[Fot] Oil pressure and tolerance on valve clearance

[Duncan Charlton]

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.