[Healeys] Valve clearances

[WILLIAM B LAWRENCE]

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><mailto: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<mailto: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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