|THE MINI MAG. Volume 2 No.3|
Vol.2 Home Page | Index Page
| BROKEN PISTON RINGS:|
Quite often a loss in performance is caused by a loss of compression. To determine the precise reason for the loss, do the following checks in this sequence.
1. Check tappet clearance. Adjust if necessary.
2. Take a compression reading from all cylinders. All readings should be similar.
3. Put about 5cc of engine oil down each plug hole.
Spin the engine over several times to distribute the oil over the bore. Take readings again. If there is no appreciable increase in compression pressure the problem could be burnt valves. If two adjacent cylinders record much lower pressures but are similar, the head gasket could be faulty.
If cylinders show a marked increase in compression pressure the problem could be broken rings. These can be caused by a flooding carby, revving an engine when cold, gasket leakage causing a hydraulic lock up, pre-ignition or incorrect oils.
The standard Mini piston is a 4 ring unit set up in one of these sequences;
Top ring. - Compression ring ---- Compression ring
2nd ring. - Compression ring ---- Scraper ring
3rd ring. - Scraper ring ---- Scraper ring
4th ring. - Oil control ring ---- Oil control ring
The B.M.C. advised oil for use over 32deg Fahrenheit (0 Celsius) is SAE 20W or 30W. Most Mini owners, thinking they are doing the right thing by their car, run 20W 50, 30W 60 or even 40W 70 (branded as HPR40 which in itself is misleading).
The engine oil also services the transmission, gearbox and differential. These heavier and commonly used grades definitely give a quieter engine and transmission, reduced oil consumption and better oil pressure at higher temperatures, less oil leaks, but unfortunately this is to the detriment of the piston. The oil is just too thick to get past the third ring and adequately lubricate the top ring. This results in additional drag and can ultimately widen the ring groove to the point of breaking the ring and destroying the groove.
These oils are very good, but not always suitable for a Mini. They are perfect for modern short stroke engines running three ring pistons which includes nearly all non BMC / Leyland vehicles. They are not suitable for four ring pistons in long stroke engines, often used at high revs resulting in very high piston speeds, in engines designed in 1953.
Some mechanics leave off valve stem seals when building a motor in order to get additional oil to this dry area. Note that the original Mini spec oil seal is an “O” ring, a far less efficient device than the modern neoprene and metal collar seals often used on re-builds.
If you have re-built an engine using four ring pistons resist the temptation to use these heavier oils until oil consumption becomes a problem.
If you have pulled your engine down and found the top rings broken and grooves destroyed don’t despair completely. In some instances the grooves can be re-machined and spacer rings fitted. If your re-built engine isn’t going to be raced or worked really hard, try re-building deleting the top groove.
Second groove becomes compression ring, third ring scraper ring and fourth ring oil control. You may get a little more piston slap and possibly a slightly increased oil consumption but you can have a good reliable engine for many years.