Is my oil filler cap defective?

1972 BGT 18V582. overdrive. Right Hand Drive UK Spec. No Emissions control.

I started a thread last week because my '72 BGT engine bay was being sprayed with oil following a recent top end rebuild.Having carefully checked for leaks and found none (thanks for all the tips guys) I now believe it is coming out through the oil filler cap. The car is fitted with an alloy rocker cover and the filler cap is a chrome plated circular one with tiny hole in it. The moss catalogue does not seem to show what filler cap goes with the alloy rocker covers, so I presume it is the correct one.
The oil is spraying out the tiny hole in the cap in the form of a (steam like) mist and onto the underside of the bonnet (hood). If I block the hole with my finger it comes out from around the sides of the cap.

Do you think I just need to buy a new cap?
Do I have the correct cap?
Could there be something more serious lurking below?

Hoping it's just a defective cap. Ian

Ian,

Sounds to me like you've got a build-up of pressure in the crankcase. It is normal to get a bit of this - combustion gases escape past the piston rings ("blowby") and get into the crankcase; via the oil drain channels this then gets up into the head as well. Too much of this and you've got a problem, probably piston rings, but a bit is normal.

The pressure is relieved via a rubber hose to each carb, where it is mixed with the air-fuel mixture and burned in the engine - you can see the hoses in this photo (not my car, I hasten to add!):


Could your breather be blocked, or even left out and plugged? If the breather is working correctly, and there's still enough pressure to blow a lot of oil out of the cap, then you may have a more serious issue.

Richard & Sammy ('73 Black Tulip BGT)

Sounds like your crankcase is pressurizing. The cap has a hole for ventilation.

Thanks Richard, perhaps not the news I wanted to hear but better to know anyway. I will check these breather hoses tonight and see can I find anything suspicious. The pic confirms that my cap is indeed the correct one for the car. Ian

The North American 1972s had sealed filler caps - but their rocker covers had a tube that hooked into the carbs/evaporative loss system. Is your rocker cover stock?

Sounds like you've got some sorting/inspecting to do on the crankcase vent.

Ian, I have an alloy valve cover Moss p/n 224-508 on my '73 and it isn't vented as the original stamped metal one was. This necessitates the use of a vented filler cap such as the Moss p/n 460-102 or 460-105. Both have full venting of pressure with a bit of a "steel wool" like substance in the cap to stop the oil spray issue that you are experiencing.

Here's what Moss says about the alloy valve covers:
"Muffle valve train noise while dressing up your engine bay with a heavy cast alloy valve cover. These valve covers do not have breather vents, and are not suitable for 1968-'80 cars without modification."

Ian Check the hose that goes into the Y junction just before the pipes that lead into the carbs. Mine was blocked solid. Unfortunately that would be classed as a result and not a symptom. The real question would then be why did it block? My answer is now sitting on the garage floor and my car is now VERY light on the front end.

Russ

I've got the alloy cover on my 67 MGB, with the vented cap and have do leakeage problems. On the race cars I like to use the stock tin cover when possible but on the Vintage and full prep SCCA 1275 motors with roller rocker arm we have to use the alloy roacker cover and even then we have use a die grinder to reliver area insdie the cover for rocker arm clearence with the roller rocker arms. On the race engine, I've had zero luck leaving the cap vent open, so I expoxy the vent hole up from the bottom of the cap and we normally add a breather tube to the valve cover. Over the years there has been atleast a dozen companies out there who have made alloy valve covers and each one had it's different cap requirements, most of them new are supplied with a filler cap, did you buy them as a set. If this set up has been on the car for some time and now all of a sudden you are getting blow-by and oil leakage for the filler cap, you may have a problem with the engine, if ring seal for any reason is compromised then crankcase pressures will rise and the extra crankcase pressure will find a way out, one way or another, if this set up is new to the car, and the cap and valve cover not obtained together, the it could be as simple as the wrong cap.

Ian,
Further to my previous posting I guess that I forgot to make the point that the entire circumference of the cap just below the top of the cap is a vent so that it relieves any pressure easily and it has the steel fiber material in the cap to stop spray from escaping. As Hap said, if the set up is new to the car and the cap and cover weren't purchased together then it could be as simple as using the type that I described. If, on the other hand the valve cover and cap were there before your top end rebuild that's another matter.