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Rebuild time? Excessive blow-by?


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My rebuilt 12J used to get warm, but not into red, on long hill climbs in summer as it was a bit strangled with the wrong exhaust.  It cracked all four pistons and pressurised the sump, though it didn’t burn anything.  Another set of pistons and a correct exhaust cured both ailments permanently.  No engine is “unstressed”, some are just over stressed in mild conditions.

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Prior to this, oil consumption was not excessive, and no there wasn't excessive smoke after start.

I've replaced the head gasket before, so no big deal there.  I have not, however, had the pleasure of replacing the pistons or rings, so now, I'm going to ask a really dumb question.

I'm assuming the pistons or rings can be replaced with the engine in place?

This is a bit of a slippery slope -- should I just plan on pulling the engine and overhauling it?

-Evan-

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56 minutes ago, evanmc said:

I'm assuming the pistons or rings can be replaced with the engine in place?

Yes they can. Head off, sump off. Remove the big end cap and push the piston from underneath up and out. 

These are nice easy engines to work on. If you can be with out the vehicle for a couple of weeks then a rebuild will be fun and worthwhile. 

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Just to add to what Monkie said... Yes it can be done in vehicle, but it's far easier to make a nice job in the warm on a bench (or even better an engine stand), especially if you've never done it before. There's no dark art: just be methodical, clean and follow the manual/specs carefully. 

The other advantage is you get to button everything back up before refitting to the the vehicle, so much less fiddling and swearing in the cold.

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To play devils advocate... Is a leakdown test going to tell you much you don't already know or won't for sure if you've got it in pieces? Like I said before: whatever the fix ends up being, you'll end up removing the head anyway. You can probably have it done in the time it takes to rig up a proper leakdown

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On 12/11/2019 at 4:54 AM, evanmc said:

Prior to this, oil consumption was not excessive, and no there wasn't excessive smoke after start.

I've replaced the head gasket before, so no big deal there.  I have not, however, had the pleasure of replacing the pistons or rings, so now, I'm going to ask a really dumb question.

I'm assuming the pistons or rings can be replaced with the engine in place?

This is a bit of a slippery slope -- should I just plan on pulling the engine and overhauling it?

-Evan-

I had no smoke and no oil significant consumption/loss with those cracked pistons, just oil being forced out of various seals and gaskets and making a mess.  Such faults don’t always result in smoke, just like head gasket failures don’t always result in cross contamination of oil and coolant.

You can replace the pistons and rings in situ, and it’s pretty straight forward.  The clincher is whether the bores are OK - light honing should be possible with the block in place, but certainly not a rebore.  You won’t know if that’s necessary until the head is off, unfortunately.  But removing the pistons will allow you to check the crank journals (you can also remove the main bearing caps to check the main bearings and journals too).

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  • 2 weeks later...

I finally had the time to take the head off -- the gasket is has some bad spots, certainly, but the unexpected find was cracks in the hotplugs.  Each one of them had cracks.

No cracks were on the head of the pistons, although the surface of two of them was beat up a bit.  They come out next.

-Evan-

 

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On 12/14/2019 at 2:25 AM, Snagger said:

I had no smoke and no oil significant consumption/loss with those cracked pistons, just oil being forced out of various seals and gaskets and making a mess.  Such faults don’t always result in smoke, just like head gasket failures don’t always result in cross contamination of oil and coolant.

You can replace the pistons and rings in situ, and it’s pretty straight forward.  The clincher is whether the bores are OK - light honing should be possible with the block in place, but certainly not a rebore.  You won’t know if that’s necessary until the head is off, unfortunately.  But removing the pistons will allow you to check the crank journals (you can also remove the main bearing caps to check the main bearings and journals too).

OK, I finally have the pistons out, and the bearings are worn down to the copper in several places.  The pistons look OK, although I'm not sure what I am looking for -- no obvious cracks, certainly.  But given the damage I show in the pictures above, should I consider replacing them as well?

As for the main bearings, I don't quite understand how I can check the bearings there with the crankshaft in place.

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On the pistons checking the width of the ring grooves is worthwhile . Are the skirts good and smooth - no significant scuffing ?  Do all four pistons look like that on top ? It's had some debris through while running for sure .

