2012 Toyota Prius excessive piston skirt wear?

[1911CHAMPION]

I am assisting a friend of mine in doing a head gasket job on his 2012 Toyota Prius with 184,000 miles. He purchased the car from a local Honda dealer back in November 2023 and has accumulated some miles on the car without issue until about a week and a half to two weeks ago. The symptom was engine knocking and a misfire on start up as well as it failed the cooling system combustion/hydrocarbon test within seconds of the car being started at a local Belle tire shop. There had been a misfire previously, but that was resolved with the new set of spark plugs and ignition coil packs.

The head gasket was completely burned out between the two cylinders closest to the crank pulley.

Now the real question is: how much slop or tolerance is there supposed to be between the cylinder bore and the piston and piston skirts on a car with 184,000 miles?

There seems to be an awful lot of rocking back-and-forth, which would indicate to me that there is excessive piston skirt wear. This is the first Toyota Prius head gasket job I have done, so I’m not versed in how “loose” per se things are supposed to be with this kind of mileage. There doesn’t appear to be excessive cylinder barware, and there is still a healthy amount of crosshatch pattern visible in all areas.

I will upload a YouTube video demonstrating how much movement there is in the piston inside the cylinder bore and attach the link to this post or comment with a link to the video.

Thanks!

 

[eljefino]

That ain't good.

I've done some in-frame ring jobs of higher mileage (150k) saturn motors. While the bottoms of the piston skirts were "shiny" and missing their machining marks, they didn't rock like this, and they didn't make noise.

If you guys aren't really good at evaluating everything I'd be thinking about a new motor.

 

[1911CHAMPION]

Here’s another video I took.

 

[Cujet]

As I am sure you are aware, you cannot measure the piston skirt to cylinder wall clearance by rocking the piston. The top of the piston is machined consider smaller in diameter than the skirt area. As the top expands when hot. The ring pack wears over time and there is less stability "up top". Over time, this tends to get worse on some engines and can be heard as the classic piston-slap sound, most often on cold startup.

Given enough time with unstable pistons, the rings will fail to seal well, blowby and oil use will result.

What I think you'll find is that new pistons and rings in a fresh bore will also rock about a bit. Just probably a bit less than you are seeing.

I wish I had a spec for this on that engine.

Excessive oil use? If so, as a bare minimum, a re-ring is in order. If everything was 100% fine until the head gasket failure, maybe fix just that?

Clearly coolant is in the oil. Did it get there by going past the piston rings? Do all pistons come up to the top 100% correctly? In other words, did the engine hydrolock?

 

[Oillover]

If you guys aren't really good at evaluating everything I'd be thinking about a new motor.

+1
At least you'd be certain you are getting something with a warranty that'll last.

Also, I'm not sure as to what would cause this issue.
My initial guess was extended OCI's, but I'm not a licensed mechanic.

 

[atikovi]

If you guys aren't really good at evaluating everything I'd be thinking about a new motor.

That's more than the car is worth unless you mean a new used motor.

The way to check is with a bore gauge and mic the pistons. How hard is it to drop the pan, unbolt the rods and shove them out the top? Might as well replace them at that point as well as the rings and bearings.

 

[Timmastertech]

Toyota allows a bit over 6 thousandths of clearance. Nominal bore size is 3.169 to 3.170 with a wear limit of 3.174. Piston size is 3.1677 to 3.1681. As far apart as you are, pop a piston out and measure things if you are concerned.

 

[Nessism1]

Top of piston expands more than bottom when hot. That test is not conclusive and should be dismissed.

 

[jhellwig]

Put it back together. It could crap out as soon as you start it on something else or run another 200k. No way to tell.

A 12 year old 180k car is one loose wire connection away from a pile of junk. So is a new car for that matter.

 

[1911CHAMPION]

As I am sure you are aware, you cannot measure the piston skirt to cylinder wall clearance by rocking the piston. The top of the piston is machined consider smaller in diameter than the skirt area. As the top expands when hot. The ring pack wears over time and there is less stability "up top". Over time, this tends to get worse on some engines and can be heard as the classic piston-slap sound, most often on cold startup.

Given enough time with unstable pistons, the rings will fail to seal well, blowby and oil use will result.

What I think you'll find is that new pistons and rings in a fresh bore will also rock about a bit. Just probably a bit less than you are seeing.

I wish I had a spec for this on that engine.

Excessive oil use? If so, as a bare minimum, a re-ring is in order. If everything was 100% fine until the head gasket failure, maybe fix just that?

Clearly coolant is in the oil. Did it get there by going past the piston rings? Do all pistons come up to the top 100% correctly? In other words, did the engine hydrolock?

There was no evidence of oil consumption. Plugs, except for the one that came out of the second hole, all looked great. And the one the came out of the second hole only had a slight amount of darkening around the porcelain insulator. I’ll go back out to the shop in a little bit and snag a couple pictures.

 

[Chris142]

Standard piston to cyl wall clearance is only like .002-.004 on most any non performance engine . Cant really see that small amount with your eye.

What you have is way too much. It need rebuilt or replaced.

 

[Cujet]

Standard piston to cyl wall clearance is only like .002-.004 on most any non performance engine . Cant really see that small amount with your eye.

What you have is way too much. It need rebuilt or replaced.

The rocking does not measure piston to wall clearance, as that is only measured at 90º to the piston pin, just above the skirt lower surface. In the picture below, we can see how piston designers will shape the piston to achieve relatively even fit when hot.

Another note, engines that may do many cold starts and come to full power quickly require different piston designs, along with significant additional clearance, as the cylinder bore will heat up more slowly than the piston. Standby generators come to mind.

 

[Exhaustgases]

Guess you found the knocking. Something has worn a bit.

 

[1911CHAMPION]

This spot on the engine block itself where the head gasket was blown out between the two cylinders, is a little concerning to me.
Try some 2,000 grit wet paper to take it down flush and slap it back together and send it?
He and his wife don’t have the kind of money to have the block all machined or to replace the entire engine for that matter. I’m trying to do my best to save him as much as possible on the repair but we will for sure be putting new timing chain, tensioner, and guides on it during reassembly. But the oil milkshake that we drained out of it is definitely a bit concerning.

 

[spasm3]

This spot on the engine block itself where the head gasket was blown out between the two cylinders, is a little concerning to me.
Try some 2,000 grit wet paper to take it down flush and slap it back together and send it?
He and his wife don’t have the kind of money to have the block all machined or to replace the entire engine for that matter. I’m trying to do my best to save him as much as possible on the repair but we will for sure be putting new timing chain, tensioner, and guides on it during reassembly. But the oil milkshake that we drained out of it is definitely a bit concerning.

View attachment 221882

Is that pic, the block surface , or the block with the old gasket on it ?

 

[1911CHAMPION]

Is that pic, the block surface , or the block with the old gasket on it ?

Block surface

 

[1911CHAMPION]

Here’s the same pic, not zoomed in.

 

[spasm3]

Block surface

Here’s the same pic, not zoomed in.

View attachment 221884

I would be worried about the head gasket failing again. That block needs resurfacing.

@Trav would have a valuable opinion here.

EDIT: looking again at the zoomed up pic, left side , I'm not sure there is not a crack there!

 

[Bailes1992]

That looks cracked to me. What does it look like on the inside of the bores?

 

[1911CHAMPION]

That looks cracked to me. What does it look like on the inside of the bores?

I will rotate the engine over tonight and get some pictures. I’ll also check that the pistons are all the same height in the bore at TDC and BDC for the two inside holes. As previously mentioned by someone, how did that coolant get into the oil. All good points.