Guest datsun75 Posted November 5, 2001 Share Posted November 5, 2001 Im rebuilding my chev 350 engines bottom end now. I just purchased my stock std. bore piston rings and measured the top ring on the cylinder ridge and the gap is around .044 to .048. I know the motor has been rebuilt before but hasnt been punched 30 over. What should I do buy rings that I file down myself to a .022 size gap. I wandering if Im going to need to punch it out 30 over. Any ideas? Quote Link to comment Share on other sites More sharing options...
Tim240z Posted November 5, 2001 Share Posted November 5, 2001 Even more important, what is your piston to bore clearance? Too much gap and you'll have piston slap. If you are unsure, or not comfortable with the setup, and since you have to put in new pistons anyway, go 30 over and be sure. Tim Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 5, 2001 Share Posted November 5, 2001 Thats an option I think, it depends on how much will need to be honed to clean up the cylinder. I would also measure the ring gap in a few places (towards the middle and near the bottom of the ring travel area in the cylinder). Reason being is to check for taper in the cylinder (or measure using an inside mike in the bores), grinding them at .022 thou. might be ok at the top of the cyl. but if it tapers down then that clearance will be tight as you can imagine at the bottom of its stroke. Unless its a real budget constraint, I'd sure try and bore it and get it right, it'll save you a trip into the engine later. Just an opinion of course. Regards, Lone Quote Link to comment Share on other sites More sharing options...
Guest datsun75 Posted November 5, 2001 Share Posted November 5, 2001 Thanks Tim and Loan! I'm going to be searching for a local machine shop now. This all takes time and Im not a patient person. LOL! I got to get this thing on the road. I did measure piston to cylinder wall clearance but only at the top and it was easily .008 This is on a ridge that hasnt been reamed yet. Yikes! We took a measurement of the ring gap on and below the ridge and did the little formula the "how to build chevy small blocks" book explains. Measuring with a feeler gauge, I came out with a difference of .0073 The book indicates that a difference over .006 is saying that you need to punch it out or plan on a motor that will not last longer than 50K miles without burning oil. Hope I haven't lost any of you on that. ha Later, datsun75 Quote Link to comment Share on other sites More sharing options...
labrat Posted June 16, 2002 Share Posted June 16, 2002 "We took a measurement of the ring gap on and below the ridge and did the little formula the "how to build chevy small blocks" book explains. Measuring with a feeler gauge, I came out with a difference of .0073 The book indicates that a difference over .006 is saying that you need to punch it out " Does anyone have this formula for finding piston to sylinder bore clearance handy? I know this is an old thread, but its just what i'm looking for... Thanks Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted June 17, 2002 Share Posted June 17, 2002 Labrat, Your question is not as easily answered as one might think as there is no real "Quick-Rule of Thumb" for piston to cylinder clearances. There are many variables that determine what your piston to cylinder wall clearances should be as well as your piston [ring] to cylinder wall clearances should also be. Now having said that there are no real rules of thumb, there are some hard fast rules (to be considered in general rules) and they are: Piston To Cylinder Wall Clearances: 1)Chevy's Cast Alluminum a)Street Use: .0015-.0025" 2)Chevy's Forged a)Street Use: .004-.006" b)Competition Use: .0055-.0065" 3)Chevy's Hypereutectic a)Street Use: .0015-.0020" 4)TRW's Hypereutectic a)Street Use: .0015-.0020" 5)TRW's Forged a)Street Use: .003-.0055" b)Competition Use: .0055-.0065" The Ring Gap: 1) Top Ring: .004" for every Cylinder Diameter Inch 2) 2nd Ring: .003" for every Cylinder Diameter Inch 3) Oil Rings: Low Tension -vs- High Tension. Low tension is for Drag Racing while High Tension is for Street usage and Auto Cross. The determining factors are not only how the piston was mfg'd (Cut by the mfg's Lathe) but also what material (alloy) was used: cast, forged or hypereutectic ect. ect. The same goes for the piston rings; what material are the rings made of(?). The reason for such an evasive answer is simply that each material has a different expansion rate than another material. The ring gap and the piston clearance will depend on the instructions of the mfg'er. You may want to talk to a local machinest in your area. Or call one of the many performance venders tech lines for answers on the pistons and rings they sell. Or, you could go to your local book store & just skim thru the chapters on Pistons in some of the "How To" performance books: this will give you a good idea of what is acceptable -vs- not acceptable. Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted June 17, 2002 Share Posted June 17, 2002 I've found that the best source for proper piston to cylinder clearance is the manufacturer of the pistons you purchase. Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted June 17, 2002 Share Posted June 17, 2002 posiably helpful info http://www.antiqueautoranch.com/montana500/speedpiston.html http://www.kb-silvolite.com/speclear.htm http://www.babcox.com/editorial/ar/ar80125.htm http://www.picknowl.com.au/homepages/harrals/tech/piston.htm http://www.babcox.com/editorial/us/us20114.htm Quote Link to comment Share on other sites More sharing options...
