L28 Rebuild Help For 1978 280z Honing vs Boring

[Patcon]

14 hours ago, Racer X said:

The method of getting the fuel/air mixture into the combustion chamber isn't relevant. What is important is that the quality, and the octane of the fuel/air mixture is correct for the application. If normally aspirated, a dished piston will only lower the compression ratio, and therefore the power output. If turbocharged, a dished piston would be more appropriate, as higher compression ratios don't work as well with forced induction.

 

Milling the tops of pistons that aren't designed and manufactured to be machined isn't a good idea. Custom made forged pistons are available with the potential for machining valve reliefs included in the design.

The dished pistons in this engine were used originally to lower compression and improve emissions at a time when manufacturers were under pressure to comply with the ever tightening emissions standards. Dished pistons were also used in the turbocharged L series engines used in the early 1980s Z cars.

 

My point was the flat tops will raise the compression and might make it impossible to pass Smog. I don't think Nissan would have added dish pistons to lower the compression for emissions if they could have run flat tops

[Ownallday]

Just wanted to get back to you guys with some pictures. I went ahead and removed the pistons and cleaned them up. Crank looked pretty good overall. Went ahead and polished the crank by hand and looked even better, zero scoring that can be felt with the finger nail or thin plastic gauge. For the most part the journals seem to be even based on how my co worker measured it.

Cylinder walls my co worker went ahead and honed it and if you ask me looks very good. Measured the bores and they are still at a perfect 86mm like when I measured before the hone. Going to take to a machine shop in a week or two to have them clean up and do some fiber measurements.

[Zed Head]

Did you guys use a ball hone?  Looks scrapey.  The angle seems shallow like it wasn't moved up and down fast enough.  Not an expert.  You can still see the scratches.  

Don't have the experience to say if it will seal well with the scratches but it doesn't look ideal.  I think that you're on the edge of "go cheap and hope" or spend the money and do it "right"?  If you have the means of tearing it down again easily, I'd probably just put some new rings in it and see what happens.  If you have limited space and time maybe you should spend the money and make sure it's right.  It's a dilemma. 

Another way to describe it - if you like working on it do the rings and take a chance, if you like driving it spend the extra money on an overbore. 

[Ownallday]

5 minutes ago, Zed Head said:

Did you guys use a ball hone?  Looks scrapey.  The angle seems shallow like it wasn't moved up and down fast enough.  Not an expert.  You can still see the scratches.  

Don't have the experience to say if it will seal well with the scratches but it doesn't look ideal.  I think that you're on the edge of "go cheap and hope" or spend the money and do it "right"?  If you have the means of tearing it down again easily, I'd probably just put some new rings in it and see what happens.  If you have limited space and time maybe you should spend the money and make sure it's right.  It's a dilemma. 

Another way to describe it - if you like working on it do the rings and take a chance, if you like driving it spend the extra money on an overbore. 

Yeah he used a ball hone. He went pretty dang fast if you ask me. I think a lot of it has to do with the camera making it look sharper than it really is because I see what you mean but in person looks different. Based on my eye though and trying to feel for scratches etc you can't feel anything.

For now I'm going to take the chance with new rings, bearings etc and see how the car performs. Worst that can happen is I burn oil and have to pull engine again which I'm okay with. $3500-$4000 is a lot compared to $300 and for the price difference I'm willing to sacrifice some time and extra work if need be.

[Racer X]

The cross hatch angle should be around 30 degrees. I still see vertical scratches in the bores too. You mention the bores are still 86mm. What is the difference, from a measurment inline with the crank, and at 90 degrees to it? Be sure to also measure near the top, halfway down, and at the bottom of the stroke. You will find a difference, in diameters, and at each of the three locations from top to bottom. There is a specification for the amount of elongation of the diameter, and the taper from top to bottom. Check the service manual.

The cylinder block deck still looks pretty rough, and would need cleaned up prior to installing the cylinder head. Have you measured it for flatness? I believe the allowance is something like 0.003".

How about the cylinder head? If the lower end is an indicator, the head will need work as well.

 

As Zed mentions, how many times do you want to pull the engine and disassemble it, and how much money do you want to spend before deciding that a proper rebuild would have been the better way to go initially?

 

[dutchzcarguy]

Sorry but i smiled behind my hand.. when you sayd that the diameter was still 86mm..  It would be shocking if it wasn't!

Normally you shoud measure it in 1/100 mm i've seen (Swiss made) parts that were made on 1/1000 of a mm! (NOT of inches!)

My bet is that the 300$ are gonna be as i call "learning money" and there is nothing wrong with that, but be prepared to pull the engine again..  Those proffesional engine builders.. most know what they are talking about and your engine has a LOT of miles on it. so it really needs a rebore.

