On my most recent purchase, the engine was rebuilt by the previous owner. It's pumping oil out and it's dripping/running from the back of the sump.

Any ideas where is the most likely orifice this oil is leaking from?

Oil filter appears tight and I have tightened the oil cooler connectors ...

Any help is appreciated

At the back it could be crank seal - or on the side it could be the push rod tube seals.
Worth checking the vacumn in the engine at idle with tube of liquid to see if oil filler inside is working...

Hi Chris
Thanks for that
Did not think of the filler tube and have been researching that recently .... doh
Engine is out this morning thanks to good weather (no garage). Just stopping for a cup of tea.
Push rod tubes appear ok at first glance.... that leaves the crank seal ... how difficult is that?
Suspect this is due to rebuild as all seals have been replaced, but can't say for sure yet.

cheers

It's the seal ... what do Haynes say ....

any tips anyone!

You can change it in situ.Mine came out easily,if its to tight to gently pry out,carefully drill two small holes in it and screw some self tappers into the holes and gently pull it out.Put some grease on the drill to catch the swarf.

Thanks Dave

I take it the breather can be checked by removing and blowing through to ensure it works one way only .... Is that correct, one way only It should pump air out to the carb

If the breather is faulty and I repair/replace it, I take the seal is ruined and will still have to be replaced

The engine is out and on cardboard on my kitchen floor.

This link will show you how gru Sean fixed an engine breather. In answer to your question you can't test it as you ask besides getting face full of oil,whatever turns you on
viewtopic.php?f=7&t=4333&hilit=breather

Thanks for the link Banjo

The way I test mine and others is with a see though flexible pipe, some insulation tape, small container for liquid (i use water) and a tape measure. Put insulation tape around the end of the pipe so it is big enough to form a seal with the dip stick hole. Container water on the floor infront of grill and place tube into water. There should be sufficient distance between top of water and dip stick tube not to allow any water to go into engine. Start engine and let idle - good distance up the tube (distance from top surface of water to water in tube) is around 40cm. If nothing or bubbles then filler is f--ked. Some will say that it could be bores/ pistons but check filler first...

viewtopic.php?f=7&t=4169
My test and my little solution to prove it was f--cked before I bought new one.