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techtalk:evo:engmech03f [2021/07/21 01:14] hippysmack [Straightness and Roundness of OEM Cylinders] |
techtalk:evo:engmech03f [2024/01/03 21:01] (current) hippysmack |
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| ====== Straightness and Roundness of OEM Cylinders ====== | ====== Straightness and Roundness of OEM Cylinders ====== | ||
| - | Article by aswracing of the XLForum. ((http://xlforum.net/forums/showthread.php?t=15735)) \\ | + | Article by aswracing of the XLForum. ((https://www.xlforum.net/forum/vendors/nrhs-high-performance/10024-straightness-and-roundness-of-cylinders?t=15735)) \\ |
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| + | **This applies mainly to factory HD cylinders**. And it is meant as an example between production cylinders and aftermarket cylinders with a "hands on" machinist. \\ | ||
| + | The factory cylinder is a production line piece. Like all manufacturing, the idea is to deliver something that works at the lowest cost possible. \\ | ||
| + | But when you rebuild a motor, or convert an 883 to a 1200, you have opportunities to do things better than the factory did. \\ | ||
| + | This is one of those opportunities. There are several others. It's really no different in the world of cars. \\ | ||
| + | A good machinist can always get things a whole lot better than typical production line stuff. | ||
| It is not a highly unusual result (for stock cylinders to be out of round). \\ | It is not a highly unusual result (for stock cylinders to be out of round). \\ | ||
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| Actual phone conversation I had once: \\ | Actual phone conversation I had once: \\ | ||
| Customer: Just got my cylinders back and they don't measure right. \\ | Customer: Just got my cylinders back and they don't measure right. \\ | ||
| - | Me: What kind of torque plates did you use? | + | Me: What kind of torque plates did you use? \\ |
| Customer: What kind of what?... (followed by a long conversation explaining how bad a factory cylinder distorts and how essential the torque plate is). | Customer: What kind of what?... (followed by a long conversation explaining how bad a factory cylinder distorts and how essential the torque plate is). | ||
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| Today I took some pictures of a new take-off stock cylinder, as we did some measurements on it. What we did is measure this cylinder in 1" depth steps. \\ | Today I took some pictures of a new take-off stock cylinder, as we did some measurements on it. What we did is measure this cylinder in 1" depth steps. \\ | ||
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| We're literally 8 thou out of round down here. Holy smokes. \\ | We're literally 8 thou out of round down here. Holy smokes. \\ | ||
| {{:techtalk:evo:engmech:cylinder_bore_-_face4inchesdown_by_aswracing.jpg?direct&250|}} {{:techtalk:evo:engmech:cylinder_bore_-_thrust4inchesdown_by_aswracing.jpg?direct&250|}} \\ | {{:techtalk:evo:engmech:cylinder_bore_-_face4inchesdown_by_aswracing.jpg?direct&250|}} {{:techtalk:evo:engmech:cylinder_bore_-_thrust4inchesdown_by_aswracing.jpg?direct&250|}} \\ | ||
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| + | I should point out, too, just how bad these numbers are. \\ | ||
| + | With the cylinder this far out of round, you're basically asking the rings to change shape as they go down the bore. \\ | ||
| + | That's not going to happen to a very large extent at all. Instead they're going to leak. \\ | ||
| + | |||
| + | On top of that, consider that a cast piston is designed to be fit to about a .001" clearance in the bore. \\ | ||
| + | Forged pistons are typically .002" to .003" depending on the application. Too tight will cause scuffing, too loose will cause piston rock and poor ring seal. \\ | ||
| + | So with a cylinder that' s this crooked, how do you get the right clearance? On what measurement do you base your clearance? It's kinda ridiculous. \\ | ||
| + | |||
| + | (So called) "seasoning" that people talk about is real. \\ | ||
| + | It's been a long time since I had any metallurgy instruction so I can't explain it, but I can tell you I can sure as hell measure it. \\ | ||
| + | I really notice it on connecting rods. Take a perfectly round big end and put it together. Pull it out in 10K miles and it'll be out of round. \\ | ||
| + | Now lap the races and resize it and put it back together. Take it apart in another 10K miles and voila, it's still round. \\ | ||
| + | That happens with cylinders, too. They seem to distort some during break in. Pull'em out, bore & hone'em, get'em perfectly straight, and then they tend to stay straighter. \\ | ||
| + | |||
| + | The factory cylinders have very little aluminum backing up that liner. They twist and distort like crazy. \\ | ||
| + | The iron cylinders are amazingly stiff, but even the aluminum jobs are better than the factory pieces. On the 883 or 1200 cylinder, it makes little difference. | ||
