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techtalk:evo:engmech03f [2021/07/21 01:14] hippysmack [Straightness and Roundness of OEM Cylinders] |
techtalk:evo:engmech03f [2021/07/21 01:23] hippysmack [Straightness and Roundness of OEM Cylinders] |
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| Article by aswracing of the XLForum. ((http://xlforum.net/forums/showthread.php?t=15735)) \\ | Article by aswracing of the XLForum. ((http://xlforum.net/forums/showthread.php?t=15735)) \\ | ||
| + | **This applies mainly to factory HD cylinders**. \\ | ||
| 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). \\ | ||
| I've seen worse in fact, and I've seen better, too. It's something of a crap shoot when you grab a production cylinder. \\ | I've seen worse in fact, and I've seen better, too. It's something of a crap shoot when you grab a production cylinder. \\ | ||
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| {{: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|}} \\ | ||
| + | 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. \\ | ||
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| + | 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. | ||
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