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12/11/2019 10:24 PM  #1


My math is out of whack!

The instructions for gapping the rings say that for a 4.000" cylinder to multiply .0065 to get the results. And then for every .001 increase in the diameter the end gap is increased by .003. My dia has been increased .030. I'm coming out with a .116 end gap? That seems a little bit too much??? Can some one check my math?


70, ragtop 351W/416 stroker Edel Performer heads w pro flow 4, Comp roller 35-421-8. T5
 

12/11/2019 10:45 PM  #2


Re: My math is out of whack!

.003*30=.090   (0065*4.00)=0.026      together you get .090+.026 =0.116   That's just under 1/8"
You are increasing the diameter 30  - .001's

The math is ok, but I cannot guarantee the validity of the algorithm.

Last edited by lowercasesteve (12/11/2019 10:46 PM)


Original owner - 351w,T-5, 4whl disks, power R&P
 

12/12/2019 7:02 AM  #3


Re: My math is out of whack!

That's too large a gap.  I would contact the manufacturer and verify.  A typical endgap would be along the order of 0.026", and I usually go a bit wider on a performance engine, especially if I might run forced induction or nitrous at some future point.  Still, 0.030-0.035" should be plenty.  Though, I saw a dyno test once where they tried gaps all the way up to 0.090" and found no measureable difference in power.  You have to consider that the endgap in only exposed for a length of 1/2 the piston to wall clearance, which is typically no more that 0.004-0.005" total, or at most 0.0025" length times the width of the gap.  When you start checking different endgaps by figuring the area off that measurement you see a variance of 0.00002" between an endgap of say 0.026" and 0.035".  Then it becomes obvious why this doesn't effect power production; its basically insignificant.

Bottom line, call the manufacturer and see what their tech department says.  Unless these rings grow some insane amount I'm betting you're going to want to be a lot closer to 0.026". 

 

12/12/2019 9:33 AM  #4


Re: My math is out of whack!

I bet you the instructions are worded poorly or you are misreading them.

Any chance of posting a pic of them?

 

12/12/2019 1:09 PM  #5


Re: My math is out of whack!

Shouldn't you have instructions for a 4.030 set of rings? It sounds like you are trying to use standard bore rings in a 0.030 overbore motor.

 

12/12/2019 1:17 PM  #6


Re: My math is out of whack!

I think it may be poor wording or a misinterpretation.  This is from a hastings web page:

=16pxAn important fact to remember is that the manufacturers rigidly adhere to these tolerances and that the ring gaps are inspected in gauges accurate to .0001" at the cylinder diameter the ring is manufactured for. Any increase in the diameter of the cylinder is being used in, over the designated size, results in an increase of approximately .003" in ring gap for each .001" increase in cylinder diameter.

What they are saying is that if your cylinder is not bored to spec, you can end up with a significant ring gap error.  It states that the gap "error" increases by .003" for every .001" inch in diameter increase in bore diameter over spec.  This has nothing to do with calculating ring gap.  If your bore is 4.030 (30 over) multiply 4.030 times .0065 and this will give you the top ring gap of .0026 inches or 26 thousandths.

Are you checking gaps on rings that are supposed to be fit out of the box or are you working with file to fit rings>  The actual gaps and multipliers should come from the manufacturer and not web pages.  If they say .0065 then go with that.

This wiseco page provides some info on filing rings.  Note that their multipliers are different than yours and this is because they are manufacturer specific.

http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf


 

 

12/12/2019 1:35 PM  #7


Re: My math is out of whack!

Or, from a different perspective - if you attempt to use a standard bore ring (for a 4" bore) and your actual bore is 4.030, you would end up with a gap that is .116, which is a significant error and it will leak, etc.

My engine guy told me to always measure ring gap even in the rings are not file to fit just to ensure the bore is correct and that they rings match the part number on the box.

You should measure the gap with the ring about 1" down in the bore and you need to make sure the ring is square in the bore.  If you don't have a tool to push the ring flat down the bore, use a scale and check the distance down the bore at at least 4 locations around the clock.

On the FE I just built my ring gaps on a 4.090 (40 over) bore were .022 on top and .018 on the second ring.  On the oil rings I verified that the end gaps were not less than .015 and they didn't require any filing.  The ring kit was a .005 over file to fit set.

 

12/12/2019 2:24 PM  #8


Re: My math is out of whack!

HudginJ3 wrote:

The instructions for gapping the rings say that for a 4.000" cylinder to multiply .0065 to get the results. And then for every .001 increase in the diameter the end gap is increased by .003. My diameter has been increased .030. I'm coming out with a .116 end gap? That seems a little bit too much??? Can some one check my math?

Could .001 be a typo, instead be .01. Would make more sense, normal increases are .01.

Last edited by wsinsle (12/12/2019 2:25 PM)


If this forum can't fix it, it isn't broke.
 

12/12/2019 3:00 PM  #9


Re: My math is out of whack!

The math is fairly straight forward.  It all has to do with pi and circumference.  A 4.000 inch bore has a circumference of 12.566 and a 4.001 bore has a circumference of 12.569 inches.  The difference between the two is .003".  So, given the same ring, the ring will have a gap that is .003" wider if the bore is oversized by only .001"

 

12/12/2019 3:11 PM  #10


Re: My math is out of whack!

Yeah, again I think he has set of 4.00 standard bore rings and needs 4.030. For 4.030 the math *does* make sense...

 

12/12/2019 3:33 PM  #11


Re: My math is out of whack!

GPatrick wrote:

The math is fairly straight forward.  It all has to do with pi and circumference.  A 4.000 inch bore has a circumference of 12.566 and a 4.001 bore has a circumference of 12.569 inches.  The difference between the two is .003".  So, given the same ring, the ring will have a gap that is .003" wider if the bore is oversized by only .001"

I get it now, the math don't lie. Bare with me I'm no engine builder, I thought he was trying to determine what the gap size should be.