As for the crank.. it can be as shiney as a new chrome part, but that tells me nothing. Use a micrometer to test it. And look in a service man. what it should be.. (And you have a good chance you find out that the crank also needs a new surface and new bearings.)

Yes i know i'm a bit harsh (i learn't that us Dutchies are a bit directer than you americans.. 😉 )  but it's now that you could save yourself 300+ dollars and a lot of unneccessary work 😮  (Think of all those bolts that have to be unscrewed and screwed on again.)

I never said it's a cheap hobby... 

[dutchzcarguy]

13 hours ago, Zed Head said:

The angle seems shallow like it wasn't moved up and down fast enough.

I agree.. Ownallday, take a look at the angle and look of the "scratches" in my original (120Kkm) engine..  it's totally different.

On the other hand i want to hear from you what the end result would be after just a hone and new rings.. 

[Ownallday]

On 6/30/2023 at 12:58 AM, dutchzcarguy said:

I agree.. Ownallday, take a look at the angle and look of the "scratches" in my original (120Kkm) engine..  it's totally different.

On the other hand i want to hear from you what the end result would be after just a hone and new rings.. 

I will get back with how the engine performs once it's complete. Made a lot of progress since. Measured everything and everything checked out. Crank was good, wall measurements were good, thrust was good, no warpage on the deck, everything was within spec. Recently checked with a plastic gauge with the new bearings and everything was pretty much perfect within spec. Crankshaft is back in the the block and installed.

I am now stuck at the installation of the piston rings. I have a standard size set of rings from Hastings and can't seem to find information on ring gap size. Are aftermarket rings gapped differently than OEM? My top ring gap measures at 0.017 with a feeler gauge. Any insight would be appreciated on this part.

[Zed Head]

Hastings implies that their rings are not meant to be filed to fit unless they are racing rings.

https://www.hastingspistonrings.com/tech-tips-faqs/

[Ownallday]

5 minutes ago, Zed Head said:

Hastings implies that their rings are not meant to be filed to fit unless they are racing rings.

https://www.hastingspistonrings.com/tech-tips-faqs/

So basically the rings are meant to fit without any gapping? They seem to be gapped slightly larger than what OEM recommends which I'm seeing is 0.015 or 0.016

[dutchzcarguy]

13 hours ago, Ownallday said:

I'm seeing is 0.015 or 0.016

Please put Inches (In?),miles (mls) or kilometers(km) behind those numbers as we are a bit international.. i don't mind  (actually i do but hey what can i do) that you use those (damn) inches.. 🙊 As Japanese cars are most in metric.. (exceptions are pipes and tires ..)

Again i'm interested how it will work, Can't believe that all the numbers were with'in the tolerances.. I think it's going to use alot of oil..

Hihi.. last week someone overhere was selling a L24.. he said he had a nice looking painted L24 and showed some pic's.. as i asked what he did to the cylinderwall he said that he had polished them with 2000 sandpaper!  🙈😮 And yeah he also told me that a carguy said it looked good!  Yeah it's a polished turd..  I told him he better not listen to the carguy as he has no clue! Now that engine needs a complete revision that's for sure.

Edited July 11, 2023 by dutchzcarguy

[Ownallday]

3 hours ago, dutchzcarguy said:

Please put Inches (In?),miles (mls) or kilometers(km) behind those numbers as we are a bit international.. i don't mind  (actually i do but hey what can i do) that you use those (damn) inches.. 🙊 As Japanese cars are most in metric.. (exceptions are pipes and tires ..)

Again i'm interested how it will work, Can't believe that all the numbers were with'in the tolerances.. I think it's going to use alot of oil..

Hihi.. last week someone overhere was selling a L24.. he said he had a nice looking painted L24 and showed some pic's.. as i asked what he did to the cylinderwall he said that he had polished them with 2000 sandpaper!  🙈😮 And yeah he also told me that a carguy said it looked good!  Yeah it's a polished turd..  I told him he better not listen to the carguy as he has no clue! Now that engine needs a complete revision that's for sure.

Okay, for the most part all the ring gaps for the top compression ring measured at 0.45mm - 0.48mm while the bottom compression rings measured 0.4mm. According to the factory service manual, the rings are to be between 0.23-0.38mm so my compression rings from Hastings are slightly larger but after looking around on their website it's within spec for what they recommend with the ring diameter.

Me and all my co workers think it'll run fine and won't go through oil. Anything can't be worse than how my previous engine was burning oil. Had to add a quart almost every 600-800 miles.

I did take a engine building class years ago so I'd like to think I'm not that stupid to sand my walls down haha. My next step is to check the head after I get the rings installed. I'm missing one of those damn exhaust liners but supposedly it'll run fine.