Commando oil pump prv

Hi Paul,

Working from a very dodgy memory, I seem to remember the way the length was adjusted was by fitting spacers until, when shaken, there was a slight rattle. So the gap you are seeing sounds about right.

Regards

Tony

Hi Paul,

For future reference, it is much better to update your post as this puts it back to the top, rather than creating a new post referring back to the previous one.

Tony

By the time that the Commando 850 was being produced Norton Villiers Triumph were claiming that many of its engine parts were being machined or manufactured so accurately that there was little variation in finished specification. Thus making some engine gaskets and shims  unnecessary. This included the Pressure Release valve spring shim(s).

Replacement PRV springs were all supposed to be 1.171" long with a compression modulus that kept the warm oil pressure at 45 to 55psi. Your spring length is just fractionally longer and would probably push your engine crankshaft pressure to the higher end of the working rate.

Hi,

Mr Pedantic here, it does not keep the oil pressure at 45 to 55psi but limits it to that.

At tick over, it could be down in single figures.

Tony

Apparently from somone who tests these things, even with no shim and just the spring it would still enable safe oil pressure to be achieved which is encouraging. Some owners strip a running engine to find there was no shim, and no signs of premature failure. 

Shell bearing manufacturers suggest around 8psi / 1000rpm which would seem more than adequate. My modern car has oil pressure relief set at 35psi, however the bearings are designed to be lead free (not ideal) and last the life of the engine, therfore designed accordingly.

The actual lubrication interface of the shell bearing depends on many criteria, pressure is just one of them, oil type, temperature, gap, bearing material, shell design all contribute to how well this area is lubricated.

In 27 years of Norton ownership, and I suspect others like PH with even longer ownership, have not heard of an incident that caused the bearings to fail due to no shims fitted or worn oil pump. Sudden failure of the oil pump, yes, but even that is rare. 

Some owners have gone down the 'logic' route of filling the PRV piston with extra shims believing this would help push up the internal oil pressure and thereby forcing extra lubrication around the crankshaft bearings. 

A great idea but unfortunately thwarted by the oil pump and timing cover seals.  Both of which  tend to dislike more than 55psi in the lubrication system and consequently stop sealing very well  when there is.  Either can allow oil to leak oil into the timing cover when hit by high oil pressures. The cheap all plastic versions of the timing cover seal,  with no inner cuff spring, can wear out or distort very badly thus causing more problems that solving.

John Hudson was a great fan of multigrade oils, especially GTX. With cold starts this oil got pushed around the system and bottom end quickly. While not being heavy enough to trigger the PRV too much. John claimed that a straight SAE 40 or 50 oil would cause PRV issues on engines fitted with a double speed oil gear set. 

On Commando engines the oil pump seal is the size of a tap washer and there is is built-in system that splits the feed from the pump and recycles the oil back to the in line. Now why do you need an oil pump seal that big???

The attached drawing (with thanks to Bruce Tinworth) explains much but does not apply to Commando engines.