The mains can be checked by loosening all the caps just a little and then removing one at a time to check , most wear will occur on the shell in the cap , while the big-end will show more on the shell in the rod. With care you can slide the upper crank shell around to check as well .

For the price of parts I'd re-shell it completely as you have it ready to do .

cheers

Steve b

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Pistons look useable if the sides are ok,  I've seen worse.   I use a narrow steel rule to help nudge upper main bearings round the crank enough to get a finger on them, remembering they only go in one direction due to the locating tab.

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The sides look fine.  Some minor wear, but nothing major scuffing.  Some minor scuffing on the sides

But, I might have spoken too soon on the piston heads looking OK.  When I cleaned them up, I see the faintest of cracks on two of them.  Not sure how well this comes out in the pic.

See there just below the V?

I measured the cylinders fo roundness.  Two disclaimers -- first day with the telescoping gauge and micrometer, and I only measured once. Things came to an abrupt halt when I dropped my gauge in the cylinder and can't get the out.  Have to wait until I have a helper.

The shop manual indicates a max of 0.0015"

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Looks like there's a crack that runs right from the point of the V through it and out the side (bottom in the pic). Stand the piston up and put some petrol in the V, it'll soon confirm. How do the others look? 

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If you retrieve your gauge, measure the bores down low, then just under the lip (if there is no lip, it doesn't need measuring!).  Look for that difference and check against factory tolerances and you'll know if a re-bore is needed.  

You can get a lightly cracked piston welded and re-machined, as long as everything else is okay with them and someone who understands pistons is doing it.  Especially check the ring grooves are still square and that new rings sit tight.  Otherwise they will wear quickly.  You can get worn grooves machined and over-width rings to match.

Back in my classic motorcycle days, when we had pistons made of unobtanium, we would get miracles done with them to keep the bike on the road, even to the extent of having them peened to swell the skirt.  Piston repair is nothing to fear.  Of course, if you can find new pistons at a good price, that's even better.  A couple of light cracks like  that shouldn't cost much to fix though.

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Once a piston has cracked, it will continue to do so exponentially - the initial crack concentrates the stresses at its end, so the rate of propagation generally increases.  If it was just the indentations, I’d just ensure no dents around the edge have peeled the crown over, and if there were any burrs, I’d file them off.

The crank main bearings are easy to check - remember that compression forces and the combustion stroke are all pushing the crank the same way as gravity, so the upper shells are normally in much better condition than the lower shells, which are also more easily removed. With all the caps loosened a couple of mm, I reckon you’d be able to pull the crank down far enough to use one shell to rotate the upper out, sliding it around the crank until it can drop clear.  Just be careful not to pull the crank too far down so you don’t damage the seals at either end.

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The two pistons that showed cracks showed them right to the edge. No cracks seen in the other two -- just surface damage on the face.

Turner engineering had them for a decent price, so I'll just replace all four.  

 

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I received my order today, but it looks like the pistons I ordered, ETC6442 are not available, and they supplied the TD versions, ETC8670.

I take it these are compatible parts?

Also, how concerned should I be with measuring piston protrusion?

-Evan-

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7 hours ago, evanmc said:

I received my order today, but it looks like the pistons I ordered, ETC6442 are not available, and they supplied the TD versions, ETC8670.

I take it these are compatible parts?

Also, how concerned should I be with measuring piston protrusion?

-Evan-

Yes those pistons are fine in your engine (just not the other way round ie putting 12J pistons in a 19J). The only difference as far as I am aware is the TD pistons have a teflon coating on the crown as part of the modifications to cope with heat stress. 

Piston protrusion is of concern in a direct injection diesel engine like the tdi engines as the measurement dictates the thickness of head gasket. Not the case on your 12J indirect injection engine. 

3 hours ago, evanmc said:

And the copy I am using for the shop manual does not give torque values for the 2.5NA conrod and main bearing caps.  Am I to use the values for the 2.25?

27-30 lb/ft and 96-100 lb/ft respectively?

Thanks,

-Evan-

You are correct. 

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  • 1 month later...

I finally got a chance to put my engine back together and things seem much better now. No oil must coming out of the rocker cover, certainly.  I will check the compression after a break in period.  Didn't get a chance to do that yesterday.

But I do notice more black smoke when I accelerate than I saw before -- can I expect this diminish a bit after things have broken in a bit?

-Evan-

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