Are chrome alloy piston rings bad?

[Robotsan]

Hi all,

I'm looking at a 73 240z which had an engine rebuild a few years ago - inc a reconditioned N42 (I *think) head, but also chrome alloy piston rings were installed instead of the standard (cast iron?) ones. I spoke to the mechanic who looked after the car for several years (he didn't do the engine work though), and he said (I paraphrase) chrome alloy rings are intended for racing, and they are not seating 100% perfectly, which is allowing air through causing a very slight compression issue, so it runs a little rich and causes a little puff of smoke out the exhaust.

Does that make sense for anyone with the knowledge here?

He says its not a problem for the car now (apart from a little smoke and less MPG), but it does mean with these rings installed, the engine can't have webbers or any other triples installed, as the engine then wants too much air and the car can't run any richer. He knows because the previous owner got him to install webbers on it and it didn't play nicely so they were removed.

So if I was to purchase the car, I would likely have these changed and the engine thoroughly inspected. Question I have is what approx cost am I looking at to do that?

The mechanic I spoke to did refresh a lot of the rest of the engine bay - new alloy rad, all pipes replaced, water pump, brake master cylinder and almost everything else, and a machine shop did refresh the replacement head. He said the engine was running really well apart from the aforementioned issue, so hopefully there wouldn't be any nasty surprises if I did have it stripped down and rings replaced.

 

[jonbills]

There's some interesting comments on chrome rings here https://auto.jepistons.com/blog/piston-ring-materials-explained
TBH, I'm not sure I'd listen too much to that 'mechanic'. His comments on engines wanting too much air and not being able to run richer sound like he doesn't really understand what he's talking about.
You can get a set of rings for £60. https://www.ebay.co.uk/itm/115122941006?
But given the mechanic's level of understanding, you won't really know that it's just rings needed until you've opened the engine.
Worst case, it needs a rebore and pistons, and if you do the strip and assembly yourself, you can probably sort that for about £1k. (actually, its not the worst case- it can always be worse!)
I think we've said before, these engines are reliable and easy to rebuild/maintain - it's really the body you want to focus on.

 

[Robotsan]

There's some interesting comments on chrome rings here https://auto.jepistons.com/blog/piston-ring-materials-explained
TBH, I'm not sure I'd listen too much to that 'mechanic'. His comments on engines wanting too much air and not being able to run richer sound like he doesn't really understand what he's talking about.
You can get a set of rings for £60. https://www.ebay.co.uk/itm/115122941006?
But given the mechanics level of understanding, you won't really know that it's just rings needed until you've opened the engine.
Worst case, it needs a rebore and pistons, and if you do the strip and assembly yourself, you can probably sort that for about £1k. (actually, its not the worst case- it can always be worse!)
I think we've said before, these engines are reliable and easy to rebuild/maintain - it's really the body you want to focus on.

Thanks Jon, that's put me at ease a bit! To be honest, it's probably not his fault - his words have been filtered through someone else who really doesn't know what they're talking about (me!). I was frantically taking notes as he was talking, so I may have got it a bit wrong. He's a classic JDM specialist, so the evidence points more to my poor note taking

Are you saying it could need a rebore & new pistons because of the chrome rings? Or is that you just saying that's a worst case scenario when you don't know the history of an engine?

I've never taken an engine apart (or put one back together again!), no matter how basic they are to most people, so I wouldn't fancy tackling that job to be honest. I'd have to take it to a specialist. So I'm guessing we could be looking at a few grand in total.

 

[jonbills]

What I'm really saying is that the only things you know to be true are the observed symptoms: it's got low(ish?) compression, it runs rich and it smokes. The rest is just speculation until you take it apart. Maybe it just needs tuning and valve stem seals or maybe it needs lots of work.
I think you'll need to spend some time with it convincing yourself it's not just adjustment of the carbs or valves or something, before you go to a 'specialist' .

 

[Robotsan]

What I'm really saying is that the only things you know to be true are the observed symptoms: it's got low(ish?) compression, it runs rich and it smokes. The rest is just speculation until you take it apart. Maybe it just needs tuning and valve stem seals or maybe it needs lots of work.
I think you'll need to spend some time with it convincing yourself it's not just adjustment of the carbs or valves or something, before you go to a 'specialist' .

Gotcha. Best not to assume anything. Sage advice, thanks Jon. Off to see the car this afternoon, will report back!