Tappet Clearances

Well, I don't use a Maney cam, but one of my own. Steve said a simple rule of thumb is to reduce the standard valve clearance by 5 thou if you're using steel pushrods and an ally barrel

It's worked fine for me for the last 30 years.....

WZ507 said:

On the subject of “zero lash” tappet adjustment, here is another data point to consider. Although this is a non-Norton application, I believe the fundamentals to be essentially the same as those being discussed here. For decades Harley Sportsters, XLH and XLCH, employed a near nil tappet setting where the factory tappet setting instruction stated that the pushrod could be “turned freely with finger tips, completely around, without trace of bind”. Thus, some minute amount of clearance existed in the valve train to allow the pushrod to rotate and the valve to seat.

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Norton ES2 is the same, free to spin but no lateral movement.

robs ss said:

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If you go down this path I would leave at least 1 or 2 thou, otherwise how do you tell the difference between zero and a valve hanging open?
Cheers

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Cellophane from a cigarette pack.

If you slacken the lock nut and wind the adjuster down until it just binds on the tip if the valve - that is zero clearance. I would not run with zero clearance.

I have Maney alloy barrels, standard rods and pw3 cam.
I was instructed to halve the tappet clearance specified for whatever cam, which on a PW3 is 11thou inlet and exhaust, so i run about 6thou on each and motor seems very happy for the last 8 years...

Hagarr said:

Yes exactly what I was after MexicoMike.

If anyone can provide a scanned copy would be great.

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acotrel said:

If you slacken the lock nut and wind the adjuster down until it just binds on the tip if the valve - that is zero clearance. I would not run with zero clearance.

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The engine does NOT ‘run with zero clearance’.

It is SET at zero when COLD.

It is set like this in order to have the CORRECT clearance at operating temperature.

I have often pondered the reason for certain valve clearance
For example a BSA b25 has an alloy barrel,alloy pushrods
The recommended valve clearance is 8 thou in 10 out I believe
But according to the Rupert ratio book of the single he recommends just enough clearance you can turn the pushrods when the valves are closed/engine cold
I tried this on my b25 and sure enough it ran fine
I checked the clearance when the engine was hot and sure enough there's plenty of clearance
And a mechanicly quiet engine to boot
So just how much clearance does an engine need?
Should we be setting our engines with "just enough" clearance when the engine is hot?
Are we giving away a bit of valve lift ?

I have always foolishly (?) assumed that the manufacturers of the engine knew what the appropriate valve clearance for that (stock) engine should be...

I have a PW3 in my commando and have often wondered why they require 11 thou, is it to do with the cam having a faster 'ramp' than the stock cam?

Listen to Seeley 920, listen to Fast Eddie.

When using iron barrels, with steel pushrods is when Steve told me to halve the clearance specified for the cam.

It isn't that unusual to set zero clearance when alloy barrels and steel pushrods are involved, as already said, the barrel will be expanding more than the pushrod.

Indeed, my 1956 AJS 500 single has its tappet clearances set to zero, but in that case with a 'warm not hot' engine according to the manual.

Steve told me what he means by 'zero' is that you can just rotate the pushrod with your finger.

AJS manuals say something similar.

baz said:

I have often pondered the reason for certain valve clearance
For example a BSA b25 has an alloy barrel,alloy pushrods
The recommended valve clearance is 8 thou in 10 out I believe
But according to the Rupert ratio book of the single he recommends just enough clearance you can turn the pushrods when the valves are closed/engine cold
I tried this on my b25 and sure enough it ran fine
I checked the clearance when the engine was hot and sure enough there's plenty of clearance
And a mechanicly quiet engine to boot
So just how much clearance does an engine need?
Should we be setting our engines with "just enough" clearance when the engine is hot?
Are we giving away a bit of valve lift ?

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Tappet clearances can change valve timing. The difference in lift is probably less important. One of the things I like about using methanol for racing, is you do not usually cook the motor. So tappet clearances are probably less important. Even so, if you run methanol very lean, the motor still gets quite hot. You just do not hear it clicking as it cools-off after a race. I have only ridden one classic race bike which was on petrol in recent years. It was still quite good, but not quite as rapid, as it was when it was on methanol. The difference it makes in a four-stroke is usually not much, but in a two-stroke it is something else.
If you ding a valve seat in a four stroke motor, there is usually a marked drop in performsnce. You probably get the same thing happening if the valve is held slightly open by a tight tappet.

marinatlas said:

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...

www.accessnorton.com

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Thanks

Tom P said:

Thanks

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And why do 4s cams require so much larger tappet gaps ?

Ian James said:

And why do 4s cams require so much larger tappet gaps ?

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Clearances are not only to ensure there is adequate clearance to prevent the valve being held open. Clearances are designed to control where and when the follower makes contact with the ‘ramps’ to start the lift, and then loses contact when closing. This part of the cam / follower interface relationship is crucial to maintain valve train control and reduce wear.

Comnoz spintron testing thread showed the importance of this topic well.

Fast Eddie said:

The engine does NOT ‘run with zero clearance’.

