Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - pressurized rocker cover

Still trying to analyse why my rocker cover is pressurizing. To recap I have an excessive amount of oil coming out of the rocker breather pipe. Certainly more than I would expect-drips from air filter and exits via the side panel vents.My update.
Following advice from here and elsewhere I've carried out the following tests.
Compression test (cold) range from 3 pots just over 130 to one at 140 psi so I considered acceptable.
A blow by test ( at a compressor setting of 95 psi) only resulted in forcing the piston down and thus opening valves allowing air to escape via inlet ( hear it from carb) or outlet (hear it from exhaust pipe). Turning and holding the starting handle reduced this escape to virtually nil.
The only other "clue" I have as to rocker area being pressurize is that whilst my dash guage shows a fraction over 40psi a secondary guage attached from block to head shows around 75psi.
So the next question is could my problem lay with a fault in the oil pump/filter area.
God this is so frustrating!!!!
JK Mazgaj

Since the head casting has plenty of room for the oil to return to the crankcase and also internal windage to blow through., Iíd be inspecting the side cover gasket to ensure that the holes venting to the road draft tube is not occluding free movement of windage to the tube. With the engine running at road speed put your hand over the road draft tube. Is there pressure? Is the inside of the tube wet or dry? If it is dry Iíd suspect that the tube itself is not somehow restricted. Your Compression numbers seem good not knowing your bore size, head milling, cam profile and theoretical CR. It doesnít sound like like excessive blow by past the rings or a burnt head gasket.

What air cleaners are you using? What car? Are you using the original oil bath ( pre TF) or a modified oil bath with a dry filter, open pancake elements or Vokes type.

If using an original oil bath are you sure your air cleaner isnít over filled and under bonnet windage isnít pulling oil from the air cleaner housing into the under bonnet area. Is the air cleaner body solid and without pinholes or cracks?
W A Chasser

Thanks WA. This problem occurs with air cleaners removed and even holding a hand infront of the rocker to filter outlet on the rocker cover there is quite a discharge/spray. When referring to the draft tube - a term I've not met before, are you referring to what I know as the rocker cover breather pipe (from which the oil is spraying) or the tappit cover breather pipe on the block side? The engine is a 1250 TF with extensive work by the previous US owner which included new crank, rebore,etc.The problem only occurred immediately after the winter lay-up following difficulty in starting (may have been stale fuel)
JK Mazgaj

Are you certain that pressure is the problem? Could simply be that oil is spraying the breather pipe from below. I had this issue once and fabricated a shield from stainless sheet which remedied the issue.
Steve Simmons

With the engine running at even low revs oil trickles from the rocker breather pipe
JK Mazgaj

Are you running an original rocker cover? My TF pressed steel one has a deflector shield over the inside of the breather hole so it would be very unlikely that any oil could splash out.
If you have a shield in place I think that must confirm a pressure problem.
I have a replacement alloy cover which doesnít have a breather but I donít plan on using it unless I can add a breather with a shield.
C I Twidle

Thanks Chris. Great point. I've not checked but will do so 1st job tomorrow. Fingers crossed splash guard is indeed missing. Regards Jan
JK Mazgaj

Bill hit on a good point earlier, the draft tube
You asked what it was
It's the tube going down from the side plate
It could be blocked and if it is there will be lots of pumping action going on out the rocker cover breather
Itf blocked it could be either, wrong gasket, gunk built up inside or the tube itself
Do you have wasps there, they are well known for going in nice holes like that and building blockages
If you get a piece of hoe and fit to the top breather and blow in it by mouth it should easily blow out the other tube-if there's a restriction to blowing then there is a blockage down there
William Revit


I am not sure this is germane to your issue but I have to ask if perhaps the sump is overfilled with oil.

James Neel

Thanks for that James. However when the same garage referred to who first highlighted the problem fitted the crank seal modification he filled with new ten and a half pints as per spec. I must confess believing that there may have been an error I did initially drain and check but the problem still existed. Due to other commitments today I'v not had the opportunity to check the baffle/blockage that has also been suggested.
JK Mazgaj

Follow up to rocker breather and absent of baffle. Unfortunately it's there so that's not the problem Will have to wait a few days to get the car back to my workshop and the compressor to see if the side breather/draft pipe is clear. Daughters B is in there at the moment awaiting confirmation of what I think is a head problem.Awaiting a CO2 leak tester something I don't currently possess!!! Can never have enough tools!!!!
JK Mazgaj

Perhaps there is too much oil getting in there? Make sure the plugs in the end of the shaft are in place. Suggest start engine with cover off and a towel draped over the valve gear and start the engine. Look for oil gushing out somewhere- from the rear pedestal area, or perhaps a cracked shaft. George
George Butz III

A shudder just went through me--
Don't use your compressor to pressurise your engine, you'll run the risk of blowing out gaskets or your new rear seal, just a blow by mouth on a piece of hose is all you need, you will be able to feel any resistance or if it's clear out the tube
William Revit

Valid point William and thanks for the prompt. Will certainly apply that to the side cover pipe. Have used the compressor via the plug holes using leak detector to see if there was any ring "blow by" but only up to 95psi. Well below the 130odd plus psi recorded on my compression tester. As stated previously as the car was OK before hibernation this really has me currently stumped and problem analysis is getting me nowhere. Without some glimmer of hope taking the engine out will be like running around like a headless chicken. I again ask myself-and others could the oil pump cause my sort of problem. The fact that I'm showing nearly 70psi at an ancillary gauge from block to head feed and only just over 40 at the dash gauge doesn't help other than seem to prove I've to high a pressure in the bottom end.
JK Mazgaj

Interesting that you have two different pressures
70lb while it's cold is ok but would need to drop a bit as it warms up
Is that at idle or road speed
Where abouts is your dash gauge plumbed in--It has to be in the bottom banjo fitting in the block and not the top one on the head
and why 2 gauges

But to answer your question ,no , oil pressure wouldn't normally be the cause of a problem like you have
William Revit

To be quite honest I can't remember the location I installed a temporary gauge. I seem to remember it was to the flexible pipe from the bottom banjo. It was initially installed because the dash guage was only reading a fraction over 40 on startup and I needed to check the dash gauge was reading true. I've carried out so many different tests over quite a period of time whilst also converting my LHD TF1500 to RHD and installing a 5 speed box. Having virtually finished the 1500 - first 100 odd mile test run a couple of weeks ago I've now time to re-apply myself to the 1250. Perhaps a failing of mine trying to do to many things at once but in my defence wanted one to use this Summer!! Will just have to start all over again in the hope I've missed or incorrectly tested something. Lovely cars these T Types though arn't they.
JK Mazgaj

This thread was discussed between 02/06/2019 and 05/06/2019

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.