malaysia back in the 80'S, with the help of a small shop, Hong Kong was the closest Volvo agent, and they didn't have that many part's available, it took forever to obtain part's, so we found way's around the problem, turns out BMW ring's fit, a Volkswagen pump worked, and the local tin smith made a new gasket, by copying the old one, back together, and got more year's out of her, it was a great experience, part of cruising back then, bonding with the local's.