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MG Midget and Sprite Technical - oil pressure after rebuild

So I'm at the point where I'm just about to start my 1275 after a very long layoff. I've filed the block and the oil cooler and have been turning the engine over on the starter for up to 5 seconds at a a time to try to get the oil circulating before I hook up the dizzy. However, I was seeing no oil pressure at the gauge so I removed the capillary at the block and found there was no oil coming from there when I turned the engine over.

Should I see the oil pressure go up just on the starter? If so, what could be stopping pressure building up?
graeme jackson

Yes, you should be able to get oil pressure just on the starter.

Have you removed the spark plugs?

Did you prime the oil pump?
Dave O'Neill2

I had the same problem with a rebuilt engine that had sat about for a year or so.

I primed the oil pump again - there is a brass plug at the back of the engine - horizontal - underneath the rear exhaust port. However if you have a very late engine (with a bolted plate over the fuel pump boss) it was removed.

Yes get the pressure up before starting as Dave noted.

R.
richard boobier

Another way of priming the oil pump is to remove the oil cooler hose from the back of the block and pour oil into the engine through the union, while turning the engine backwards by hand.
Dave O'Neill2

When I took the rear oil cooler hose off there was a lot of oil there so the pump was obviously working. Ended up removing the oil pressure relief valve to see if it was sticking and cleaned it up a bit. Turned it over some more and got oild squirting out of the capillary connector so the next thing is going to be to run some welding wire through the capillary to see if it's clogged.
Got the engine running though :-)
graeme jackson

Graeme,

Your pipe to the oil pressure gauge may not be blocked as it can take a while for the oil to flow to the gauge and start to register. This can best be seen with translucent plastic oil pressure pipe. I expect if you disconnect it at the gauge you'll get a rapid flow through the pipe if not blocked and then you can reconnect it.
David Billington

>>I expect if you disconnect it at the gauge you'll get a rapid flow through the pipe if not blocked and then you can reconnect it<<
yes and if you don't reconnect it properly you'll get oil on the gearbox carpet and if that's black you'll only notice when you put your hand on it (damned braided hoses with spike finish left by PO, tidy first reconnect second) :)
Nigel Atkins

I do wonder what is in the gauge as it appears to let the air out of the line but not the oil, I don't know how they achieve that. I wonder if some micro porous material that lets air out but not oil but these things have been around for how long? at least 60 years. The first time I helped install an oil pressure gauge was in a mates VW Beetle in high school back in around 1979 and that had a translucent pipe from rear to front and you could see the oil creeping along the pipe and it didn't register on the gauge until the oil got to the gauge. Enquiring minds want to know.
David Billington

David,
this even quotes for enquiring minds, I don't know if it's enough but have a look - http://www.mgb-stuff.org.uk/enginetext.htm#gauge
Nigel Atkins

Graeme,
I'll drop around with a spare gauge to eliminate that if you like? I'll drop around anyway just to see how you're getting on, which is a lot better than me ;0)

MGmike
M McAndrew

The actual gauge has a C shaped thin walled tube, flattened to an oval cross section. As the pressure inside increases, the tube inflates, changing the cross section towards round and slightly uncoiling the C. This operates through a gear and lever to move the needle. Exactly the same mechanism as for the temp gauge.

I don't think there is any way of purging air fully out of it, other than slackening the connection at the back of the gauge. I don't think the very small amount of (compressible) air in the gauge itself has much influence on the accuracy at the pressures involved.

A non functioning gauge can be as simple as a lose needle which is just a press fit onto its shaft.
Guy Weller

I had a clear line on mine but bled the air out at the gauge by lossening the fitting. Just did it because I didn't like the look of air in there. Then I decided I didn't like the non factory line so went copper. Fussy I know.

What makes people think some magic in the gauge bleeds the air out? Won't it just stay in the line, compress with the pressure of the oil and make no difference to the reading? The worst it could do is act as a pressure transient dampener and slow a sudden rate of charge in pressure e.g. start up. Never ever going to notice that though.
Greg H

Exactly as Greg/Guy say. I bleed the line to the gauge, but any air left in there is of no consequence. The will compress so far and then no more, and not effect the oil pressure readings. Actually I even think it''s a waste of time bleeding the line to the gauge, as my oil line weeps a little at the juction with the pipe coming from the engine. The oil weeps out and air seeps in. But I still get a solid reading at the gauge.
Lawrence Slater

I'm aware of the operation of a Bourdon tube gauge but on the installations I've seen where a translucent tube was used the oil always seemed to make it to the gauge in time and the connection didn't leak oil afterwards. That seemed to indicate that the air was able to get out of the system as otherwise it would compress in the tube and the oil would only get part way to the gauge as the volume in the tube and gauge is very small.
David Billington

Sorted! tried connecting my airline to the capillary - still no results so I pulled the gauge to see if I'd blocked the fitting with the little leather washer. Noticed something shiny in the end of my brand new (from Moss) cappillary - they'd brazed a new brass fitting to the copper pipe but hadn't cleaned the excess solder out - ended up having to drill it to get rid of the solder. Now, at 2500 rpm I have a nice healthy looking 80+ Lb/foot showing on the gauge....



graeme jackson

yes but how accurate are these gauges ?




no, I'm joking, just joking

it's always nice to see the needle race around, sometimes I thing was there a delay or did I just blink

. . . I love blinkin' I do :)
Nigel Atkins

David I wonder if over time the air is going into the oil?

Nigel that delay is probably a clagged oil pump. ;p Avoidance works well, just don't look at it until the pressures up.
Greg H

I was going to put I prefer blinkin' then for some reason a thought dropped into my mind, I can't remember the last time I tapped the rev counter just after start up - does the dial now instantly work or have I got so used to it that it doesn't register?

I would say I'd check next time but I'll forget
Nigel Atkins

This thread was discussed between 16/03/2013 and 18/03/2013

MG Midget and Sprite Technical index

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