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MG MGB Technical - When to rebuild

My 67 GT just turned 90,000 miles. It does not smoke and is running really well.

Check the compresion and #2 is about 25 below the others. Does not come up with addition of oil to cylinder. I had the carbs a bit lean and may have burned a valve. Figure I will pull the head this winter.

I do not know if the engine has ever been worked on. Car only had 76,000 when I got it a few years ago.

Other than looking at how much ridge there is at the top of the piston and cleaning the piston to see if it is stock, any other way to tell if engine has been gone over?

What is the thinking of the group on just having the head rebuild and putting it back on without doing anything else to an engine with this milage?

PS. Oil pressure is at 65 and comes right up at startup.
Bruce-C

Bruce,

have you checked the valve clearance before you did the test?

Ralph
Ralph

If it is a valve (which seems likely) you should hear a regular 'chuffing' in the exhaust or intake. After doing what Ralph says and testing again (hot, all plugs out, throttle wedged fully open) and it is still the same I'd just do the heads as everything else looks fine. I also wouldn't bother about hardened seats and different valves as that can cause more problems than it solves (recession being rare in practice), just replace burnt valves and recut seats as required, grinding in the others. May not be bad enough to need new valve(s)/recutting anyway.
Paul Hunt

Bruce, I agree with most of Paul's assement. However, when I did the valves on my '67 I had hardened seats and valves installed so that I could eliminate the possibility that I would be replacing the stock parts again. The fuel that we have today, especially in California, is so unpredictable that I won't trust it to cool the exhaust valves. I work on many vehicles and encounter reduced valve clearances on too many occasions where the milage dose not warrant it. RAY
RAY

I agree with just doing the valves, the venerable push rod B series is not too demanding here, other than the weight of that cast iron head. My input on the valve seats is to point out that if you do not fit hardened inserts you can never have an insert drop out in service.
Stan Best

If you do any long distance driving at highway speeds over 70mph make sure you have the hardened seats installed.

It's not unusual (especially if you have the OD) by us to go 750mi in one day to a meet doing 75-85mph all day. After such a trip one of valves almost always requires adjustment if it does not have a hardened seat. And of course this is always in the summer with 90ºF and up temperatures.
werner haussmann

Why would the hardened seats drop out if they were done by a competent machinist? I have had a total of 3 heads done on my Mgs and never had problems, are there some rogues about?
Jim BGT

My roadster quite often gets used for several hours at speeds at or over 70 when travelling to runs around the UK. Some years ago on a round Britain trip I did 2500 mile in five days, I've also done long trips to and through France. I've never had to adjust my valves from one years end to the next. I very much doubt mine has hardened seats as it is a 73-vintage Gold Seal engine, I've had the car 20 years, and the UK only lost leaded in late 1999. I have used LRP or Super unleaded with Castrol Valvemaster (unboosted) ever since. If you are having to adjust the valves (open up the gaps again I assume?) after 750 miles that indicates severe recession, or something else is happening. With that amount of recession you would very soon come to the end of the adjustment of the rocker screws.
Paul Hunt

Castrol Valvemaster - "Lead replacement fuel additive for use with lead-free fuel. Protects engine against valve seat recession under all driving conditions. Keeps fuel system clean and protects against corrosion"

If you have not replaced the exhaust seats with hardened seats, by all means use a fuel additive to protect the valve seats as Paul suggests. But don't go without either when driving with high rpm and load.
werner haussmann

Having owned my B for over 36 years I have seen some machine shops that are not that familiar with the proper procedures for installing hardened inserts. Still, I would err on the side of caution and have the seats replaced. To just have the valves and seats reground is false economy. RAY
RAY

This thread was discussed between 27/09/2008 and 28/09/2008

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