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Re: Valve clearance on '74 MGB

To: wkilleffer@comcast.net, mgs@autox.team.net
Subject: Re: Valve clearance on '74 MGB
From: Barney Gaylord <barneymg@MGAguru.com>
Date: Tue, 09 May 2006 13:00:13 -0500
At 07:27 AM 5/9/06, wkilleffer@comcast.net wrote:
>....
>The books all say to adjust to .015 cold, ..... I used a 
>clik-adjust, .... and a feeler gauge. ....

Click adkist will not work at all if the adjuster screw hangs up in 
the threads.  When it does work it is only accurate to +/- one 
click.  The primary attraction for click adjust is, it's supposed to 
be quick to use, and is may compensate somewhat for badly worn rocker arms.

A feeler gauge is as accurate as the flatness of the mating surfaces, 
and depends somewhat on knowing how to use it.

You can adjust the valve gap on a stock MG engine without using 
either a click adjust or a feeler gauge.  Rotate the pushrod with 
your fingers.  Turn the adjuster screw down to close the gap until 
the pushrod stops turning (zero clearance).  Note the orientation of 
the slot in the adjuster screw, and think of a position at right 
angles to it (make an "X" with your fingers).  Rotate the screw 
anti-clockwise 1/4 turn (or 91 degrees if you prefer), and lock down 
the jam nut.  That's it, resulting in a perfect .015 inch gap.

It isn't magic.  It is a result of 24 threads per inch and 1.42 to 1 
rocker arm ratio.
The math is:  1/24 x 1/4 x 1.42 = 0.0148
You would have to miss the rotation angle by 6 degrees to have an 
error of.001 inch.

>....
>This morning, I drove it to work, and the valves chattered ....
>
>Did I adjust things to be too loose? ....

I wasn't there to know if you did it right.  There is a lot of 
tolerance for crankshaft position when the valves are closed.  You 
can adjust the valve any time the tappet is sitting on the base 
circle of the cam, which spans more than 1/2 rotation of the cam, or 
more than one full rotation of the crankshaft.  You can do it when 
the piston is within 1/2 stroke of TDC on the compression 
stroke.  You can do that with the spark plugs in place just by 
bumping the starter and letting the engine stop where it likes (near 
1-2 stroke).

Using the rule of 9's, when one valve is open (by any amount) you can 
adjust the complimentary valve.  1_8 , 2_7 , 3_6 , 4_5.  With plugs 
in, give the engine a bump.  It will stop near half stroke with two 
valves open.  Adust the two compliments.  In four steps you're done.

>My commute's about four miles each way. Will driving it that 
>distance do damage?

The valves hwould have to be REALLY far out of adjustment to cause 
any damage on a short trip.

>Also, is it true that the valves can be adjusted to .013 when the 
>engine is hot/warm? ....

The gap difference between hot or cold is about .001 inch, and the 
gap gets smaller with heat.  So .014" hot is about the same as .015" cold.

Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com




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