Rebuilt engine burning oil

Hello, and firstly I'm sorry for such a long post; I've tried to be as detailed as possible to answer any questions before they come up, but also because it's been ongoing for a year and a bit, so there's a bit to cover.

I've rebuilt my Maza B6 1.6 engine after the pressure relief valve in the oil pump failed (first time rebuilding an engine, took a lot of help from the HPA YouTube among others). There was no major damage to the internals, and they were all very serviceable, although the bottom end bearings were starting to look a little second hand. In the rebuild I replaced all gaskets and seals, new piston rings, on original pistons, ACL race bearings on connecting rod and main caps, MLS 1mm head gasket, and a full head reconditioning with a 20thou shave (done by a machine shop). I used OEM parts wherever possible. I've also now got a coolant reroute, and an electric water pump. Prior to the build, apart from the oil pump issue the engine was running without fault not burning any oil and running faultlessly.

After a faultless startup and breaking in the rings following a procedure recommended by one of the top MX5 shops in the USA I took the car on a 1000km running in road trip where I used a running in oil and didn't go over 4000rpm. Nothing out of the ordinary happened except that it consumed about 800mL of oil by the end of it which I more or less put down to oil finding its way into new places in an previously dry engine. There is no external leakage, and nothing on the garage floor. I've put an inspection camera through the clutch fork boot and no oil in the bell housing, but when I put the inspection camera through the spark plug holes I saw a small amount of wet oil on the crown of the piston, and it's the same on all of them. I've taken the intake off of the throttle body and had a peek inside and it's about as clean as when the engine went together.

I've done a compression test which got me 200psi (+/- 10) across all 4. I went and got a leakdown test kit, and found that I've got a pretty uniform 20% (5% difference from highest to lowest) loss across all 4 which was larger than I thought there would be, but pretty uniform nonetheless. There's a small amount of smoke coming out the exhaust only in heavy acceleration, but not significant enough that I can see it in rear view mirror, but people following me can. And there doesn't seem to be a difference in this smoke between long periods of idle and then throttle and short periods of idle and then throttle which might be indicative of valve stem seal potentially. Coolant is clear with no milkiness or bubbles.

While scratching my head about this, I decided to give it the old “Italian tuneup” and went on a 500km drive with some friends in much faster cars, and spent a lot of time very high up in the rev range. On this drive it consumed almost 500mL of oil.

After posting on forums, and speaking to many people I got to eventually deciding to pull the engine apart again to check what was gong on. What I found was that there was caked oil on the top of all pistons, mostly on the exhaust side of the crown, but could be relatively easily wiped off. The rings were all in the correct orientation (in terms of top/bottom), and no overlapped ends of the oil control ring. The skirts of the pistons did not have any oil staining, and there was some vertical marks in the bores, but nothing you could feel. I removed one exhaust, and one intake valve and found that the exhaust valve was quite pitted, but there wasn’t any lateral motion in either of them. However on the exhaust side when you put your finger on the top of the valve guide and remove the valve there was no “pop” where there was on the intake side. I did take the head back to the same machine shop that rebuilt it to ask them to disassemble and check valve guides, and valves, and they gave it the all clear. To be clear, this is a very reputable machine shop, and this has been going on for a year and a bit and they’ve been nothing but helpful.

To answer some questions that have come up in prior discussions:

-I use 10W30 full syn oil, always have.

-All seals, rings etc, were OEM. Bearings were ACL

-Cylinders were honed only, and I’ve been assured by many people that this removes piss all material, and so won’t change the bore dimensions

-PCV valve is functional, and I’ve replaced it with a turbo model PCV valve, which is supposed to have a stronger spring I guess?

-Since the rebuild it’s travelled a bit more than 4000kms

Below I’ve attached my spec sheet. I’ve measured the bores with snap gauges, which have their inherent inaccuracies, and I’ve measured pistons with a micrometer, at the point which the FSM says to, and I’ve measured bearing clearances with plastigauge. I realise that the pistons are out of spec, however I’m reasonably confident (although not absolutely certain) that the engine has never been apart before. I realise also that ring gaps are slightly out of spec, and this was as out of the box, no grinding ends. I’ve spoken to a number of people about both of these, and been assured that they’re surely inconsequential.

So the point that I’m at at the moment is what to do? I can’t keep driving the car burning this much oil because it will damage it eventually. I am replacing the head with one I’ve been building up (bigger cams, stronger springs), and don’t want those valves to be damaged like on the current head on account of oil burning. The only other thing which wasn’t in spec was relating to pistons, so I’m wondering if I just bore out to accept an oversized piston? Any other suggestions greatly appreciated because this has already cost me an arm and a leg, and saving money at this point would be nice.

