Morris Minor Owners Club

i rebuilt the engine on primrose exactly a year ago, and its sounded lovely ever since, i also installed an oil pressure gauge, which i quickly have become obsessed with.
Anyway, me and primrose are doing 70 miles a day and approx 300 miles a week at motorway speeds (or as close as a primrose will do)
however, i have noticed that when i start the car up, the pressure reads a good 65psi, and maintains this for a good 40 mins until i get to the 2nd roundabout on my way into uni, when it slowly creeps down to 40 psi. when ever i have to stop after that for light, junctions etc, the pressure drops down to 20psi, but climbs back up to 50 when ever i take off and maintain a steady speed.

now, i am confused with this. the engine for the first time ever dosent have any oil leaks, no oily exhaust etc. when i rebuilt the engine, i put a new crank, piston heads, piston rings, new oil pump, the usual really (except i got the conrod caps mixed up, which means the engine has a slight rattle when its started first thing in the morning).

so, what could be the cause of this changes in oil pressure? or could it simply be the gauge is "iffy"

(except i got the conrod caps mixed up, which means the engine has a slight rattle when its started first thing in the morning).

What do you mean by this? Did you run the engine with the conrod caps mixed up? Or just assemble it and realise the mistake before attempting to turn or start the engine?

What oil are you using and how long/many miles since the last oil change?

20 on idle is OK, but 25/30 would be better!

The con rod cap thing is certainly worrying! Are they STILL mixed up - or did you sort it out, leaving some damage on the shells ? But these pressures are just about 'ok' - not too worrying. Although in summer I would expect them to go lower with the warmer weather - and then indeed it will be a worry. You could try packing the relief valve slightly - but your start-up figures suggest it is actually pretty much correct, and so little to be gained. If not using 20W50 oil at the moment - certainly DO so before summer comes along.......

If the engine rattles when you start up that is an indication that the ends have some play in them. Turn the engine over on the key with no choke to build up some oil pressure and then use the minimum choke that it will start on and keep the revs down until the oil pressure has built up.
You need to drop the sum and examine the big end bearings for wear patterns and if necessary swap the caps around to get the best fit. When you bolt the cap to the rod you should not be able to feel any ridge between them

ill drop the sump tomorrow and have a good look at what's going on in the bottom,

if one has damaged the shells, how does one tell the size of the crank?
the new one i put in had been re-ground so i havent the foggiest what size it is.

The shells are marked on the back with the oversize - 10 or 20 etc

Before dropping the sump it might be worthwhile removing the oil pressure relief valve to check its condition. There must be no marks or scratches on the side of the valve plunger. The original plungers were better than some of the ones available now, which are aluminium and can pick up on the cast iron block. If there is any doubt use some fine wet 'n' dry to clean the outside on the plunger, clean thoroughly and replace it. Check the oil pressure cold and hot.

Assuming that nothing changes then drop the sump. The conrods and caps are usually marked with 1,2,3,4 or dots that indicate the same thing. Also you must ensure not only must the cap match the rod, but it must be on the right way round. This means that the location tags on the big-end shells must be opposite each other. The rods also need to be the right way round on the crank as well.

For size, if there is nothing indicated on the back of the shell, or it is marked with 'STD', then it is standard, otherwise there will be something like 010, 020 etc, which means 0.010" undersize, 0.020" undersize etc., and shows that the crank has been reground.

If there are any marks on the crankshaft big-end journals, the crank will need grinding, and so long as the shells are no more than 0.020" undersize, there should be enough left for another regrind.

Drop the centre main bearing cap and look at the crankshaft journal and the shell, if both are good, then clean and re-assemble with plenty of engine oil on the crank and shell. If not a regrind!

If the crank is OK, then look at the caps and shells. Are the caps on the correct rods? Are there any marks on the shells?

If you can take some pictures and post them, that will help the feedback to you.