If this forum can't fix it, it isn't broke.
 

12/12/2019 3:47 PM  #12


Re: My math is out of whack!

He is - but in the initial post two different things were combined to determine gap.  The .003 for every .001 increase in diameter is not a value to set or alter the gap - it is a value of the error that is created if the bore is out of spec.  The .0065 times bore diameter is the correct method if that is what was provided by the ring manufacturer.

I had wondered where the .003 came from and it is just the increase in circumference - ran some numbers to verify.  Sometimes values that are thrown out into the interverse are rules of thumb and sometimes they are based on math.  In this case, it is pure math.

 

12/12/2019 4:32 PM  #13


Re: My math is out of whack!

The rings must be file fit based on his initial post where he is talking about "gapping the rings".  You don't gap the rings unless you are going to change the gap, and to do that you need file fit rings.  Otherwise you are just checking the end gap.

In my experience for about $25 the machine shop will file fit the rings, and this is what I've had done for the last three engines I've built.  The time required to check, file, and recheck just one set of rings makes that $25 seem like a bargain.  They have an electric machine that just does them all, knowing that their bore sizes don't vary by more than 0.001" typically.  The idea of fitting rings to individual cylinders is pretty absurd at those tolerance levels, as there would literally be no benefit.  They just set the gap based on the smallest cylinder and if any are a little larger an additional 0.001" of end gap isn't going to matter. 

Now, I still check every gap on assembly just to make sure nothing went wrong, but that takes very little time once you get good at it.  Spiral the ring in, and square it with a piston upside down.  I use the oil ring groove as the marker to know I'm in square. 

I've found you can spiral the ring into the bore by hand, but DO NOT try to put them on the pistons like that.  Use a ring expander, they cost like $10.  Failing to do this will result in a broken ring, or one compromised that will crack and fail in operation.

Then use a tapered ring compressor to install the pistons & rings.  These compressors ensure the rings don't hit the deck going in, which can crack or break the rings, unlike the old ratcheting coffee can compressors.  I find I can push the pistons into the bores with two fingers using a tapered compressor vs. having to use a mallet with the other style.  I'll never use anything but a tapered compressor again. 

 

12/12/2019 6:55 PM  #14


Re: My math is out of whack!


Sorry I’m late getting this out.  I had to fly all afternoon.


70, ragtop 351W/416 stroker Edel Performer heads w pro flow 4, Comp roller 35-421-8. T5
     Thread Starter
 

12/12/2019 7:09 PM  #15


Re: My math is out of whack!

Bad placement of the "it should be observed" as it appears to be part of the gap calculation.  It would have been more clear if they had put it in a separate paragraph.  Go with the .0065 times your actual bore and you'll be good to go for the top ring.  Have fun!

 

12/12/2019 7:26 PM  #16


Re: My math is out of whack!

Right
4.030" x .0065 = .0262"

Hypereutectic pistons sometimes run the top ring closer to the top of the piston where they get hotter, requiring a larger end gap such as the ones you have.  

Normal cast and most forged SBF pistons use 0.004" per every 1.0" of bore, or around 0.016" to 0.017" gap.   The only hyper pistons I ever had that called for the big gap were on some Keith Black pistons in a 350 bowtie I built for a friend at work.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

12/13/2019 7:15 AM  #17


Re: My math is out of whack!

Now, here's the real question: how on the ragged edge is this engine going to be?

Second question: How much do you trust your tune up?

Here's why: A little wider gap isn't going to hurt anything on the top ring.  No measurable decrease in power, no increase in oil consumption.  BUT, it does provide cheap insurance if the tune up ever gets lean and excessive heat is built up causing the top ring to expand more than proper mixture tolerances say it should.  If those ring ends but up its adios muchachos.  It sounds crazy, but an extra 0.005" could be the difference between life and death under such circumstances. 

 

12/13/2019 11:52 PM  #18


Re: My math is out of whack!

I think the later part of the instructions were just trying to state that the circumference of the bore is the diameter of the bore times PI (3.1415).  Don’t loose sleep about that part of the instructions.  All it really means is that if you purchased pre gapped rings for a 4” bore and tried to use them in a 4.030” bore the Ring gap would be .0942” too big.  That make sense?

I’d stick with the numbers below unless you feel the need for power adders...

Diameter x.0065 = ring gap
4.030" x .0065 = .0262"

 

12/14/2019 8:23 AM  #19


Re: My math is out of whack!

The only question I'd have about using rings meant for a 4" bore in a 4.030" bore engine would be are the rings specifically designed for the application, or just longer to account for the increased bore size?  Here's why: What determines how well the rings seal isn't the endgap; its the radial tension applied to the cylinder walls.  The difference in sealing from an 0.026" gap and a 0.035" gap is meaningless.  Obviously you'd prefer the correct part for the application, but in the event say you opened the package and installed a couple already and they can't be returned I would be only concerned with the answer to the question "are the rings different bore to bore or just longer for overbored engines?"

Since we're doing math, here's some that'll illustrate my point about endgap:

PTW clearance 0.005", half of which is 0.0025".  Ring endgap of 0.026" creates a hole of 0.026 x 0.0025 or 0.000065 in/sq

PTW clearance of 0.005", half of which is 0.0025".  Ring endgap of 0.035" creates a holes 0.035 x 0.0025 or 0.0000875 in/sq

A difference of 0.0000225".  Yes, that's 2.25 HUNDRED THOUSANDTHS of a square inch.  That's about the cross sectional area of a human hair. 

 

Board footera


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