Er...huh? Clearances are indeed determined by the shape of the opening
slope of the cam -- Larry's website is quite informative on how to
determine appropriate lash (which I had to do a few months ago for a
Ford racing engine with an unknown camshaft in it). My reply was to the
general question about the way manufacturers list valve lash /tolerances/.
By the way, the cold valve lash specification for my 990 cc 1936
Matchless OHV V-twin was /zero/. It had a combination of materials:
aluminum crankcase, cast iron air cooled barrels and heads, aluminum
rocker boxes, steel roller cam followers, aluminum rocker arms and
2-section pushrods made of both aluminum and steel.
Duncan
On 6/3/14, 4:54 PM, Michael Porter wrote:
> On 6/3/2014 7:03 AM, Duncan Charlton wrote:
>> I've seen anywhere between 70 and 100 psi on our racing engine when
>> warmed up and on the track but never as low as 50. I have read that
>> 70 to 80 is best but I never had any leaks of the engine or the oil
>> cooler with 100 psi. By the way, where are you measuring the
>> pressure? The oil line to my gauge was tapped into the oil gallery
>> on the left side of the engine rather than the stock location to get
>> the most accurate reading of what's going on in the oil passages.
>
> 50 psi does sound a bit low. Still, the important thing to determine
> is if the low pressure is due to excessive clearances or leakage past
> the pressure relief or pump problems (or some combination of those).
> The old rule of thumb is 10 psi per 1000 rpm, so at full chat, 50 psi
> isn't enough pressure.
>
>>
>> I don't think it's unusual to see a range of valve adjustment
>> although it most often seems to be stated thus: "0.010 to 0.012". I
>> looked at a list of specs for Briggs and Stratton small engines and
>> most of their engines are given an allowance range of 0.002" to
>> 0.004": http://tinyurl.com/m2vynmd
>>
>> I understand it can make a difference in power if you set valve lash
>> at the small end of the range since the valve opens earlier and
>> closes later but the amount of time the valve sits on it seat is
>> shorter, so there is a tradeoff in loss of time for heat transfer.
>
> Larry Young could speak better to this, I'm sure, but comparing the
> lash of other engines is just an apples and oranges exercise that's
> more likely to mislead than help. For instance, the stock cold
> clearance on the old VW aircooled engines was 0.006". However, because
> of the engine's construction, its hot clearance was 0.015-0.016", and
> it's the hot running clearance that matters. There's a very small
> advantage in decreasing the clearance balanced against a much bigger
> chance of ruining the lobes or the lifters on the lobes' initial
> ramps. That's the primary reason why the clearances on racing cams
> (with steeper ramps) are generally quite a bit larger than stock,
> IIRC, a situation that gets worse with increasing spring pressure.
>
>
> Cheers.
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