Low oil pressure

Thu Nov 25, 2004 10:40 pm

I put another motor in my cub. Works fine, no oil burning or ticks, but also very low oil pressure. I have lots of pressure for the first 5 minutes, after that it starts to drop. I put new oil filter and another gauge but it didn't change things. My next step would be to put new main bearings in it.would I need to put new rod bearings in it also. It would probably be a good idea. Would this cure the problem? There is no blow by so I'm not going to change the rings or fiddle with the valves. I don't want to take the motor out so I'll just drop the pan and do it from there. Is this a good idea?


Thu Nov 25, 2004 11:38 pm

You won't care for my answer, but if this is a tractor you intend to use and keep for a while I would either borrow a micrometer or take it someplace that can mike it. If the bearings or that badly worn the chances of the crank needing to be ground are very good. I would also go with rod bearings too.

Thu Nov 25, 2004 11:44 pm

Also, don't forget to replace the oil pressure regulating valve & spring. Two inexpensive items that can help quite a bit where oil pressure is concerned.

Fri Nov 26, 2004 10:06 am


I had one like this one time and this is what I did.

I dropped the pan and used plasti-gauge to check the clearance's on the mains and all of the rod bearings.
This is a cheap way to see the condition of the crank and how much clearance exist.
If you need to tighten it up a little, you can maybe find some .001 or .002 undersize bearings.

This can be done with the engine in the tractor.


Rod and main journals

Sat Nov 27, 2004 9:16 am

Question. Does, did IH stamp their part numbers on the back side of the rod and main inserts?

Allis Chalmers stamped their part numbers on the back of their main and rod inserts. This makes buying the correct replacement inserts quite simple. You plastigage the journals, checking for clearence and taper. You then cross reference the old insert part number to find out the journal size.

Allis also placed shims between the journal base and cap. With the new insert in place you plastigage then add or remove shims to obtain the proper clearence. A very nice feature.

Question. Cam bushings. Cub engines - Do the cam bushings need to be replaced when you replace the rod and main bearings? On Allis engines the oil is first feed to the cam bushings then to the rod/main bearings. Worn cam bearings result in a loss of oil pressure.