It is SET at zero when COLD.

It is set like this in order to have the CORRECT clearance at operating temperature.

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How do you know the tappet clearance is only going to increase with temperature ? In an alcohol motor the valves get hot first and it is difficult to get heat into the rest. Some petrols also have significant latent heat of vaporisation. The clearances might depend on how quick the rest of the motor heats up ?

acotrel said:

How do you know the tappet clearance is only going to increase with temperature ? In an alcohol motor the valves get hot first and it is difficult to get heat into the rest. Some petrols also have significant latent heat of vaporisation. The clearances might depend on how quick the rest of the motor heats up ?

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Aluminium has a greater coefficient of expansion than steel or iron. So any engine that replaces steel or iron components with aluminium is going to see increased expansion of those components and a knock on effect to any design tolerances.

When your barrels are alloy, and your pushrods are steel, the barrels are gonna expand more than the pushrods. This is bound to impact clearances when the original design had the exact opposite! It’s pretty much as simple as that really.

acotrel said:

How do you know the tappet clearance is only going to increase with temperature ? In an alcohol motor the valves get hot first and it is difficult to get heat into the rest. Some petrols also have significant latent heat of vaporisation. The clearances might depend on how quick the rest of the motor heats up ?

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How about just checking the valve clearances on a motor that's up to temperature?

baz said:

How about just checking the valve clearances on a motor that's up to temperature?

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Yes, exactly, any lash we set at room temp is intended to make the opening/closing events occur at a particular position on the ramp when hot. According to Prof Gordon Blair, a generic constant velocity ramp is ~ 25 cam deg long (50 crankshaft deg), with a rise ~ 0.3 mm (0.012”). With respect to valve lash Blair says the following.

The valve lash, when the engine is running hot, will be set to about 0.2 mm (0.008”) or 0.25 mm (0.010”), which will translate to an equivalent ‘cold’ valve lash setting which must be determined as a function of the relative expansion characteristics of the cylinder head and the cam follower mechanism.

Being able to visualize the foregoing information is often times the easiest way to absorb it, so let’s take a look at what a generic ramp, as described by Blair, looks like and then compare the generic ramp to ramps on Norton CDO, 2S and 4S cams, that I believe have factory recommended cold lash settings of 0.006”, 0.008”, and 0.016” respectively. The Norton cam data is courtesy of forum members Comnoz, LCRKEN, and Snotzo.

The blue line in the 1st plot is provided as a reference so the reader can easily distinguish the constant velocity portion of the ramp (where the red and blue lines run parallel to each other) from the flank of the cam where the opening rate increases dramatically. According to Blair, proper hot lash places the opening event at 0.008” to 0.010”, which as you observe here in the plot, allows ample time on the constant velocity portion of the ramp to take up the slack and elastic component in the valvetrain, yet is very close to transitioning from the ramp to the flank of the cam, thus minimal time is spent on the ramp before higher lift rates are encountered on the flank.



Now let’s have a peek at the 2nd plot of all Norton cams, which allows us to see where, along the ramp, opening occurs with the factory cold lash settings. As you’ll no doubt note, the CDO lobe has a ramp that is essentially identical to the generic ramp described by Blair, so 50+ years ago the CDO cam had “modern” opening/closing ramps, so hats off to Norton for the nice ramp design. You will also note that the CDO specified cold lash of 0.006” places the opening event somewhere near the mid-point of the ramp if not slightly before. We would expect that when hot, CDO valve lash would loosen slightly to move the opening further up the ramp, i.e., closer to the flank and closer to the “Blair recommended” hot lash setting of 0.008” to 0.010”.



The 2S and 4S cams have significantly different ramp designs than the CDO cam and for obvious reasons have larger valve lash specifications. I am uncertain of why the unusual 2S and 4S ramp designs would be employed given that the CDO ramp is a state-of-the-art ramp. Perhaps if Snotzo might be looking in he may be able to inform us in this matter, as he has been involved in cam/valvetrain design spanning many decades. At any rate you can clearly see that if the 4S cam were lashed like a CDO cam, the opening/closing events would seemingly go on forever and prior to encountering the flank would provide essentially no inlet airflow (even though the valve is no longer seated), and on the closing side the extra duration and very low lift would act like a leaky valve that doesn’t want to close, thus the large factory lash setting of 0.016” is required to get the opening/closing events to occur close to the flank of the cam.

The data employed here for the 4S cam was taken by hand (not on a cam analyzer), and therefore may not reflect as accurately as we would like, the precise ramp shape. As shown, the curve suggests that at a lash of 0.016” there would be essentially no constant velocity ramp and that opening would commence abruptly on the flank of the cam. To provide a more accurate image of the 4S ramp design a plot from Snotzo, presumed to be actual design data, is shown and suggests that at a lash of 0.4 mm (0.016”), denoted by the blue vertical line, initial opening does in fact occur on a constant velocity section of the ramp which is in close proximity to the flank.



Hopefully the above discussion and images contribute something useful regarding our understanding of hot and cold valve lash and what we are actually hoping to achieve in a hot engine via proper cold valve lash settings.