MG Car Repair: MGB valve spring questions, cotters, spring questions

Question
Hi Howard,

I own a 1968 MGB with the GF engine.  The #7 valve had one broken valve cotter, one damaged valve cotter, and the valve spring cup was damaged.  The associated rocker arm looked ok, just like its mates.  I purchased replacement cotters, and cup, from Moss.  Immediately I noticed the cotters were constructed significantly thicker than the cotters that were damaged, broken, or normal with the other valves.  The replacement cup appeared identical to its mates.  I installed these new cotters and cup; the net effect now is that the valve spring is more compressed, by about 3/16 of an inch, than its neighbors.

1.  Is this difference significant? Do I need to change all the other "old and thin" cotters to the "new and thick"?  I don't know if this is necessary information, but currently the motor is being rebuilt for a number of significant problems.  After the motor is completed its duty will be putting around town, under 60 mph; it will NOT be raced.

2.  What would have caused the one cotter to break?  Is there a likely explanation, or is it just "one of those things"?

Thank you again for your informative answers!

Regards,
Dan

Answer
Hi Dan,
To see which is wrong do this, Install the valve in the head and the two cotters in the collar on the valve without the spring. Then accurately measure the distance between the head and the collar part that the spring seats against. It should be 1.44" This is called "Installed spring height" (if you can measure it with the spring installed that's ok too but it is more difficult to measure.) If the new one is close to that, then all the others are wrong and also must be replaced. Cotters (keepers) must NEVER be mixed up after they have been used in an engine and is the most common cause of failure when they are just mixed up and reused. They can normally be reused if kept in pairs ONLY.  Installed spring height is very important and MUST be close to correct on all valves. The correct spring pressure is also very important so if that is in question you should replace them also. The only way you should reuse a valve spring is if you have a valve spring pressure tester. If you are 100% sure they are the correct MG springs for that engine, you can just accurately measure "Free" length and confirm that they are all the same and to specs. (1.92")
Howard