'Valve Clearance' - What is it?

[alt]

Forgive my stupidness, but before I recently bought my BF90 4Stroke, everything I had was 2stroke.<br /><br />A recent term that has been coming up is 'Valve clearance'. What exactly is meant by this term? (Is it the exhaust/inlet valves on the head?) And how does someone go about altering it? (I heard sometimes it can lead to problems running the engine) And is it a hard job to do?

 

[rodbolt]

Re: 'Valve Clearance' - What is it?

valve clearence is the gap between the stem and the actuator.<br /> on rocker arm motors its normally a stud with a lock nut, on most overhead cam motors that do not use rockers its usually a shim between the cam lobe and the valve shim holder or "bucket"<br /> its to compensate for the thermal expansion and contractrion of all the valvetrain parts and head/block.<br /> best I remember the BF 90 was an overhead cam with cam followers,rocker arms, and adjusted with the stud and locknut method. <br /> to loose and valve timing becomes late and excessive cam wear can occur because the lift ramps have passed before the valvetrain slack is removed and the valve tends to pound on the seat at closing due to the closing ramp is not allowed to close the valve. to tight and it can hold a valve slightly off its seat causing compustion gas to burn it and affect cylinder sealing.

 

[vikingham]

Re: 'Valve Clearance' - What is it?

Thanks, Rodbolt! I had always heard the term and never knew what it meant - great summary.<br /><br />As a follow-up....the mechanic I've been working with said that the sound of the engine is a good indicator of whether the valve clearance is OK or not. Is this your experience, too? From your description, I could see how it could be.

 

[rodbolt]

Re: 'Valve Clearance' - What is it?

nope<br /> unless they are hydraulics and you hear tapping that lasts more than 30 seconds after engine start not much can be told.<br /> some older motors with over head cams used hydraulic posts on the opposite side of the valvestem with a cam Follower that actuated the valve. most modern overhead cam designs use eithe a rocker arm,sometimes refered to as a follower or are direct acting meaning the cam pushs directly on the valve assy. over the years cylhead and valvetrain arragements have fascinated me.<br />its amazing the configurations that have been tried.<br /> I saw a kit some years ago to convert a 454 to a 4 valve per cyl engine and still use a stock block/cam arragement all the way to a kit for a 5.7 that converted it to twin overhead cams with 4 valves per cyl. oneday I may have to play with that 5.7 arragement as I have sucessfully built 5.7"s in the past with pushrods that would turn 8500.<br />and its been since the 80's that I made an attempt at anything other than stock. good thing some of the big block merc stuff is still exciting .<br />the draw back to rockers is the fact the tip wipes across the stem in an arc. creates a lot of friction and sideloads on the stem. direct acting cams get around this by placing a "bucket" over the stem and a shim to adjust cam clearence on the bucket. the bucket rides in bore and the bucket absorbs the sideloads not the valve.<br />couple that with two smaller lighter valves that can open and close quicker with less spring pressures and cool better you can spin it faster.<br /> I am still toying with the idea of an electronic actuating system using push pull solinoids and ECU controled that not only can be varied in valve timing but in duration and overlap.instead of spring pressure to close the valve I would use electricity. couple that with no frictional horsepower robbing cams and life would be good.<br /> a failure would most likly be catastrophic .

 

[alt]

Re: 'Valve Clearance' - What is it?

rodbolt - You seem to know what you're talking about! Thats for the explanation, I also did a search and came up with loads of info... I could be reading all week!

 

[vikingham]

Re: 'Valve Clearance' - What is it?

Thanks again, Rodbolt - you're the best! And yeah, I guess with, what, 120 years of engine history, people have probably tried every combination and more....good luck with the electronic system - and hey, catastrophic failure modes have a long history, too!