Q: Second Ring Gap

[240260280]

1974 (and 1970) FSM states 2nd Ring gap for 260z (or 240z) is  0.15mm to 0.30mm (Avg = 0.23mm)

Recent dyno tests now recommend the 2nd ring gap be larger than the first to reduce ring flutter caused by inter-ring build-up.

The FSM average top ring gap of  0.30mm matches the  0.0040" per inch of bore rule of thumb.

The new data suggests that for the above rule of thumb, the 2nd ring should be 0.0045" per inch of bore which equals 0.37mm 2nd ring gap.

What are you all using...old or new?

 

 

[Zed Head]

1 hour ago, 240260280 said:

1974 (and 1970) FSM states 2nd Ring gap for 260z (or 240z) is  0.15mm to 0.30mm (Avg = 0.23mm)

Recent dyno tests now recommend the 2nd ring gap be larger than the first to reduce ring flutter caused by inter-ring build-up.

Not building anything, just curious about these "recent dyno results".  Are they published somewhere?  Is there a link?

 

Check out the invisible ad that showed up when I copied some text to delete it.

[240260280]

McCormick, Anderson, Mayhew, Rychlewski, New Developments in Piston Rings for the Modern Diesel Engine, SAE 750769

http://www.diagnosticengineers.org/journal_ articles/Ring Gaps vs Knowledge Gaps.php

 

Edited March 24, 2018 by 240260280

[Captain Obvious]

I have no experience with ring gaps, so I don't have any input into your original question, but I am curious...

You keep mentioning the average center of the range for the specs of the gaps. Is that your target for when you put rings on?

Why don't you aim for the narrow end of the spec? I mean, as the cylinder walls and ring wears, won't that gap just get wider over time? That's about a six thousandth wide spec... Seems like aiming towards the narrow end of the spec would be a better place to shoot for than the center.

Like I said, I have no expertise here, but just wondering.

[240260280]

10 hours ago, Captain Obvious said:

I have no experience with ring gaps, so I don't have any input into your original question, but I am curious...

You keep mentioning the average center of the range for the specs of the gaps. Is that your target for when you put rings on?

Why don't you aim for the narrow end of the spec? I mean, as the cylinder walls and ring wears, won't that gap just get wider over time? That's about a six thousandth wide spec... Seems like aiming towards the narrow end of the spec would be a better place to shoot for than the center.

Like I said, I have no expertise here, but just wondering.

 

Good point.   The reason I went for the middle is to be safe and the Datsun FSM middle of range value aligns with the rule of thumb value for street cars.

If the space is set too close,  a period of too much sustained heat (like track racing) may expand the ring ends into each other and bind the piston.  Also, the Datsun rings I removed were chrome whereas the replacements I purchased are molybdenum  coated iron so they may expand differently.

The rule of thumb coefficient changes for the application as max temperature and duration of  max temperature can make the ring expand even more. For example the gap for a turbocharged track car is much greater.

 

Here is a nice page to introduce most aspects of a piston ring:

http://garage.grumpysperformance.com/index.php?threads/piston-ring-gaps.2837/

 

[madkaw]

I wonder if newer concepts concerning rings can be applied to older engines(pistons), blocks, etc... 

I went with recommended gaps for our engines with stock pistons. The second ring is a smaller gap , though I went closer to the wider end gap averages