Slight discoloration on cylinder walls towards the top (see pics)

[jc1000]

Hello, I just pulled the cylinder head off my M20B27 with 126k miles. The cylinder walls look and feel really nice and smooth and the engine rotates super smooth, BUT I did notice the slight discoloration towards the top of the cylinders. It is almost like a bunch of greenish-brown spots. It feels perfectly smooth to the touch but just looks strange - at least to me. I know that there are faint horizontal lines from where the piston rings reach their highest point, but it is more the "spottiness" that I am confused by.

Is this normal? Is it a sign of an issue? I'm hoping to leave it untouched if I can but will do whatever I need to.

[digger]

looks like light corrosion, i wouldnt leave as regardless as engine is out of the car and they do not look anywhere near mint. i dont know what the goals are but normally you would measure to see how much wear there is and decide from there whether it needs proper honing or a simple dangleberry if on the cheap and minimal effort is required to get back on the road.

[jc1000]

looks like light corrosion, i wouldnt leave as regardless as engine is out of the car and they do not look anywhere near mint. i dont know what the goals are but normally you would measure to see how much wear there is and decide from there whether it needs proper honing or a simple dangleberry if on the cheap and minimal effort is required to get back on the road.

Thanks digger. I know it is hard to say just by eyeballing it, but does it look like I have a decent shot at staying within spec? Or is just too hard to tell without measuring?

[e30davie]

What was the reason for disassembly? did it run good before? compression alright?

Obviously the best thing is to do it properly, but if you cant feel the marks...and it ran alright before.....I've assembled worse and ran it for years.

As a newzealnder mate of mine once said - If in doubt, rev it out. (I suggest against this - mostly)

[digger]

Thanks digger. I know it is hard to say just by eyeballing it, but does it look like I have a decent shot at staying within spec? Or is just too hard to tell without measuring?

As asked by e30davie why is it apart? how did it run before? what's the intention for it e.g. proper rebuild, just need it running again.

[jc1000]

What was the reason for disassembly? did it run good before? compression alright?

Obviously the best thing is to do it properly, but if you cant feel the marks...and it ran alright before.....I've assembled worse and ran it for years.

As a newzealnder mate of mine once said - If in doubt, rev it out. (I suggest against this - mostly)

Unfortunately, I bought the car non-running and the engine was already partially torn down - though the issue was either spark or ignition-related, nothing in the bottom end (I confirmed with the seller). I wasn't interested in running it stock anyway so I tore it down the rest of the way. So no idea on compression, unfortunately.

My plan is to do the 2.7i budget stroker: 2.7 eta bottom end (which it came with) with the 885 i head (which I already have). I was hoping to not even need to pull the pistons - which I suppose I could still do - but digger and others have convinced me to at least have it mic'd at a machine shop. Do it once do it right, right? :)

The question becomes: if they are going to pull the pistons, I may as well source some short-skirt i pistons and have them deck the block a few mm to boost compression. Of course, that is only if the cylinders are still in spec (fingers crossed).

[e30davie]

If you look closely you can see the worms escaping the can. But if you've got the money, do_eet.

Admittedly I've always run "questionable" engines after they have passed a compression test. We've all got stories but I had a 4age that had been sitting for years and years. Compression tested - very low..... So I set it up in my garage with a battery and a dodgy fuel pump, and ran the engine on a tyre - hell of a lot of fun, but quite scary. but compression came good! i guess i cleaned up those rusty bores. Ran that engine for 3yrs before I sold it...it was always a little smokey at the top end...

also, never trust a seller ;)

[McGyver]

I just went down this route.

Chances are that the cylinder walls are close enough in spec and you can source used pistons for cheap that'll also be in spec. I'm sure this will run for years without issues.

I decided to get oversized pistons and re-bore the cylinders. It's not terribly pricy and I know I'll get a good seal.

https://www.rmeuropean.com/Make/PartNumberSearch.aspx?pnsearch=11251714810

[jc1000]

I just went down this route.

Chances are that the cylinder walls are close enough in spec and you can source used pistons for cheap that'll also be in spec. I'm sure this will run for years without issues.

I decided to get oversized pistons and re-bore the cylinders. It's not terribly pricy and I know I'll get a good seal.
https://www.rmeuropean.com/Make/Part...ch=11251714810

Thanks, this is helpful! How much did you go over? Is 0.5mm standard?

[McGyver]

First over is 0.25mm. These pistons are NLA.
Second over is 0.50mm. This was more than enough to clean up perfectly. The pistons I linked are essentially OEM and the cheapest I could find at $101/each. Way cheaper than custom pistons.

[jc1000]

First over is 0.25mm. These pistons are NLA.
Second over is 0.50mm. This was more than enough to clean up perfectly. The pistons I linked are essentially OEM and the cheapest I could find at $101/each. Way cheaper than custom pistons.

Sorry, what is NLA?

[McGyver]

No Longer Available

[jc1000]

I just went down this route.

Chances are that the cylinder walls are close enough in spec and you can source used pistons for cheap that'll also be in spec. I'm sure this will run for years without issues.

I decided to get oversized pistons and re-bore the cylinders. It's not terribly pricy and I know I'll get a good seal.
https://www.rmeuropean.com/Make/Part...ch=11251714810

Was your build the same as mine (budget stroker) or just an M20B25?

[digger]

Bear in mind the additional stroke means the rings will run over additional part of the cylinder wall

[jc1000]

Bear in mind the additional stroke means the rings will run over additional part of the cylinder wall

Yes, in fact, the more I think about it, the more I am leaning towards just buying OEM replacement (i) pistons. I am considering just honing over .5mm as I am not finding any stock 84mm pistons, but see lots of 84.5. Plus, then I will have a perfect seal.