Specs:

Pistons at second disassembly:

Intake valve:

Exhaust valve:

Did you install new valve stem seals? Something like this:

https://www.good-win-racing.com/Mazda-Performance-Part/61-3556.html

Any chance the valves / guides were changed to a smaller stem diameter, but stock seals were used?

Do you think Oil is draining out of the head properly?

From the carbon patterns on the pistons, and the exhaust valve build-up, it certainly appears to be drawing oil down the exhaust valve guides. I know many people have the idea that the exhaust ports will (almost) always have positive pressure and so oil can't be sucked into the ports, but there can certainly be a lower pressure in the port than in the valve train area of the engine.

As David suggested, it could be slightly worn, or marked, guides that still fall within tolerances, but it's more likely to be a probolem with the exhaust valve stem seals - you did use them, didn't you? The most common problems are

a/ the seals are the wrong size for the valve stem - not a common issue, but sometimes spec's change over an engine's run. Can't find any info' on that, but it's possible if there was a turbo' version of the engine with sodium filled exhaust valves with e 6.5, or 7mm stem, and the gasket set was for that - but I didn't dind anything to confirm that theory.

b/ the seals were incorrectly fitted, either slightly canted leaving part of the 'seal' exposed and as oil pools in the top it runs down the guide, but this is unlikely to affect all the guides. The otyher intallation mistake is using too much force/pushing them down too far, this can cause the top of the seal to 'flair' and leave a gap between the lip and the valve - possible, but?

c/ the sealing lip was damaged during the fitting of the valves, either torn or the springs that hold them closed (if used) - but, again, ALL the exhausts?

d/ the seals were the wrong size for the 'guides outside diameter, and they came loose and instead were 'stuck' to the valve stems. This may fit all the issues you're having, especially as when they are 'stuck' on the stems they're effectively pumping a small amount of air and oil down the guides as they're opened. I have seen this happen when loose 'push-ons' were used that didn't have the required grip.

e/ it may be that a "generic" seal was used, one that was slightly taller than the OEM, and on the exhaust they were just tall enough for the retainer/collets to de-form the seals.

f/ something else that doesn't come to mind

I had a similar case when valve oil seal was sitting too high on the valve guide causing the oil injection inside the combustion chamber... The engine was very smoking.

Thankyou for the prompt replies.

Although I don't know the history of the car prior to my ownership, I did buy it with 80k kms, and it didn't have any signs of modification when I bought it. Certainly I don't believe the engine has been apart.

I don't think oil is pooling in the head, as the two times I've removed the valve cover since i've not noticed it. Not noticable through oil fill hole either.

Gord, to answer your points:

a) There were no forced induction options for this engine. The first forced induction model came out almost 10 years later. Although it's possible the VSS has changed in design, as I've had to replace a couple of intake valves, and the faces are different, although stem dimensions are same.

b) hard to say, as I didn't do them. I'd like to think that the reputable machine shop I went to would know how to install a VSS, but maybe not? Visually they looked like they were installed fine.

b and C) as you've said, unlikely to have affected all valves, but also it does point to a systemic error if that's the case, which would be consistent with a shop doing it.

d) what would you say is an acceptable "stretch" for the VSS over the stem? Also you might have to clarify this point a bit more please, are you saying that the lip of the VSS is stuck on the stem and acting as something of a pump through which it pumps air/oil in and out of the valve guide, whereas isolating it from the area above the VSS?

e) OEM VSS's were given to the shop, whether they used them is another thing. But they did look correct

Shota do you recall what the error was? Wrong seal? Incorrect installation?

The error was wrong combination of valve guide heigh along with oil seal on it that didn't match the camshaft knobs profile.

d/ it was a long time ago, but I think it was a short "generic" seal that slightly too tight for the guide and that worked it's way off. In that case, when stripped down, we figured when the valve was fully opened the seal was sticking there, and when the valve was closed there was a gathering of oil splashe on the valve stem, then when the valve opened in operation the oil was partly carried down by the valves motion, and partly the seal's downward movement displaing the air between it and the guide top, as the valve opened, increasing the air pressure there and helping to 'blow' the oil down the guides.

Off hand, can't really suggest anything definitive, that would apply to all cylinders, just possible issues.

Thanks for your inputs. No one has pointed out the short block discrepancies, does that mean that they are mostly insignificant, and that the oil is more likely coming from the head? I'm thinking to just pay extra attention in assembling the new head, and when swapping it in crossing my fingers and toes and arms and legs that it's fixed. Because re-doing the bottom end will be expensive.

From what I can make out of the bore walls, I'm disinclined to blame them for the problem - one cylinder, maybe, but because of the pattern, and that it's affecting all cylinders approximately the same, it suggest something that applies to all the cylinders, probably exhaust guide related.