Engine rebuilt - blowing oil

[iaincockburn1]

Hi! Just completed a full engine rebuild - cylinders rebored (new pistons and rings), crankshaft reground (new shell bearings), cylinder head overhauled (new valve guides, valve seats reground, new oil seals). The cylinder head and engine block faces were lightly skimmed. All of this performed by a highly reputable local engineering works, who also provided the piston rings and oil rings from a good manufacturer.

It's running smoothly with no adverse noises BUT it's blowing blue smoke throughout its rev range, worst when revving but evident also on tickover. All 4 cylinders are affected as evidenced by the oily spark plugs.

I have removed the cylinder head, rechecked the valve stem fit in the (new) valve guides, the position of the oil seals, checked for warping of the cylinder head face, no evidence of cracks in the cylinder head or block, checked and replaced the head gasket, torqued up to 45lb/ft again, all to no avail - no improvement and no obvious faults found.

Any ideas please? Have I overlooked something? For example, should the piston oil scrapers be fitted in a certain way? Is it simply a case of waiting for everything to bed down??

[Murrayminor]

The blue smoke suggests burning oil as your investigation has found.
As you have checked the inlet side the only other thing as you mention is the bores and piston rings.
Have you carried out a compression test?

What oil are you using?

[philthehill]

The rings should be fitted thus.

img175.jpg (383.24 KiB) Viewed 1867 times

The rings are normally marked top and should be fitted so.

What type of valve stem seal have you fitted?

The top hat type seals only on the inlet guides are best. Use ESM 948cc/1098cc engine valve guides with top hat seal locating groove.

https://www.morrisminorspares.com/engin ... l-p1209547

Newly rebuilt engines do need to be run in so always create a small amount of blue smoke but yours sounds excessive.

If everything is as it should be all I can suggest is that you run it in and see what happens.

Phil

[Murrayminor]

I think the rule of thumb is if the car smokes on start up the issue is with the top end but if it smokes on acceleration its the bottom end.

A fresh engine will take some time to "bed " in as mentioned.

[liammonty]

It does sound excessive, though as Phil says, a certain amount is to be expected to begin with. Have you left the car idling for prolonged periods? It's quite easy to glaze the bores immediately after a rebuild - an issue that I once had. In my case, it had to be stripped again and the cylinders honed which cured the issue. Running in is tricky as you need to avoid working the engine too hard, but you also need to avoid continuous engine speeds and too much light work in order to allow everything to bed in properly.

[Myrtles Man]

You may find this of some interest:-

https://www.youtube.com/watch?v=xpoglovyy_8

[ianmack]

An interesting video, but he is working with new Honda parts which are machined to an accuracy unheard of a couple of decades ago. If you assemble an engine with a mixture of old, new and repro parts and using a local machine shop there’s always a chance of a tight spot and running in makes more sense.

[iaincockburn1]

Ah - many thanks for your responses especially the graphics. No, I did NOT check to see if the scrapers were orientated as outlined in the diagram, I assumed they were simply flat against the cylinder wall. Yes, the compression rings were correctly fitted but the included instructions did not mention the scrapers! Dammit - sump down and oil in the face I suspect.....

Will let you know the outcome....

[iaincockburn1]

Hi, me again. Just beginning to lose confidence.....!
I recall that the edges of the compression rings were stepped, not chamfered. The instructions that came with the rings stated fitting them with the narrower radius on the top, other websites suggest fitting them with the narrow radius downwards (to help clearing off the oil film on the downward stroke). Any thoughts?

This turning out to be a steep learning curve!

[philthehill]

Only the top piston ring is stepped. This is to get around any wear ridge at the top of the bore if fitting and using new piston rings in old bores.

If you fit them with the step down there is the possibility of breaking the piston ring when the ring hits the wear ridge.

The rings are not chamfered as much as shown in the schematic - the extreme chamfer shown is there for effect but the rings are still chamfered.

The chamfer should be as the schematic so as to scrape the oil of the bore walls on the down stroke.

If the chamfer was the other way around oil can get past the rings and into the combustion chamber giving the conditions you are experiencing.

[BobHood]

Quick question, were the bores honed as well as being bored when the rebore was done?

I ask because reboring the cylinders will leave them with a work-hardened surface. This means the new rings won't be able to 'bed in' very easily. That's why it's normal practice to hone the cylinders as well. Honing breaks through the hardened surface to the softer metal behind, and this, along with the cross-hatch finish, allows the rings to bed in more easily.

[iaincockburn1]

Hi again, a quick update.

The bores were rebored and honed before I reassembled the engine. The cylinder head was lightly skimmed, and new standard valve guides (not the mini type with the extra ridge) were fitted. The valves were refaced and lightly ground in. When I took the head off the first time to try and trace the source of the oil I checked the valve stem fits in the new guides - they were not too loose. The new O rings were refitted and valve shrouds were fitted on the inlet valve just to be extra sure. Despite all this the engine still smoked oil especially when warm so I started to assume it must be the non standard piston rings (stepped, not chamfered). I should also add that the smoking would get a lot worse for a few seconds if I blipped the engine after a little idling. Oh, and the engine was not left idling for long at any stage (just enough to check for oil leaks and water leaks after it gets hot) and has undergone a couple of journeys of 30 miles each to try and bed in - but to no avail regards the smoky exhaust.

I decided to order the standard piston rings (flat face on #1, chamfered on 2 and 3) and took the head off again in advance to dropping the sump to drive the pistons out. However, as soon as I removed the head I noticed a teaspoon of oil sitting on the top of each of the pistons. This can only have come from the cylinder head but how? Or is this a normal appearance after a cylinder head is lifted and I am barking up the wrong tree? I would really appreciate your advice!!

[philthehill]

Regarding the valve stem seals/rings.

They are next to useless in keeping oil away from the valve stems. The shrouds are not particularly effective either - BMC discontinued their fitment to later 'A' series engines.

The only effective way to keep oil away from the inlet valve stems is to fit the later valve guides and top hat seals.
The exhaust valves do need some oil down the valve stem/guide as with the use of unleaded fuel the valves run hotter and there is the possibility of the valve stem picking up and sticking in the guide. Therefore do not fit top hat seals to the exhaust valve guides.
Induction suction draws oil down the inlet valve stem/guide into the cylinder especially after blipping the accelerator when induction vacuum is increased, engine revs are high and the throttle closed.

You may have oil coming past the rings especially with new bores/rings but I would get the head/valve guides/seals working satisfactory - refit the head and run the engine.

[iaincockburn1]

Hi again

Sorry for the long delay but progress has been slow given all that is going on at the moment.

So - the big answer?

Well, I removed the cylinder heard again and installed the later valve guides (with the slot for top hat oil seals) and of course the top hats themselves, just to the intakes and I think this has worked. I can't test this properly as we are in the midst of a lockdown but it seems much better with no trace of a smokey exhaust, at least on idle or the occasional blip on the throttle.

The piston rings were fine - I got it wrong. Each of the compression rings were flat to the piston bore, with only a bevel on the top inward side of the ring. There was a teaspoon of oil on the surface of each piston head when I removed the cylinder head and this clued me as to where the problem was. I left the exhaust valves and guides unchanged, as recommended.

Many thanks for all your advice, it was much appreciated. I guess the valve stems were a little worn, despite the new guides, and oil was being sucked in due to the improved fit of the new rings on each intake stroke?