Valve clearances, why so different.

Hey guys,
I have pondered this question many times. I have asked Motor Mechanic friends of mine, but never really got
an answer that left me satisfied.
I know there are members of this forum with the Technical background that can answer this....
Why does valve clearance differ so much on similar engines.
By similar, I mean, lets say, Norton twin cylinder.
The book says for a standard 750cc 6 and 8 thou. or Combat 8 and 10 thou. 4s cam 14--16 thou.
Why.? why the difference. Even I can understand basic metallurgy (?) that when metal gets hot it
expands, in this case... Cam loubs, followers, pushrods, rockers etc, but why do high lift cams need more
clearance than low lift cams.
What would happen if one were to set the clearance to tight, and why.
Please explain. Thank you all learnned gentlemen.
AC.

Crikey ! :shock: .......................... :|

as you already know ,set the clearance at zero and the valve will be pushed off its seat when hot.

john robert bould said:

as you already know ,set the clearance at zero and the valve will be pushed off its seat when hot.

Click to expand...

Yeah but the point in question is why different cams have such different clearances. Surely a 4S won't have any greater expansion than a Std cam.

Yeah but the point in question is why different cams have such different clearances. Surely a 4S won't have any greater expansion than a Std cam.[/quote]

Well yes, I understand what you are saying, but the point is you need the tappet clearance set at zero when engine is at a hot working temperature, or the vale will be lifted off their seat at the base of the cam when engine gets hot.
If you think you have a problem, on the full race cam the clearance set cold is a whopping .015-.016 thousands!

The action is said to be more violent , also the throttle is thought to be used more openly :lol: raceing . ,
The valve needing to seat firmly in lesser time , is ' let sit ' , closed , firmly , and opened suddenly . To get maximum
use of dwell , The time its near fully open .Also so securely closeing the cylinder in and under Compression .

Essentialy its all coming good way up in the powerband , or operateing effeciently there , low ends less important .

Cams with quitening ramps , where the valve is seated more quitely or progressively tend to get less determined
degree wise , at excessive r.p.m.s

The Race Cam aims to accelerate the getting it open and getting it closed . Quiteness isnt a prime consideration,
Though set correctly there is not intollerable free play in the mechanism .

Some early narrower duration cams may produce as much power , but the powerband is liable to be peakier .

Quite a bit of study was done , they tell us , in the mid 70s on lost motion / discontinueos control of valve ,
useing ossciloscopes and the like .


The improvements in Valve Spring materials and design , from things like Fords LeMans progamme , flowed on .

Progressive rate centerless ground wire utilised gave superior control to previous Triple Springs used on ' Square ' (lobe)
Race Cams , Like the G 45 Matchless , which broke the rocker spindle holders in the Alloy Head , or the E 49 Charger ,
The triple springs imposeing high loads in the actuateing mechanism ( Valve Gear ) and unnessearily ( in comparison )
absorbing horsepower .

So Dewkatis used mechanical closeing , for all of the above , the light closeing springs fitted in Bevels beeing purely to assist starting .The mechanism and map being fully determined mechanically .

Ancient ' monosoupape ' ( mono valve ) aero engines , the valve was opened by the piston sucking it open against a light spring. Staggering redlines of 2.000 rpm were recorded pre W W 1 . The Peugot twin cam 4 valve was to reduce inirtia . In 1908 . 8)
Metalurgy was more relevant in ' high speed internal combustion engine ' design , being the practical limitation , along with manufactureing & fitting tolerances .

So Tecnically , NOW , 40 years later , we can build a better Commando Engine . At a Price. unfortunately. unless we get the Volume Up .

After reading the replies to Aussiecombat's question as to specifically why are different clearances required with different camshafts, I hope someone can give a clear answer.

I too have always not understood why a higher lift camshaft would need looser clearances at the rocker arm than a lower lift cam. I mean, lift is lift, and all else being equal why more clearance needed between cams?

On my ole Chevy V8's I'd set lash by ear when hot and running [with some oil splash], so would that be safe/wise to do on C'dos?
Its not an academic question to me as Ms Peel will make a lot of heat with a fairly radical drag race cam inside best engine parts, materials and build one can obtain nowadays. This articles says set to spec loosely when cold to start then once hot let the ear set more perfect.
http://www.diamondbackengines.com/techn ... alve-lash/

The valve lash setting is determined by the profile of the quieting ramps cut into the cam and the expansion rates of the pushrod and barrel and other components. It really has nothing to do with the overall lift of the cam although using more or less lash does change the lift by that amount.
The quieting ramps are slow ramps that take up the valve lash before the steep part of the ramp takes over. If you have a cam with long slow ramps like a 4s and you reduce the valve lash setting the valve will end up with too much off the seat time in comparison to the seated time. The valve does not cool well when it is off the seat so it overheats and may fail. Jim

ludwig said:

Bernhard said:

.. you need the tappet clearance set at zero when engine is at a hot working temperature, or the vale will be lifted off their seat at the base of the cam when engine gets hot..

Click to expand...

Not nessecarely so ..
The relation between valve clearance and increasing temperature is not linear .
I mean : when the engine warms up from cold , valve clearance increases , then decreases, increases again ,etc ..
It may well be that on some engines , at working temp , the valve clearance is higher than cold ..

Click to expand...

