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  Chicagoland MG Club:Driveline
Broken Valve Spring Cap Notice
September 28, 2003 - Streamwood, IL

Dave Bralich’s MGB street machine was demonstrating a nasty case of valve float at a mere 5000 rpm, and also a bit more valve noise than normal. A peek under the valve cover was enough to revealed the problem as a broken alloy valve spring cap. See cover photo. When the spring cap breaks it releases all pressure on the outer valve spring, and the engine continues to run using only the smaller inner spring, which does not have enough force to bring the valve home fast enough at high engine speed. Fortunately there was no other damage to the expensive cross flow aluminum cylinder head.

There was a bit of a challenge removing and replacing the dual valve springs without removing the cylinder head, but with the right tools it’s not too bad. Photos and notes for that are on the web site (URL below). In short, air pressure was applied to the spark plug port to hold the valve in place while a special tool was used to compress and remove the valve springs from the top only.

The job was easy, but the broken part is a big deal. This is a light weight aluminum part used to reduce the mass of the valve train so the engine can run to higher speed without encountering valve float. This is particularly important in an engine with a high performance cam shaft, which is intended to run faster than a factory stock engine. If this part was to break when the engine was running several thousand RPM, there are several ways to encounter significant engine damage.

This is the second reported case of this failure in our club this summer. Barney Gaylord had two of these alloy parts break in one day while driving his MGA in the Flying Low Rally on July 6. In that case also there was no other damage to the engine, as the parts apparently failed during normal cruising speed, and the valve float was noticed immediately the first time the engine was run to 5000 rpm. Close inspection of these alloy spring caps reveals that the parts that broke were machined with a thinner edge than most of the other parts, and it was this thin edge that led to the failure of the part. Barney’s engine had at least three of the eight parts with this thin edge condition, and two of those failed after 22,000 miles of running. Dave’s engine had only one of the eight parts with the thin edge, and that one broke with 15,000 miles of running.

So we have this predicament. We know that at least two shops in the area have installed these parts in perhaps a dozen of our members’ cars, and no one knows how many have been fitted by the car owners. If you have an MG which has been fitted with these alloy valve spring caps, it would be a good idea to pull the valve cover to check. The outer edge of the parts should be 1/8 inch thick. If you have any which are thinner, it would be best to consider changing the parts before they break. Also, whether your parts are good or bad, please contact Barney Gaylord, who is collecting statistical data to determine the approximate likelihood of another part failure of this type. So far we have two failures in the only two cars known to have run them between 15,000 to 22,000 miles. With no other data in hand, this appears to be a 100% engine failure rate within that mileage range, but of course the data base is so far quite limited.

Barney Gaylord - 630-505-0866 -

More notes and photos at:

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