[Healeys] Valve clearances

Chris Dimmock austin.healey at gmail.com
Wed Feb 13 23:47:21 MST 2019


Isn't part of the reason that the valve dissipates heat into the head when it is seated, so part of the reason for valve clearance is to allow the valve to cool?
Maybe try a 4 thou valve clearance and see how it goes? 
I probably won't!! 
:-)
Chris 

Sent from my iPhone

> On 14 Jan 2019, at 10:23 am, Michael Salter <michael.salter at gmail.com> wrote:
> 
> This is another subject that I often wonder about. 
> If the valve clearance is there to ensure that the valves are fully closed when the engine is hot then why, when I check the clearances on a hot engine after a dyno run,  are they hardly any less than they were cold?
> I can appreciate that the valve train components grow because of the heat but surely the block and head and rocker shaft pedistals also grow and thus there is little if any net change.
> I also appreciate that the exhaust valve stem get really hot but ????
> Just wondering....
> 
> M
> 
>> On Sun, Jan 13, 2019, 6:42 AM WILLIAM B LAWRENCE <ynotink at msn.com wrote:
>> I disagree that valve clearances are not critical. As the engine comes up to temperature the metal parts of the valve train (valves, lifters, push rods, the head itself and the rocker shaft towers) increase in length and the valve clearances are calculated to assure that this expansion isn't allowed to affect the sealing of the valves on their seats. If the valves are set too tight they reach a point where they won't touch the seats and therefore won't seal. Hot gases rushing through the gaps will quickly erode the valves. Also increasing the duration and/or lift of the cam has the effect of the  decreasing the length of the opening and closing ramps on the back side of the cam This reduces the length of time (dwell) the valve remains on the seat. Increasing the valve clearance restores the dwell time the valve needs to transfer its heat load to the cooling system through the valve seat and the cylinder head. Too little dwell time or the lack of proper valve sealing are two causes of burned valves.
>> 
>> In my own experience I installed an Isky cam with a grind a little more aggressive than the 100M cam and set the clearances to 0.012" per the manual. Within about 2,000 miles the exhaust valves were trashed. I then took the time to read the Isky installation instructions which specifically state the valve clearance needs to be set to 0.018". I re-installed the head with new valves and set them to Isky's spec and never had another problem with burned valves.
>> 
>> Follow the cam grinder's recommendations and you will avoid trouble.
>> 
>> Bill Lawrence
>> BN1 #554
>> From: Healeys <healeys-bounces at autox.team.net> on behalf of Bob Spidell <bspidell at comcast.net>
>> Sent: Saturday, January 12, 2019 11:10 PM
>> To: Michael MacLean; healeys at autox.team.net
>> Subject: Re: [Healeys] Valve clearances
>>  
>> I should note the rockers are pretty noisy at 0.015; IIRC I tightened them up just a bit.
>>> On 1/12/2019 2:39 PM, Michael MacLean wrote:
>>> I'd go with .016.  You can always tighten it.  Better loose than tight.
>>> Mike MacLean
>>> 
>>> On Saturday, January 12, 2019, 11:01:32 AM PST, Bob Spidell <bspidell at comcast.net> wrote:
>>> 
>>> 
>>> Makes sense (for some reason).  I put a DWR1 cam in my BJ8 which, according to DWR, 'is a little better than the BJ8 cam and so works well in any standard or mildly tuned car' but I think the 0.015" applies to all profiles on the spec sheet.
>>> Bob
>>>> On 1/11/2019 8:48 PM, John Rowe wrote:
>>> 
>>> Simon,
>>> 
>>> Modified cams require the large clearance (don’t ask me why?) So go for .015 that is what mine is.
>>> 
>>> John Rowe
>>> 
>>> Qld
>>> 
>>> Australia
>>> 
>>>  
>>> 
>>> From: Healeys [mailto:healeys-bounces at autox.team.net] On Behalf Of simon.lachlan at alexarevel.plus.com
>>> Sent: Saturday, 12 January 2019 9:14 AM
>>> To: Healeys
>>> Subject: [Healeys] Valve clearances
>>> 
>>>  
>>> 
>>> The car is a MkII BT7.
>>> 
>>> The BMC manual says adjust the clearances, cold, to 0.012”.
>>> 
>>> Denis Welch said I should go 0.015”, cold, after fitting their “Fast Road” cam.
>>> 
>>> The rolling road guy said 0.016”, warm.
>>> 
>>>  
>>> 
>>> I have the radiator out right now so I can get a nice socket onto that nut and have a go at the clearances.
>>> 
>>> Stick with 0.015”, cold?
>>> 
>>>  
>>> 
>>> Thanks,
>>> 
>>> Simon
>>> 
>>> 
>>> 
>>> 
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