Mandlrel bend necking fit-up - how to deal with it?

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I am building a set of headers with some mandrel bends. Unfortunately the mandrel bends have some serious necking which means that when I match up sections there is a distinct mismatch in pipe size. This turns into a bit of a nightmare in that they leave a serious lip inside the header pipe.Any tips on how to make this work?EDIT: Doh! Misspelled "Mandrel" in the subject and no subject edit feature..
Reply:Those bends aren't mandrel bent, just regular exhaust shop bender did that.  Notice the smaller radius on the inside of the bend.Mandrel bent tubing is the same diameter in the bends as it is in the straight sections.  That's why you are having problems.  Check out Columbia River Mandrel Builders Kit for an idea on what you need.MarkI haven't always been a nurse........Craftsman 12"x36" LatheEnco G-30B MillHobart Handler 175Lincoln WeldandPower 225 AC/DC G-7 CV/CCAdd a Foot Pedal to a Harbor Freight Chicago Electric 165A DC TIG PapaLion's Gate Build
Reply:Originally Posted by MarkBall2Those bends aren't mandrel bent, just regular exhaust shop bender did that.  Notice the smaller radius on the inside of the bend.Mandrel bent tubing is the same diameter in the bends as it is in the straight sections.  That's why you are having problems.  Check out Columbia River Mandrel Builders Kit for an idea on what you need.
Reply:Plus that's regular aluminized steel pipe. It won't take the heat that headers do and will rust out in no time. That's why the (cheap) headers from summit and other places only last a year or so. Might as well do it correctly and use stainless.
Reply:A mandrel bender uses a die in the center of the tube & the tube is slid over it as it's bent.  That way the inside diameter remains the same.Google "Mandrel Bender" & look at some video's.  That will help understand what the difference is.As BC said, header tubing won't last long if it's aluminized tubing.  Stainless is the best way & it's cost effective if you're building them yourself.MarkI haven't always been a nurse........Craftsman 12"x36" LatheEnco G-30B MillHobart Handler 175Lincoln WeldandPower 225 AC/DC G-7 CV/CCAdd a Foot Pedal to a Harbor Freight Chicago Electric 165A DC TIG PapaLion's Gate Build
Reply:Originally Posted by MarkBall2A mandrel bender uses a die in the center of the tube & the tube is slid over it as it's bent.  That way the inside diameter remains the same.Google "Mandrel Bender" & look at some video's.  That will help understand what the difference is.As BC said, header tubing won't last long if it's aluminized tubing.  Stainless is the best way & it's cost effective if you're building them yourself.
Reply:I've seen the Hooker bends & they seem quite nice.  I have no idea what Columbia River bends are like, they were the first website that came up.There is usually some change in the diameter with mandrel bends, but not much.  Nowhere nearly as much as the one's in your photos.  Usually less than half the width of the tubing being bent, so matching should be much better.Good luck & keep posting the outcomes.  We like pictures.MarkI haven't always been a nurse........Craftsman 12"x36" LatheEnco G-30B MillHobart Handler 175Lincoln WeldandPower 225 AC/DC G-7 CV/CCAdd a Foot Pedal to a Harbor Freight Chicago Electric 165A DC TIG PapaLion's Gate Build
Reply:I just did some headers with mandrel bends, and fit-up is indeed critical.  I can't imagine trying to do it with sub-par bends.  Keep us posted on your progress.
Reply:I was having the same problem, but maybe not quite so pronounced. I had a machinist friend make me a tapered "plug"; about 1/4" smaller than the inside dimension on one end, and about 1/4" larger than the outside dimension on the other end. Before fit-up, I tap the plug into the end of the exhaust tube to round it out. It will sometimes need a littlemore persuasion, but a little work with a small hammer does the trick.Bgbkwndo.
Reply:I think you may have found the one use for this tool:http://www.harborfreight.com/medium-...der-37353.html Attached Images
Reply:Originally Posted by rlitmanI think you may have found the one use for this tool:http://www.harborfreight.com/medium-...der-37353.html
Reply:from my experience with those tools, they wil not change the diameter of tubing, they only expand a crushed / deformed tool back to round.  You might try with a torch?  I destroyed the tool trying to expand some pipe to create a slip fit.  And that was a better quality autozone loaner tool than harbor freight.  (high end chinese vs low end chinese)miller syncrowave 250hobart handler 140home made 400 amp engine driven in progress...
Reply:Er... The harbor freight tool makes a good heat sink for welding up bicycle bottom brackets.  The bottom bracket is the large horizontal tube at the bottom of a bicycle frame that holds the crankset.  Its threaded, and warps easily. The way that tool is made its of little use as a real tubing expander.
Reply:Here's how you deal with this type of mismatch..Always align the outside radius of the bends so that the lip occurs on the tight inner radius. This is done for two reasons...When bent, the metal is stretched and is thinner on the outside than the inside. A gap/step is very easy to fill (and make look pretty) on the inside and much more difficult to fill  and make look pretty on the thinner walled outside. And secondly when exhaust (or whatever) flows down the tube it's mass and inertia will cause it to  crowd the outer radius wall, the wall is what's defelcting the flow and making it change direction, so it's much more important that the outside radius be lip-freeand more smooth than the inside radius.Welding/Fab Pics: www.UtahWeld.com
Reply:First, those harbor freight hand expander tools are pretty much worthless.Second, if the only fit-up problem is tubing end is oval shaped compared to another end being mated to, and not perfectly mating all the way around, that is easily addressed by first lining up one spot and tack welding it.  Now, use a c-clamp to squeeze the tubing to bring another area into desired alignment, and tack weld that.  If needbe, you can repeat squeezing in different places to align the tubing one small tack or stitch weld at a time, working your way around the tubing lining openings.  However, if your problem is one tube is a different diameter than another, this technique won't fix that, and you'll still be left with a mismatched diameter "step" at the end.  However you can cut a slightly larger circumferential opening if you cut at a slight angle.  It is best for flow to cut perpendicular to the tubing though.Even a perfectly done mandrel bent will have the inside radius of the bent part thicker than the outer radius, so it's never going to be totally perfect (either the inside, the outside, or both inside and outside of the tubing being mated up probably won't perfectly fit).  A slight misalignment is usually acceptable.  If you want about the best quality mandrel bends there are, I'd say order them from burns stainless:http://www.burnsstainless.comThese are not "budget" mandrel bends, though.Last edited by jakeru; 09-12-2012 at 03:49 AM.
Reply:That's why you are having problems.
Reply:Like mentioned those aren't true 100% mandrel bends. Got to get good stuff to get good fitup.
Reply:Originally Posted by MikeGyverHere's how you deal with this type of mismatch..Always align the outside radius of the bends so that the lip occurs on the tight inner radius. This is done for two reasons...When bent, the metal is stretched and is thinner on the outside than the inside. A gap/step is very easy to fill (and make look pretty) on the inside and much more difficult to fill  and make look pretty on the thinner walled outside. And secondly when exhaust (or whatever) flows down the tube it's mass and inertia will cause it to  crowd the outer radius wall, the wall is what's defelcting the flow and making it change direction, so it's much more important that the outside radius be lip-freeand more smooth than the inside radius.
Reply:Yupp, those are not mandrel bends.