FAQ Archive: Valve Guides on MGB

QUESTION:

I hope you can help me. I had the head rebuilt on my 1969 MGB with hardened valve seats. The engine ran fine for a few minutes then started skipping. I removed the valve cover only to find a valve stuck open. I removed the head and took it back to the machine shop. The machine shop is a reputable shop with an excellent reputation. He checked the head and polished the valve stems and checked the valve guides. I reinstalled the head and it worked perfect for about 5 miles then it started skipping again. I removed the valve cover only to find another valve stuck open. The valves are adjusted cold to the specification called out in the owner’s manual. I have removed the head again and plan to take it back to the machine shop. The oil was changed as part of the head work and the engine averaged around 60 PSI of oil pressure. I have been working with MGB’s for many years and have never had this problem. Have you ever experienced this problem? Thanks.

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JOHN’S RESPONSE:

The problem you have is with the valve guide, not the valve. There are two types of valve guides: cast iron and manganese bronze, the latter type are a coppery color. The cast iron guides require half a thousandths clearance between the valve stem and the inside of the guide; the manganese bronze require four times that, or 0.002″. The little O rings are not sufficient to control oil consumption, so with the manganese bronze valves especially, use FELPRO umbrella oil seals SS 70373. Your problem is not uncommon. You just need to open up the valve guides some.

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FOLLOW UP:

Thanks for your assistance. I spoke to the machine shop about the head and he told me that he had to replace all the exhaust valves because they were rubbing the guides. The new guides were the cast iron type with a smooth guide surface. He told me that he plans to cut ridges in the guides and taper the lower part of the guides (apparently they were flat on the bottom). The new guides were from Federal Mogul and appeared to be cut differently from previous guides for MGB according to the machine shop. He also plans to open the valve guides some and install the small o-ring on the valve stem. Hopefully this will correct my problem.

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JOHN’S RESPONSE:

Do NOT use the small o rings! Use the FELPRO SS 70373 umbrella seals.