Three bearing crank....strange middle bearing issue

[Graham Barker]

Three bearing engines have smaller oil jets so as to raise the pressure to feed the centre main and centre cam bush appropriately.

They normally run at about 9psi warm.

Oil pressure on my 3 bearing 1937 engine drops to about 2 psi at idle when hot and about 9 psi at 35 mph when hot. Are you suggesting that I have a problem ?
Graham Barker

[Ruairidh Dunford]

No, your car is running at 9psi, according to what you say above?

[A7novice]

Sorry for delay with photos as requested. Crank is 1 5/16 inch at big end journals.

[Chris KC]

OK so it's a 3-bearing splash-fed crank. I'm not sure but in the top photo it looks like the oil pipe union has been blanked off? And as you say, no pipework to the usual location on the centre main. Which brings me back to my question above, is there any sign of a drilling into the oil gallery next to the oil filler neck? That being the most likely alternative lubrication arrangement (for the C/M) I can think of.

There would be a plugged hole somewhere around here:

  Plug.JPG (Size: 40.53 KB / Downloads: 251)

[A7novice]

Yes, you are tight, see photo. There appears to be a third oli jet.so, looks like it has been converted from pressure fed to Drip fed. What would be the advantage of this conversion ?

[Chris KC]

Aha. 

I'm afraid I can only guess about the history / configuration of your specific engine. I suppose it may once have housed a Reliant Crank but if so (and if it was done properly) I'd expect to see other things going on too.

What I suspect may be the case here is that someone has drilled down through the main oil gallery so as to link it with the hole for the long offside centre main bearing stud. From the other side they would have removed the screw which secures the centre camshaft bush, and via this hole extended the drilling of the cam bush oil supply across to link up with the aforementioned stud hole. With this done, and perhaps some modifications on the long centre bearing studs & holes, you then have an oil supply direct from the main gallery into the top of the centre main, which is probably better than the original Austin arrangement (which was prone to fractured pipes etc. as well as supplying oil to the "wrong" side of the bearing). 

On other engines I've seen the cam bush oil supply is then blanked off; the bush is instead drip fed via a small hole drilled in the floor of the block from the valve chest (which is perfectly adequate).

If all the above is correct, you have no need to worry that your centre main is getting no oil. However as with all modified engines, you really need to have it apart and explore to understand in detail what has been done to it. Meanwhile I would advise you to take very great care if replacing any engine parts in case you upset the workings of the present arrangement. For example if your cam bush is drip fed, replacing the block with another could cut off its oil supply, should you fail to spot the additional drilled hole.

p.s. yes I know I'm a bit tight but frugality used to go hand in hand with owning an A7...

[A7novice]

Many thanks to all folks who took the time to reply, much appreciated.  I am much clearer now . Regards.

[Austin in the Shed]

It would be worth taking the centre crank bearings out at least to check their condition,and know that this modification works.
The centre crankshaft main bearing always wears out first,this allows the already weaker crank to flex and break faster.
It would be worth checking the bearing clearance with plastigauge if it looks at all suspect.