Yanmar cylinders slightly oval

[IsaakOker]

Hello,

I have a question regarding cylinder bores.

I have a Yanmar 4jh2-TE & unfortunately had to change the head gasket, etc. because of water sitting on pistons.

The engine block and head are straight but when measuring the cylinders they’re still within spec but the “ovalness” isn’t within spec.

I can’t pull the engine out and get it to a machine shop so the question is :
what are the real risks of putting it back together without perfect cylinders?
Will the oil consumption be higher but that's it or is there a real risk of the piston turning and seizing the engine and all that goes with it?

The cylinders have been measured at three heights and across. (Cylinder 1 is flywheel end).

The highest discrepancy is
0.0022 in cylinder 1,
0.0002 in cylinder 2,
0.0004 in cylinder 3,
0.0013 in cylinder 4

Thanks for any input!

[markxengineerin]

How many hours on this engine?
Zero risk of seizing pistons due to out of round cylinders
Maybe a small difference in oil consumption, what is your oil consumption now?
Most people with the headgasket off would never measure this, and ignorance would be bliss : )

[IsaakOker]

Hahah ok.

1100h

Veeeey Long story short, bought the boat after it had been sitting for years with fresh water in cylinders so rusted slightly.

Honed them very lightly just to get some hatches going and measured.

I though oval cylinders could mean risk for piston tilting and eventually getting stuck and therefore bending rod, etc.

[markxengineerin]

.002" is negligible when talking about how much misalignment is needed to "tilt and get stuck". That has probably never happened to any engine.

Honing in the engine (w/ pistons still in there?) is risky. Make sure your cleaning afterwards is EXTREMELY good, white paper towel test, lots of soap, WD-40, everything you can do to get that grit out. I think you are at much higher risk for honing debris than you are on your measurements.

1100 hours is nothing! My similar engine looked good after 8000, see this thread
https://www.cruisersforum.com/forums...rs-258953.html

[Bellinghamster]

Pistons can't rotate - they're fixed rotationally by the crank , connecting rod and wrist pins. If it wasn't consuming oil, started easily and ran well previously, it should be OK to leave it as is, assuming you didn't remove the pistons (so couldn't rotate the rings), therefore pistons and rings are "matched" to the bore shape.

Also, assuming you tried starting with water in the cylinders which blew the head gasket, then there is a slight possibility of bent connecting rods, damaged pistons or rings. Turn the engine over and measure the height of each piston at its top dead center and make sure the top very parallel to the tip of the block on its way up and down (i.e measure multiple depths around piston when is about half stroke, both rising and falling.

Edit: I'd written all that before others replied. Now understand water in cylinders was sitting - that does have the pilotential of rusty rings... And also honing debris in the rings. Unless you "gooped" around the top of the piston before honing (wax or grease).
Rust "ring" is often worst where the piston was sitting in the bore - of it was near the bottom of the stroke you won't be able to see or hone it.
Would you be able to lift the engine enough to pull the oil pan, remove pistons and inspect the lowered part of the cylinders, honing if necessary (taking appropriate measures to protect and keep grit off the crank journals)

[Cheechako]

Originally Posted by Bellinghamster

Pistons can't rotate - they're fixed rotationally by the crank , connecting rod and wrist pins. If it wasn't consuming oil, started easily and ran well previously, it should be OK to leave it as is, assuming you didn't remove the pistons (so couldn't rotate the rings), therefore pistons and rings are "matched" to the bore shape.

Also, assuming you tried starting with water in the cylinders which blew the head gasket, then there is a slight possibility of bent connecting rods, damaged pistons or rings. Turn the engine over and measure the height of each piston at its top dead center and make sure the top very parallel to the tip of the block on its way up and down (i.e measure multiple depths around piston when is about half stroke, both rising and falling.

Edit: I'd written all that before others replied. Now understand water in cylinders was sitting - that does have the pilotential of rusty rings... And also honing debris in the rings. Unless you "gooped" around the top of the piston before honing (wax or grease).
Rust "ring" is often worst where the piston was sitting in the bore - of it was near the bottom of the stroke you won't be able to see or hone it.
Would you be able to lift the engine enough to pull the oil pan, remove pistons and inspect the lowered part of the cylinders, honing if necessary (taking appropriate measures to protect and keep grit off the crank journals)

Yes, but in theory the rings can rotate in the piston grooves right? So a new ring that wears to fit the oval could I suppose rotate and wear overall to the smallest diameter of the oval over time, creating more clearance. But I'm not convinced this would be a problem for .002"

[IsaakOker]

Sorry for not giving more info initially.

I removed pistons before honing !😅

And the measurements for absolutely everything(combustion chambers, block, rod, crankshat, everything is as new)

The only measurements that aren’t ideal are the cylinders not being entirely round.

Cleaned everything out thoroughly! Ready to reassemble.

The only other big issue I have is the alternator tensor bold going though the water pump housing is stuck and head broken off so I’m going to apply head later on this week and try to loosens it that way.

Ps: going to install new rings obviously!

[markxengineerin]

Sounds like you're on the right track, please post some photos if you have them, interesting stuff!

[Don C L]

I have no idea if this applies to Yanmar diesels but Vespa engines had (and may still have) slightly oval pistons that would become round as they heated up (or so I was told, never measured a hot piston.) I know you are talking about cylinders, but oval may be the way they are supposed to be. Any Yanmar mechanics around?

[markxengineerin]

They should be round- there is a spec for it in the manual, I think .0008" from my memory, produced by boring + honing, which by default gives a round result when done well. Pistons on the other hand usually have complex profiles like you said.

[kenbo]

Make sure to gap the rings prior to installation and try to stay on the tight side of the gap spec. Also, did you hone the cylinders with a dingleberry hone or a 3-blade expanding hone? The 3-blade type is the hone to use for out-of-round bores.

[IsaakOker]

Literally was going to start tapping the rings but it appears that I have 1T & TP which confuses me as the Stainless one has bevelled edges and is slightly thinner but the manual states the first ring is thicker…

Any ideas?

https://ibb.co/d4tqDz7

[markxengineerin]

The markings face up. I believe the width of the top ring is ever so slightly thicker (1.975 to 1.990mm) than the 2nd ring (1.970 to 1.990). The manual shows a chrome coating on OD (cylinder touching) surface of top ring and it's barrel faced, "friction proof hardening process" for top and bottom surface of that ring (piston touching).

The 2nd ring shows "tapered inner cut". That looks like best way to identify to me.

Looking at your photo, the one saying IT looks like top ring to me. More photos would be good if you're not sure. Sorry for all the edits, trying to make my post clear.

[IsaakOker]

Haha. Yep, those are all the clues that I could also find.

Top ring thicker but the only one with anything shiny or thoughened is 1T but it’s thinner than the black.

The black one also has straight sharp edges which I would have though goes as second ring but it only fits in top groove.

That’s what’s confusing me.

[markxengineerin]

It looks like either ring should fit either groove, by the numbers in the manual for ring and groove widths. Are you sure the grooves are clean?
Also, what are you measuring with, are the brand new rings showing new specs?