At the risk of contridicting John Twist, it would seem to me that
excessive stem to rocker clearance (ie .025") could make for accelerated
wear on the valve train, especially during run in.
When you hear valves clattering, it means that the rocker is moving
faster when it slams into the valve stem, as it has had an extra .010 (in
this case) to accelerate before contacting the stem. Also, cams are
designed so that the last few thou. of valve opening occurs over a
relatively wider rotational distance. It's called "quieting ramps", and
the purpose is to "ease" the valve back on it's seat. The greater
clearance could have the effect of allowing the valve to contact it's
seat before the lifter has gotten to these areas, and thus the valve hits
the seat with greater force.
I can understand the reasoning behind the higher clearance, as the
higher temps encountered in running-in a new engine should (in theory)
increase the expansion of all components, but I would question an extra
10 thou. of expansion.
Personally, I usually set the valves on a new engine at a "loose"
specified setting, then re adjust after about the first hour of running
(which I generally do in the shop, or at the least with me driving). Then
I will have the customer come back at about 250-300 miles for a check,
and again at 500 miles to re-torque the head and re-adjust the valves.
I've only had one valve related failure (knock on wood) and I blamed
that on the machinest who failed to properly set the clearance on new
guides, and one stuck.
(PS. He ended up paying for it)
Rick Morrison
72 MGBGT
74 Midget
On Tue, 07 Jul 1998 09:55:19 -0400 (EDT) RAY WYGONIK
<RWYGONIK@grove.iup.edu> writes:
>Several weeks ago there was a message that talked about how the valves
>should
>make noise on an MG. The lister said when he hears a quite mg engine
>he can
>feel the valves burning. I adjusted mine to .015" per the book and
>they run
>quite.
>I wasn't aware John Twist recommended .025". I imagine this would add
>some
>noise to them.
>
>I too am interested in the wisdom of the list on this matter.
>
>---------------------------original message---------------------------
>From: IN%"Matt.Kulka@hboc.com" "Matt Kulka" 7-JUL-1998
>09:36:36.87
>To: IN%"mgs@autox.team.net"
>CC:
>Subj: Valve clearance on a new engine
>
>Hello,
>
> After nearly a year, I'm on the final stages of reassembling the
>motor
>for my '74 B. I found a very good checklist on John Twist's website
>for
>rebuilding a 5-main MGB. Unfortunately, I found it near the end of my
>project, but in time to use some of his wisdom.
>
> One thing I saw is that he recommends valve lash of .025" for
>running
>in. (.015" is normal.) I've never left the valves loose following an
>overhaul before, but I'm not willing to believe I know it all.
>
> I was wondering if anyone on the list could shed some light on this
>for me.
>
> As always, thanks in advance.
>
>Matt Kulka
>'74 B - soon to rise above doorstop status.
>
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