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MG MGB GT V8 Factory Originals Technical - Valley inlet gasket bulge

I'm looking for practical solutions as to what can cause a valley gasket to bulge. Crank case pressure is the most likely cause, but the engine in question, a 4.6 ltr was run mainly on LPG. I'm wondering if a backfire could be the cause. All suggestions are welcome. Barrie E
B Egerton

It would have to be an explosion in the crankcase for a backfire (into the carbs) to cause it. Factory cars have a oil/flame trap between each rocker cover and SU carb which should prevent ignition travelling that way and ignition oil fumes, or maybe petrol/gas fumes if a carb is flooding and neat fuel is running down the throat into the combustion chamber and past the rings into the crankcase. I think modern piston-type PCV valves should shut in the event of pressure coming from the carb rather than vacuum. Not sure about the diaphragm type as used on early 4-cylinder cars.

Simple pressure from blow-by or piston movement shouldn't cause it as there should be at least two (three with twin-carb factory cars) ways for excess pressure to escape with a PCV system - the fresh-air inlet as well as the carb suction.

What type of crankcase ventilation do you have?
PaulH Solihull

Hi Paul,I've found out a bit more info. The PO tells me that the inlet gasket was replaced & the new one bulged straight away.A new short motor was purchased & that cured the problem.I now have the old short motor.It came out of a 4.6 RR with hot wire EFI & was run 99% of the time on LPG. Apart from the gas conversion, everything was standard. Also, a compression test carried out before the transplant showed all cylinders were good. Barrie
B Egerton

Could be excessive blow-by, then, with no crankcase ventilation.
PaulH Solihull

If it's crankcase pressure, check the weep hole on the bell housing to see if oil is being blown through the crank oil seal. Also you should be able to see the valley gasket "flapping". It's a big engine, moving a lot of air, ensure that, at least, both rockers are venting.
Allan Reeling

The block/short motor is bolted to an engine stand. I hope to strip it down over the next few weeks & eventually replace my current 3.5 block with it. I was hoping to find something at this stage rather than when it's installed & ready to fire up. Thanks for your help so far. Barrie E
B Egerton

This thread was discussed between 25/09/2010 and 29/09/2010

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