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MG MGB Technical - Self de-Adjusting Valve

Funny thing, it appears that the #4 valve adjusts itself. I recently checked the valve clearance and found that this valve was a good amount out of adjustment. When I adjusted it, the nut was not loose. Then, I remembered that when I had a real mechanic give it a good servicing he had said that the same valve was a good deal out of adjustment.

Any ideas on why one valve would do this? I have already thrown out the idea that someone is secretly doing this at night when I am sleeping. Way too far fetched.

Cheers!
Jason
Jason Coolbaugh


I had the same problem on valve number 6 on my 1972 GT. After checking out everything above the head and adjusting it many times I decided to investigate. I took out the cam followers (you can just about do this without removing the exhaust manifold). Several of them were worn but by far the worst was number 6. The surface in contact with the cam was concave instead of slightly convex and the surface looked a bit like the suface of the moon. There were areas where the metal had flaked off surrounded by areas with a highly polished finish. The wear was uneven so that every time the engine stopped with the cam on a different part of the cam follower the clearance was different. (they rotate)
I know that one should replace cam and cam followers together, but the warm weather was approaching and I did not feel like a big rebuild job yet so I just replaced the worst cam followers and kept on driving. The adjustment is now much more stable and the engine quieter.
A new cam and followers may be on the agenda for the winter.
Barry Whitehead

Jason. Another possibility, if the valve clearance is closing up, is the valve sinking into the head or the neck of the valve stretching. I would examine all of the valves to see if the one causing the problem is sitting up higher than the others. Les
Les Bengtson

If your head has never been rebuilt for unleaded gas and you're using unleaded, it could be that the valve seat is burning away. That allows the valve to move upwards a few thousands every couple of hundred miles. It appears that the valve lash keeps getting tighter. This happened on my early 74 a few years ago and I had to have the head rebuilt with new exhaust seats. but check your rocker arms first, as mentioned above.
Ken Thompson

Are you using the 'rule of nine' (RON)? I found recently that several of my valves had the clearance still changing as it passed through the RON point, so unless I checked it at *exactly* the same point every time I got different results every time. Since then I have set the gaps where they are largest and had no more problems.
Paul Hunt

Could be that the push rod is bent.

Wayne
Wayne Pearson

Rule of Nine? Have no idea as I have never heard of it. Can you enlighten me?

As for a little clarification on the other thoughts.
- If the valve seat was wearing away, valve sucking upwards, or valve stem stretching, wouldn't the gap decrease?

The clearance keeps increasing. The clearance seemed to be about .003/4 greater than the others which didn't need adjusting. I am going to check again today to see how it is doing.

If one decides to replace the cam & followers, is that also the time when a valve job is done? Would like to keep the cost down as much as possible, yet do it right.

Thanks all!

Jason
1970 mgb roadster
Jason Coolbaugh

Jason, you're right, when my exhaust seat burned the gap decreased. If the cam followers wore, then the gap would increase because of the sloppiness. A bent pushrod would cause it to increase. It sounds like your valve seats are okay. If you have the rocker arm off, lay a straightedge on its edge across the valves, and they should be even. when the exhaust seat on mine burned, that valve stuck up about 1/8 in higher! But yours should be even and look for some kind of sloppiness in the valve train.
The rule of nine means that when the no8 valve is fully open, you adjust the no 1. When no7 valve is fully open, adjust no 2. and so on. They should always add up to 9.
when valves are too loose, they usually don't burn. When they are too tight then you get burned valves.
Cheers,
Ken T
Ken Thompson

Jason. What year is your car? The RB cars are generally considered to have a "soft" cam. One I pulled out about a year ago had most of the upper part of the lobe worn away and some wear on the base. With the age of these cars, it is difficult to tell what might be in any engine since most of them have been rebuilt or otherwise modified over the years. I would follow the advise about seeing how the valves sit in relationship to each other. This can be done by using an inexpensive set of dial calipers to measure the height of each valve when the rocker is loose (valve closed). Then, I would check the valve when it is fully open to see how high it is. That should give you an idea if the problem is a worn cam without having to pull the cam and lifters. I believe it is possible to pull the pushrods without having to loosen the rocker arm assembly. Remember, four of the bolts holding the rocker arm on are cylinder head studs and, if you have to remove the arm assembly you will need to loosen the head in proper sequence. This generally means a new head gasket. Les
Les Bengtson

This thread was discussed between 28/04/2002 and 01/05/2002

MG MGB Technical index

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