Valve clearance: measure at cam or valve?

JFENG

JFENG

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I see there are two camps: adj at the cam or the valve end. I have always done it per Haynes, at the valve end. Easy to do consistently. All my older BMW’s have had a little valve tick andot has never bothered me because I assumed it is a characteristic of engines which don’t use hydraulic “lifters.”

It’s my guess the OEM doesn’t want to risk scratching the cam lobe, with a hardened feeler that has a sharp edge, and therefore recommends measuring at the valve end.

For those that measure at the cam, Do you divide the OE spec by 1.3 ?
 
bfeng said:
I see there are two camps: adj at the cam or the valve end. I have always done it per Haynes, at the valve end. Easy to do consistently. All my older BMW’s have had a little valve tick andot has never bothered me because I assumed it is a characteristic of engines which don’t use hydraulic “lifters.”

It’s my guess the OEM doesn’t want to risk scratching the cam lobe, with a hardened feeler that has a sharp edge, and therefore recommends measuring at the valve end.

For those that measure at the cam, Do you divide the OE spec by 1.3 ?
Click to expand...

yes
the haynes, the blue manuals, the autobooks, all of them say the same, do it at the valve end
 
All the techies that are into engine building measure at the cam. That way any tip problem at the valve is accounted for.
I've done it this way for 15 years.
You cannot scratch a cam with a feeler gauge. Cams are extremely hard. Look what they do all day long. Push a valve with a hundred pounds or more holding it closed at up to 3000 rpm.
If you want to worry about something worry about a rocker arm breaking. Pretty common failure. Or the first rocker arms not getting enough oil, another common problem or your oil pump getting tired and have 10 lbs of oil pressure at idle.
 
There's another reason to measure at the cam: on my '72 tii, the valves stems had worn into the eccentrics which made the measured gap smaller than the actual gap, since feeler only measured across the high edges of the divots.

The 1.26 rocker ratio factor I understand, but owning bimmers since 1980 that's a figure I've never seen before.
 
Honolulu said:
There's another reason to measure at the cam: on my '72 tii, the valves stems had worn into the eccentrics which made the measured gap smaller than the actual gap, since feeler only measured across the high edges of the divots.

The 1.26 rocker ratio factor I understand, but owning bimmers since 1980 that's a figure I've never seen before.
Click to expand...
Most excellent reason for doing this. What I should have added earlier. Explained well, Thanks!
 
@bfeng

doing something for twenty or one hundred years may sound as a reason, but it isnt, that is called tradition, reasons lay underneath

as i can see it, there are serious mechanical reasons to address that it is exactly the same to perform the adjustment at any of those ends

the contact between the eccentric and the tip of the valve is round/flat
between the cam and the lobe is round/round

in both cases it is linear contact

if you have a defect/deterioration, in any end, you will not solve it by measuring at the other end, there is no way to have the stems worn into the eccentrics, as the valves rotate with each movement, and the top valve is a very hard metal, yes you can make the eccentric surface flatter, but that has no effect in measuring the gap, maybe it will make that valve gap more difficult to fix as the eccentric is round no more

the only advantage i see is that when you try to pass the feeler gauge at the valve end you find more difficult to introduce it as the rod of the valve is a vertical element, while at the other end you slide the gauge over the cam lobe, not sure about the access to that point in each of the rockers, there are many solid aluminium walls around there

i can see one point that is unclear to me as when you do it at the valve you put pressure with the feeler gauge, the kind of pressure that you have to be sure is not compressing the valve spring or you will mess up the value, well when you make it the other end you have a lever to put that pressure on the spring maybe making easier to disturb the measure, maybe ?

i can understand that the books recommend not to do the cam end to avoid using a wrong value, being 0,25mm at the valve someone may not divide into 1,26 to make it at the cam

i try to understand the reasons
 
deQuincey said:
@bfeng

i can see one point that is unclear to me as when you do it at the valve you put pressure with the feeler gauge, the kind of pressure that you have to be sure is not compressing the valve spring or you will mess up the value, well when you make it the other end you have a lever to put that pressure on the spring maybe making easier to disturb the measure, maybe ?
Click to expand...

The seat pressure for the valve spring is somewhere north of 50#. It will be real obvious if you are pushing the valve off the seat as the feeler gauge will be extremely hard to move. You can use a go-no-go feeler gauge to get a feel of what pushing the valve off the seat feels like.
 
Mike Goble said:
The seat pressure for the valve spring is somewhere north of 50#. It will be real obvious if you are pushing the valve off the seat as the feeler gauge will be extremely hard to move. You can use a go-no-go feeler gauge to get a feel of what pushing the valve off the seat feels like.
Click to expand...

appreciate the info Mike
that is real
good to know how it affects to the decision
 
i seeked the expertise of +40 years of the guy that does this at my machinist shop, not giving full credit, and only his opinion, but for what it is worth

he claims that he uses both ends with the only thing in mind of which gives more accessibility to the feeler gauge, full stop, no other considerations

he also confirms that the valve rotates so the seat position with respect to the eccentric changes,

also that always the eccentric suffers the deformation, never the valve seat,

and only catastrophic damage on the eccentric may really mean a difference, but that he wouldnt do with such a case without changing the damaged elements first
 
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