labrat Posted June 18, 2002 Share Posted June 18, 2002 Thanks for the information, it was fascinating reading, but not exactly what I was looking for. I'm looking for something along the lines of "if your piston ring end gap is X, then your piston to cylinder clearance is X" basically trying to measure my exact bore dimensions without an inside micrometer. That's what it appeared Datsun75 was doing in the previous post. On second thought, it kinda looks like he just measured the difference between the ridge circumference and the circumference lower in the cylinder (using the ring gap as a guide) to determine the wear for the cylinder, not the exact dimensions. Hhmmmm... Bill Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted June 18, 2002 Share Posted June 18, 2002 Labrat, I'm not aware of any formula for using the Ring End Gap as the deciding factor of what your Piston to Cyl.Wall Clearance should be; simply due to the fact that the ring and piston are of two seperate metal alloys which will have two seperate rates of expansion and contraction. The pistons are not perfectly round-they are to the naked eye but not to a micrometer. Your cylinder wall is pretty close to a perfect circle after the machining process and your rings will conform to your the cylinder bore; not the other way around. If you know of another way-please let us know. As far as piston to cyl.wall clearance goes; that "KB-Performance Pistons" site that GrumpyVette gave has a diagram on it of just about any clearance any engine piston will ever need; good site to read and re-read. Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
labrat Posted June 18, 2002 Share Posted June 18, 2002 Well, i wasn't so much as looking for what I needed, as what I already have. The motor I tore down only has a couple thousand miles on it, so I'm reusing pretty much everything (pistons, rods, crank, etc) just freshening it up with new bearings, rings, cam, and heads. I just wanted an easy way to "check" how much wear I had on the cylinder walls. What I did was measure the ring end gap with the piston on the ring ridge at the top of the cylinder with a feeler guage. I then measured the same gap with the ring slightly lower. The ridge should still be 4.040" (bored .040 over) as no wear occurs here. I took the difference in end gaps, divided by pi, and got the cylinder wear in thousandths. With .002 difference in the end gaps between the ridge and cylinder wall, that means less than .001 wear, or increase in piston to cylinder clearance. Basically just checking to see if it needed punched out again and new pistons. If there is a flaw in my logic here, please point it out! Thanks for the input, guys. Bill Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted June 21, 2002 Share Posted June 21, 2002 Labrat, I would have to say your method to your madness is all fine and dandy; providing you knew what the measurements were prior to the initial build up of your engine. If you did not actually take those exact same measurements prior to their initial install-then everything from here on out is only "Assumptions" not substantiated in any fact and we all know what [assuming] will get us. Still, if you want to know cylinder wear; then take your in cylinder North/South/East/West measurements at the top of the stroke, mid stroke and bottom of the stroke w/in the cylinder. Then take those same measurements on each piston and compare them to the measurements of their proper cyl's. Then and only then will you know what or how your cylinder has worn in the few thousand miles. BTW: why are you tearing down the engine after only a few thousand miles? Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
labrat Posted June 21, 2002 Share Posted June 21, 2002 Well, I bought an inside mic (one of the cheap autozone dial ones) and measured the ridge, and it was 4.040, but my mic won't fit further down in the cylinder, hence my roundabout way of measuring the cylinder wear. I'm tearing the motor down because: 1.) it ran like crap, smoked, and used lots of oil. (had 5 broken top rings) 2.) I just bought the donor car and swapped drivetains without really knowing what I had. 3.) the rear main seal was leaking and the easiest way to get to it was with the motor out of the car. While i've got it out, I might as well... 4.) I just bought a motorcycle, it's summer out, and while I have the time and alternate transportation, I might as well... I was able to mic the outside diameter of the pistons (at the skirt) and judging by my crazy way of getting cylinder diameter, determined I have .002 piston to cylinder clearance at the widest point. .0015 at the narrowest. The old bearings looked good when I took them out (only 1 was scored, and i suspect it was ran low on oil once), and there was still a suggestion of crosshatching on the cylinder walls, that's how i'm getting the fact it only had a few thousand on it. I was also told that by the "original" owner (the guy who had it before me, but not original by any means), but I didn't know for sure. If nothing else, it was 200 bux and 30 or so hours worth of peace of mind. I also got a nice new lumpy Crane energizer cam in it, so at least I know what I got now. That was worth it, if nothing else. Bill Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted June 21, 2002 Share Posted June 21, 2002 Piece of mind will go a long long way; you definately did the right thing. So many busted top rings? What in the world did the previous owner do/or not do correctly? Atleast now you car rest at nights knowing your internals are happy now; happy engines make for happy owners. Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
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