I agree, But are you talking Nortons? My Vincent as zero clearance cold,But it employs silver steel push rods..Doe's the Norton alloy push rod expand greater needing "some" addicional clearance?

comnoz said:

The valve lash setting is determined by the profile of the quieting ramps cut into the cam and the expansion rates of the pushrod and barrel and other components. It really has nothing to do with the overall lift of the cam although using more or less lash does change the lift by that amount.
The quieting ramps are slow ramps that take up the valve lash before the steep part of the ramp takes over. If you have a cam with long slow ramps like a 4s and you reduce the valve lash setting the valve will end up with too much off the seat time in comparison to the seated time. The valve does not cool well when it is off the seat so it overheats and may fail. Jim

Click to expand...

If I am understanding this correctly, the goal is the get the desired height on the cam lob, but due to the geometry of the ramp required to get that lift, you have to be concerned with how long you hold the valve open if you try to do it with smaller tappet clearances?

Yeah, basically the needed clearnce is dictated by the profile of the quieting ramp. You can fudge a little tighter but if you get too tight the quieting ramp will end up raising the valve a few thousandths off the seat long before it should and you will end up with a hot valve. Jim

john robert bould said:

I agree, But are you talking Nortons? My Vincent as zero clearance cold,But it employs silver steel push rods..Doe's the Norton alloy push rod expand greater needing "some" addicional clearance?

Click to expand...

I know as I have a VW stroker engine that I built, that if you use Chromoly PR's they have to be set at a loose zero cold and if you use a set of aluminium PR's they have to be set to .006 as the alloy pushrods do expand more than steel.
On a VW engine you have an alloy case, cast iron cylinders with a alloy head just like a Norton and they are both air cooled.
The difference is that a VW engine has a fan and the Norton does not.
The hot and cool cycles change more on a bike than a Vw and so does the expansion rates of the PR's.
Was any of this helpful or do I need to stop drinking coffee and go for a ride?

Guido said:

john robert bould said:

I agree, But are you talking Nortons? My Vincent as zero clearance cold,But it employs silver steel push rods..Doe's the Norton alloy push rod expand greater needing "some" addicional clearance?

Click to expand...

I know as I have a VW stroker engine that I built, that if you use Chromoly PR's they have to be set at a loose zero cold and if you use a set of aluminium PR's they have to be set to .006 as the alloy pushrods do expand more than steel.
On a VW engine you have an alloy case, cast iron cylinders with a alloy head just like a Norton and they are both air cooled.
The difference is that a VW engine has a fan and the Norton does not.
The hot and cool cycles change more on a bike than a Vw and so does the expansion rates of the PR's.
Was any of this helpful or do I need to stop drinking coffee and go for a ride?

Click to expand...

Similar problem with Commando engines if you switch to steel push rods. Using them requires reducing the cold clearance so you get the proper clearance at operating temperatures. For some cams, that means setting the cold valve clearance with steel rods at a "loose zero". I recall Steve Maney telling me several years ago that with some cams (might have been the 4S) and steel pushrods, you couldn't get the proper clearance when hot, even with the loose zero cold settings.

Ken

Ken, The 4s cam works with aluminum barrels and steel pushrods. It is designed to run loose. Its the 2s or PW3 that gets too loose when they get hot. Jim

comnoz said:

Yeah, basically the needed clearnce is dictated by the profile of the quieting ramp. You can fudge a little tighter but if you get too tight the quieting ramp will end up raising the valve a few thousandths off the seat long before it should and you will end up with a hot valve. Jim

Click to expand...

Jim, Are we talkin just the exhaust getting hot :?:

The exhaust will certainly be the worst but the intake can suffer from the same problem. The result can be pre-ignition followed by engine failure if the exhaust valve gets hot enough to glow red.
Mild cases over a period of time usually just cause erosion of the valve and that can happen on the intake or exhaust. Jim

comnoz said:

The exhaust will certainly be the worst but the intake can suffer from the same problem. The result can be pre-ignition followed by engine failure if the exhaust valve gets hot enough to glow red.
Mild cases over a period of time usually just cause erosion of the valve and that can happen on the intake or exhaust. Jim

Click to expand...

Jim, Please explain the "getting hot " reason, ie what happens with small valve clearance and correct clearance :?: surely if the correct clearance is 8 thou and we adjust to 4 thou the valve will be opened only a fraction of a degree early, as stock drive set ups can be 1-3 degrees variation what would a millie second make by a tappet three-four thou tight, you will have the excuse my persistance on this topic.
Regards John

Keep pestering I need to know about value train geometry and thermal issues.

comnoz said:

Ken, The 4s cam works with aluminum barrels and steel pushrods. It is designed to run loose. Its the 2s or PW3 that gets too loose when they get hot. Jim

Click to expand...

Thanks, Jim. I should have realized that. Steve and I discussed the merits of steel vs aluminum vs titanium pushrods some years back, and he mentioned the clearance problem with steel pushrods and some cams, but I had forgotten which cams he was referring to. I should have rememberd that when he came over and raced with our team in the US a few times back in 1999 - 2004, he was using alloy cylinders and a modified 4S cam profile with steel pushrods. The specified clearance for the 4S cam was .016" , which leaves a lot of room for it to loosen up if you set it tight when it is cold. Just another senior moment, I guess.

Ken