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MG MGB Technical - Valve Adjustment

I just watched the John Twist U-Tube video on valve adjustment. It was pretty slick and makes me think I'm going to attack this project next.

The only thing I didn't understand is how he was able to find the adjustment position (TDC for the cylinders) so easily. He just tapped the engine over quickly and went to the next set of valves to adjust.

I suspect it has something to do with his description of the number of degrees a valve is opened, but I thought you had to be pretty much at TDC of a cylender stroke to make the adjustment.

If needed, here is the link to the video:

http://www.youtube.com/watch?v=fezXUwVfH7U

Thanks,
BH
BH Davis

Ahhhhh........I think I get it.

If number one is open, then number 8 is closed. Since a valve is closed for 468 degrees of rotation (over one full revolution of the engine), and since you adjust valves when they are closed, as long as the opposing valve is somewhere on the opening stroke, the one you are adjusting will be somewhere within that 468 degrees, and hence, be closed.

Do I have that right?

Thanks,
BH
BH Davis

What to do is turn the engine over slowly, use a socket on the end of the crank, or push carefully in gear, until a valve has just gone fully open- you will see the rocker stop going down, and start going up.

After a little while, you get the knack for spotting when they are just about to rise again, the first time you end up pushing and pulling the car, but don't give up.

Then you use "the rule of 9" basically, count the valves from the front 1,2,3 ...8. then the one you adjust is the valve that added to the one that is fully down makes 9, eg. #3 is fully down adjust #6.

Easier is to use the fact that the valves mirror from the ends.

If the 2nd valve from the front (#2) is down adjust the 2nd valve from the back (#7).


What you are doing is adjusting the clearance when the point of the cam is facing away from the rocker, this happens when the cylinder 360 degrees away has the cam point facing the rocker = fully open.
Martin Layton

Just read your second post.

Assuming that the cam is a perfect circle, and then rises to form the lobe, adjustment anywhere on the circle will be fine. If the perfect circle is 234 degrees then you are correct. Explains why most mechanics adjust two valves at once- you will notice that when one valve is fully down there is another quite close to fully down.

I always check clearances when the lobe points away from the follower, at leat then I'm consistent!

(I have to be honest and say I sampled the video rather than watching it, seemed a complicated way to explain something simple to do)
Martin Layton

BH - Martin's explanation is absolutely correct, I would add just one thing that will make it easire to fint the position of the cam's rotation. Hang a box end wrench from the lock nut on the rocker you are rotating to get the valve wide open and watch the end of the wrench. Due to the length of the wrench the movement will be geatly magnified at the end of it as you rotate the engine. Cheers - Dave
David DuBois

Great Link. As to it being two complicated of an explanation I have to disagree. Just shows why its ok to adjust 2 valves at once. I never realized to check youtube for MGB stuff. The Wheeler Dealer episodes posted on there are great for guys like me in the USA who never ever see anything about foreign cars on TV.
James

Another way is for any given cylinder, adjust the exhaust when the intake is closing. Adjust the intake when the exhaust just starts to open.
John H

If done as most Japanese cars recommend it's quicker and easier. With number 1 cylinder at TDC compression stroke, both valves are loose, adjust valves 1,2,3 & 5. Turn engine one complete turn and adjust valves 4,6,7 & 8. I have compared this method with the rule of nine and it makes no difference on the engines I have checked. If the cam base circle is correct it should make no difference because of the valves being open for 468* as John Twist points out. Paul Hunt has said several times that his engine requires going a bit past the ideal rule of nine setting for best accuracy. I have found the base circle wasn't always ground on a perfect circle on the hundreds of Briggs & Stratton 5 hp cams I have checked for go kart racing, but on the cars I have checked the base circles have been very close to a perfect circle. In normal use the cam base circle will show little or no wear because the valve is closed and there is little or no pressure on the lifter. That is one reason a worn cam will not show up on a valve adjustment or valve clearance check check.

Clifton
Clifton Gordon

I offer a slight variation, not dependent on accuracy of cam grinding.

Line up timing marks. Either No 1 or No 4 will be at TDC on compression stroke with both valves fully closed. You can tell which as the other one will be at TDC on exhaust stroke so the exhaust valve will be open.

Check the clearances on the cylinder that is at TDC on compression. Turn the engine 180 degrees (half a turn, mark the pulley opposite the timing mark, near enough is okay). Check the clearances on the cylinder that is at TDC on compression (the next in the firing sequence, but again it will be obvious). Etc for the other two cylinders.

Because you are doing half turns to known positions then there's no messing about with having the car in gear and rocking it - just put the gearbox in neutral and use a spanner on the main pulley nut. You're doing two at a time. And if you are like me and like to double check stuff, you just go through the sequence again.

I find the rule of nine complicated (too much adding up!) and pointlessly fiddly with all that rocking the car...

Neil
Neil

As Clifton indicates some of mine need to be a bit before the strict 'rule of nine' point and some a bit after, to get the biggest gap. These are always the rear 3 or 4, but I've never found out why. The important thing to note is that even a few degrees either side of the strict 'rule of nine' the clearance is actually changing. I wouldn't expect any other 'rule' to rule that out, so to speak, so I always adjust mine now at the greatest clearance point. Whatever rule you use measure at that point, then measure again a few degrees either side of that point. If all three are the same then you can use the rule with confidence. If not, then you may wish to do as I do.
Paul Hunt 2

This thread was discussed between 04/08/2007 and 09/08/2007

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