HS40 Engine Compression Increase

#1
I have a HS40 engine I want to get a little more torque out of & want to increase the compression. I plan on removing the bump on the backside of the exhaust valve camshaft lobe for full compression but I also want to reduce the head squish a little bit too. I would rather use a thinner head gasket instead of shaving the head down if I can get one. Can someone tell me if a 0.10" head gasket will work without any clearance issues with the valves contacting the head?
 

mustangfrank

Well-Known Member
#2
I'm researching similar mods to an HS50...the "IF" means going to a copper head gasket. Heads are still plentiful and cheap, I decided to stick to milling heads. I've read a .040" shave is maximum but there are others on here with actual experiences.
 
#3
Thanks for the information mustangfrank. The HS40 & HS50 engines share the exact same head & the bump compression release camshaft is the exact same also between the HS40 & HS50 engines. So they must have the exact same lift to the camshaft & the valves open the same distance. I don't have a machine shop anywhere close to me, so I figured getting a thin 0.10" head gasket would be the easiest option. But I can not find any 0.10" head gaskets for a HS40, just OEM versions & I am kind of at a stand still now. I'll probably just leave it stock if I can not find a 0.10" HS40 head gasket anywhere soon.
 
#5
Id do the cam mod then decide if you want to make it any more after trying it. The bike in my avatar I did the cam only and its a bear to pull start. Really kicks back sometimes and iv broken a few pull starters. As for shaving the head if you dont have a belt sander suitable the sandpaper on a flat surface method works just fine. I like using the factory head gaskets rather than a thin one because they are thicker and more complient less prone to leaks. thin copper gaskets are have to be anealed and tend to not seal. The head isnt very thick you know its got some movement to it and really not intended to be used with a hard gasket. Its a great mod my bike is super strong pulls very hard I think its the strongest engine I have.
 
#6
Great information jeep2003. Now that I hear you say that the camshaft modification alone did such a big improvement, I might just leave the cylinder head stock & just remove the bump from the backside of the exhaust valve instead of my original plan. Now you got me thinking & can't wait to see how much of an improvement it makes. Thanks again for the excellent information.
 
#7
Im sure you will be pleased. Its like trippleing the cc's. With the comp release the valve doesnt close until the piston is like 1" from tdc. I dont know why they bothered making the engine so big if they were just going to let all the air out.
 
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