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techtalk:evo:engmech04p [2025/10/01 05:02] – [Pushrod Length and Valve Train Geometry] hippysmacktechtalk:evo:engmech04p [2025/10/10 00:25] (current) – [Checking Lifter Pre-load] hippysmack
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     * Camshaft base circle     * Camshaft base circle
  
-Lets say we take a pretty straight forward 883/1250 build with mild heads and cams but the heads are decked .050" and the cam base circle is reduced .030". So how long do the push rods need to be?+{{:techtalk:evo:engmech:rocker_arm_rotational_changes_by_aswracing.jpg?direct&400|}} ((drawing by aswracing of the XLFORUM)) \\ 
 +\\ 
 +Each end of the rocker arm travels in an arc. To the extent that arc is over or under rotated, the contact point moves that much more sideways and less vertically, costing you lift and increasing the valve side loading (thus accelerating wear). You want both the valve travel and the pushrod travel centered in the arc, such that at half lift, you have a 90° angle between the valve stem and a line drawn between the valve tip and the center of the rocker shaft. You want a similar 90° angle on the other side as well. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/133669-head-flow-data/page2#post2852099)) \\ 
 +{{:techtalk:evo:engmech:rocker_arm_overrotated_by_aswracing.jpg?direct&400|}} ((drawing by aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/133669-head-flow-data/page2#post2852099)) \\ 
 + 
 +As you get into high lifts, this starts becoming a concern (at valve lifts of .575" and higher). Essentially what happens is that as the valve gets pushed open farther and farther, the rocker tip starts moving sideways on the valve tip more than pushing down. You also start getting the same effect on the pushrod side of the rocker arm, where additional lift starts causing a lot of sideways motion because you're not well centered in the arc. The corrective action for this problem is to extend the valve stem protrusion such that the tip of the valve is raised. You essentially want to raise the valve stem protrusion by about half the amount you increase the valve lift, the idea being to keep a 90 degree angle between the valve stem and a line drawn between the valve tip and the center of the rocker shaft when at half lift. This gives maximum lift with the least amount of side loading possible. Often times the valve stem protrusion needs to be raised anyway when using very high lifts just to get enough valve spring travel. ((from Hammer Performance website, https://www.hammerperf.com/ttxlcaminstall.shtml#geometry))  \\ 
 + 
 +\\ 
 +HEAD DECK: \\ 
 +The SE head is just a stock head that's been molested, by decking the bejeezus out of it (for more compression) and also by cutting down the valve spring pockets (for taller springs & more travel). Ports and valve sizes are completely stock, and the flow is identical. Both of those changes cause issues (reduced piston to valve clearance, manifold fit problems, pushrod length problems, and a thin port roof that compromises your ability to port it). There are other, better ways to get more compression and more valve travel that don't cause problems and they have to be compensated for. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/64473-se-vs-nrhs-conversion/page5)) \\ 
  
 ====== Pushrod Length ====== ====== Pushrod Length ======
- 
-===== Checking Lifter Pre-load ===== 
 Hydraulic lifters self-adjust over about a .100" range, from .050" preloaded to .150" preloaded. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/160330-squish-band-clearance?t=1720575)) \\ Hydraulic lifters self-adjust over about a .100" range, from .050" preloaded to .150" preloaded. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/160330-squish-band-clearance?t=1720575)) \\
 They'll actually work outside that range but they might make noise, so it's best to stay in that range. \\ They'll actually work outside that range but they might make noise, so it's best to stay in that range. \\
  
 +===== Checking Lifter Pre-load =====
   * **Measure at the rocker box how much the lifter will get preloaded at the rocker box**. ((http://www.hammerperf.com/ttpushrodlength.shtml))   * **Measure at the rocker box how much the lifter will get preloaded at the rocker box**. ((http://www.hammerperf.com/ttpushrodlength.shtml))
     * Put your pushrods in place (remember, the longer one goes on the exhaust).     * Put your pushrods in place (remember, the longer one goes on the exhaust).
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 The lifter plunger should be preloaded between .050 and .150. Make the measurement if you're not sure. \\ The lifter plunger should be preloaded between .050 and .150. Make the measurement if you're not sure. \\
 {{:techtalk:evo:engmech:checking_lifter_preload_by_aswracing.jpg?direct&300|}} ((photo by aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/42637-push-rod-length?t=113818)) \\ {{:techtalk:evo:engmech:checking_lifter_preload_by_aswracing.jpg?direct&300|}} ((photo by aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/42637-push-rod-length?t=113818)) \\
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 Pushrod lean angle on 91-up engines. Pushrod lean angle on 91-up engines.

Trace: • EVO: Engine Mechanicals - Sub-04P • REF: Engine Mechanicals - Sub-07P