TECHNICAL INFO  

 LUBRICATION 

2003-08, Oil gallery plugs 

Here's something unusual that cropped up a little while ago that I've never commented on before - main oil gallery plugs/bungs. 
 
I'd been conversing via e-mail with a guy that was experiencing weird oil flow/supply problems with his performance street engine. Initially he'd been blaming it on the supposedly superior oil pump he'd bought. He sent it to me; I inspected it and found it to be in perfect health with no signs of causing the issues in hand. Firstly there was a claimed total oil drain down from the pump causing lack of initial supply on start-up for a few seconds, and then repeated crank bearing failures when used in anger (i.e. at high rpm for protracted periods). Unless there is something drastically wrong with the oil pump and fitment - cracked pump body, non-flat mounting surface, mounting bolts too long, split gasket, etc. - it will not drain down simply because the inlet and outlet ports are above the centerline, near the top of the pump as it is mounted to the block. Therefore it will always hold oil. A delay in pick-up is usually either down to an air leak in the pick-up system from the end of the oil pick up pipe to the inlet side of the pump. This could be a badly sealing gasket, missed-out 'O'-ring that goes between the gearbox and the block, heavily silted up pick up pipe gauze, a pin hole in the oil pick up pipe (either in the tube itself or where welding/brazing has been applied) or maybe an issue with one of these pre-oil pump filters that can be fitted into the gearbox where the pick up pipe is fixed to the side. It may also be an issue with the relief valve - the seats can come adrift. 
 
After several strip-downs, checks and re-builds - each time with the same results detailed above - carefully checking everything I outlined with no anomalies reported, I touched upon the subject of the oil gallery plugs. Initially I had just asked whether he'd actually removed the originals when cleaning the block after it had been machined. Now, this is absolutely imperative since proper cleaning of these main oil galleries that feed the crankshaft bearings and oil pressure gauge connection cannot be done without doing so. The plugs are sited at each end of the block, and are usually brass (solid plugs on pre A+ blocks unless they've been changed, or core-type cup plugs on later blocks). Weird oil pressure readings, including apparently slow oil pick up from start-up, can be caused by a blockage or gunge at the oil pressure gauge port right at the end, furthest away from the oil supply hole from the oil filter. Serious debris build-up can also cause oil flow restriction to the main bearings - particularly centre and rear ones. 
 
The reply from this guy was 'yes, I have pulled the plugs and replaced them with threaded plugs as per David Vizard's advice in his book. That way I can easily remove the bungs at each engine strip down to facilitate cleaning, which I have done'. Fair enough. Only problem here is - and I've come across this many moons ago in a block similarly prepared - the way these threaded plugs are fitted can cause chaos. The existing holes may need drilling/tapping out to use readily available grub screws as those David Vizard used where old tapered plugs now not generally available. After some careful questioning, turns out that matey-boy here had drilled/tapped the holes out to suit a straight UNF grub screw to such a depth without realizing that when he tightened the screws into place, the tapered ends where almost completely blocking the galleries off that supply the end main bearings! I instructed him to grind the tapered ends off the screws, then re-fit them using bush/bearing fit liquid only as deep as they need to go to be level with the end on the block. Doing this eradicated the main bearing failure and apparently 'slow' oil pick up issues in one hit. 
 
My advice is - just use the latest cup-type gallery plugs in all blocks - easy to fit, easy to remove. If you must use screw-in bungs, then make sure they are as short as you can get them, and when screwed in they do not cover the main bearing feed galleries. 
 
Useful part numbers: 
 
12G3053 Brass oil gallery plugs, latest 'core plug' type, 4 for large-bore block, 
2 for small-bore block and Cooper S.