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Hello all, 

i’m struggling to find any information regarding valve clearance on motorcycles. I need two different motorcycles (ie a high performance race bike and a cruiser) and their valve clearance etc. to distinguish the difference between the two engines, and why their manufacturers have done this. 
Thanks for any help! 

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Look at high lift cams , ported valves 

Racing you want that engine spinning as fast as you can ie airmix in and exhaust out as quickly and smoothly ..


Clearances change with temperature so if being pushed hard they get hotter so clearances get smaller with metal expansion and what they are made from alters their reaction 


Edited by TimR
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As above you probably need to be pretty specific, what are you trying to achieve with the data? 

Valve clearance has to be enough so under operation there is no pressure on the valve but you don't want such a gap that wear will be high when the lobe on the cam engages, cam profile is more relevant to performance.

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If you are trying to show that high performance engines have different valve clearances to low peformnace engines, I don't think you are going to find that.

1. When the valve should be closed, you want to make sure it is, having clearance is a good way to ensure that.

2. Having clearance also helps oil get between the camlobe and the cam follower to help lubrication and reduce wear.

3. Engine temperature varies, cold when unused, hot when working, very hot when worked hard. Different components are made of different materials that all expand/contract different amounts. You need enough clearance to ensure there is alwaysa gap under all operating conditions of the engine.

3. Air cooled engines run hotter than liquid/water cooled so tend to have larger valve clearance.

4. Bigger engines tend to have larger clearance because the larger components expand/contract more.

Are you sure you are not getting confused with Valve to piston clearance.

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Might make more sense to pick a bike that came out a few years ago like a Honda Rebel. Then compare the clearance on the original 1985 Rebel to the 2020 Rebel and see what the differences are. From what I've seen new bikes tend to have smaller clearance and tighter tolerance compared to older bikes. I'd guess it's because now they have to meet stricter emissions.

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