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Piston rings 401 standard |
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Dave Z 
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Joined: Jul/02/2007 Location: Massachusetts Status: Offline Points: 300 |
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 Topic: Piston rings 401 standardPosted: Aug/17/2019 at 7:35am |
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I'm having an issue getting rings for an engine I have. 1971 401 standard bore stock pistons. I'm not doing a full rebuild, just new rings and bearings. In trying to find rings I tried Summit, they were back ordered. My local O'Rielly's, where I do most of my business, didn't have a listing for my engine, but a Chevy 400 .040 over is the same size. So I ordered a set, they were Mahle/Cleveite. When they came in, I did a quick check of the rings in the bores and found the second rings to have a gap about .010 larger than the top rings. I checked with a local engine builder who I have a good relationship with and have the utmost confidence in and he advised that wasn't right, don't use them. Brought them back, got another set, same thing. Brought those back, asked for the store to contact their supplier. Answer came back, must have been boxed incorrectly at the factory, we can send another set. This seemed like insanity, so I canceled that order, I'll try another source.
Now I figure I'll educate myself on what really is correct. The TSM says top and second ring have the same gap. Maybe ring technology has changed. Checked the Hastings, Weisco and JE sites and they all call fro the second ring to have a larger gap. Ok, I have used piston rings from two different 401 engines, both standard bore. To my amazement, the second ring has a gap about .015 bigger than the top ring. Also, the second ring is about .020 wider than the top ring. Back to my friend the engine builder. He puts me in touch with the vendor he buys from. I call him and told him about the Mahle rings, he's not surprised. He says if I used them I'd probably be ok. I ask why start with worn out parts and he agrees. He says he may be able to find an old set of Perfect Circle rings, I'm waiting for him to get back to me. My friend recommends if I can't get what I need there to try Grant rings. Sorry for the long post, but what's going on here? Why does the TSM say same gap and presumably factory rings don't measure as such? What's really correct? The fact that I'm staying with the stock bore seems to be limiting my options. No, I'm not going to bore it at this time. Dave Z Dave Z
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WesternRed
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Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5799 |
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Posted: Aug/17/2019 at 7:48am |
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A lot depends on application, for a stock rebuild using the original pistons it's probably best to stick with factory specs if you can. If you are changing pistons, then go with the piston manufacturer's recommendation.
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I've finally given up drinking for good...........now I only drink for evil.
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bbgjc
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Joined: Nov/24/2009 Location: No.VA Status: Offline Points: 1660 |
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Posted: Aug/17/2019 at 8:04am |
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They do make errors when writing a huge volume of info.
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SC397
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Posted: Aug/17/2019 at 8:05am |
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I Think that I have a couple sets of standard 401 rings if you need them.
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amxron
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Posted: Aug/22/2019 at 10:22pm |
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I used .040 428 CJ rings (4.170"), file fit the end gaps for your combo.
Ron.
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AMXron
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Class Guy
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Posted: Aug/23/2019 at 6:11am |
I was about to suggest to get a file-fit set and make your own gaps. First, you never said what any of the gaps were, only that the second ring gap is larger. Secondly, a second ring gap the is .010" larger will have no effect on the ability of the engine to last or make power, as long as the top ring is within specs.
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BassBoat
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Posted: Aug/24/2019 at 12:04pm |
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Ring science has changed. Used to be 004 per inch of bore for a top ring, 003 per inch of bore for a second ring, figuring the second ring didn't get as hot and didn't close up as much. Modern thinking is driven by very thin rings, which want to flutter when there is gas trapped between the top and second ring. So second ring gap is usually the same or even larger gap than the top ring to make sure gas isn't trapped between. Assuming you are using stock 5/64 rings, that old school thinking is probably still what you want but I wouldn't worry about it as said above by Class Guy. As long as the rings don't butt at temperature, you will never notice.
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Dave Z
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Posted: Aug/25/2019 at 5:31am |
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Thanks for the replies. I'm learning here and that's what I was hoping for. Finding standard size rings specific to my application seems to be problematic, as they may be listed but are not immediately available. My comment on the second ring of the two sets for Chevy 400 rings was a quick check of new parts. Even if I went by the guidelines for a larger gap on the second ring, it wouldn't be that much bigger. If the second ring needs to bleed pressure and the gap doesn't matter, then what does it do?
Dave Z
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Class Guy
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Posted: Aug/25/2019 at 7:45am |
From Hastings website: "Second compression rings (LCRs – lower compression rings)
scrape oil and prevent it from reaching the combustion chamber. They also
provide a second seal to trap combustion gases and aid in the heat transfer to
the cylinder wall." Here is a link to a good discussion about piston ring materials and technology -https://www.enginelabs.com/engine-tech/engine/the-material-details-evolution-of-piston-ring-technology/ Extensive current research shows that ring face wear is much more a factor to sealing the engine than end gap. That is why so much engineering and time has been spent perfecting materials and design of both the rings a ring grooves. That is why good rings cost more. If you do not expect the life of the engine to exceed 50K miles (1000 hours) and engine operation to not exceed 3K RPM for 99% of the time, then ring material and installation does not matter much. Ring gap is even less of a factor within reasonable limits. I have performed a couple of dyno tests for a research project where an engine was assembled with very controlled and measured specification, tested, and then disassembled to increase the top and second ring end gaps from .015" to .040". The result was way less than 1% variance.
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billd
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Posted: Aug/25/2019 at 3:33pm |
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Don't think so much about the gap difference of .010 - think of the square inches difference once the rings are in and only a fraction of that gap is "seen" by pressure. You put the ring in and see a gap - but when it's in the piston - how much of that gap is showing? How much pressure can be lost in one gap compared to the other?
The biggest thing on these old-school engines limited in RPM (compared to modern engines turning over 6K RPM routinely) is to prevent the ends from touching and wedging as they heat up. Follow the book - no problems. Class guy was talking going from .015 to .040 - you were talking only a .010 difference - from .015 compared to .025 - a full .015 less than the difference you are speaking of. Don't over-think this on these. |
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amxron wrote: