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oil pumps & rocker spindles.

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``Reading through some past threads/posts, my attention was drawn to one from early Dec' 2019 seeking advice on the relocation of the writers oil tank, and whether the existing three start oil pump would cope with the new elevated position.                                 Over the next few days varied advice was offered concerning the older three start system and the new six start and the various merits or not of either one, but one piece in particular had what appeared to be an afterthought at the end of it, or it might have just been an opinion.                                                           I thought this particular advice to be somewhat erroneous and was surprised that no members had criticised it. (Not even Jeanette)               Scrolled rocker shafts were compared directly with twist drill bits and the fact that they suffered from an inadequate oil supply (fact) caused them to carve their way through the rockers.                   I have never found this to be a fact and my first motorcycle, a 1951 "88" was so equipped and never gave me cause for concern in this area.                              Yes the scrolled rockers certainly have a spiral oil groove about their circumference but that's where the comparison with twist drills should end.                                                                      Further to this the scrolled rocker shafts were said to have half the surface area of the plain shafts.        The plain shafts have a surface area within the rocker trunnion of just over two and one third square inches, the scrolled shaft has slightly less than two square inches.                                        The scrolled rocker shafts receive very little oil on start up and even less once the engine has started.                          On the contrary, because there is no positive feed to the rockers on the older twins, start up is actually when most oil goes to this area of the cylinder head.      Motorcycles are no longer used as essential go to work transport, instead of being used on a daily basis they might stand unused for a day or two or even a week.                                      During this time oil will make it's way from the tank through the pump into the sump, this quantity of oil after a week or so might be quite substantial, cold and thick.                                                 On start up the oil will be pumped back to the tank as a continuous flow, because of it's viscosity it will resist entering the tank and will be pushed up the rocker feed pipe to the head also as a continuous flow.                                                                                                       Once the oil has cleared from the sump the flow to the tank will become intermittent and will enter the tank as spurts of oil and air, the flow to the rockers, at this point, will become almost non existent, but despite this it does seem to work.                                      For those wanting to give their rockers a treat,  placing a finger tip over the oil inlet within the tank will cause oil to be pushed up to the rockers,  a tie wrap round the inlet partially covering the inlet hole will have the same effect.       The three start system on the early twins seemed to be adequate at the time, but why a positive feed to the rockers wasn't incorporated is rather odd, but as previously stated I never found that I had any problems in this area.                                                                                    

                                                                                                                                                                            

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I suppose that if a positive feed had been fitted with a continuos flow  ,it would have to be very small oilways to avoid overoiling the seal less inlet guides.  Very small oilways would be harder to machine and easy to block in a system that circulates bits of grot.Norton did recommend blocking the return and flushing the head regularly, so they must have known it was a bit hit miss. My exhaust side did run a bit dry and the spindles must have tightened up as they wore loose in the head supports.Cant say they wore out the rocker bushes though. The RGM locking kit seems to be working ok with the extra oil I arranged,I should have tried  new spindles as its often the steel that wears rather than the softer alloy.  On the Atlas the hard steel piston rings wore themselves out against the soft alloy piston surfaces which are still within tolerance.

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Hi,

I agree with you Ian. You don't need loads of oil going up to the head. Some bikes had no oiling system whatsoever to the rockers, other than 'mist'.

The 1970's Morini 3 1/2 and 500 are such bikes. The 1980-1990's Husabergs are another - in fact, Husabergs didn't even have an oil pump, simply relying on crankcase pressure from the descending piston to push the oil around, and the engines were good enough to win motocross world championships.

The Morini has small 'crows feet' cast into the rocker covers which collect oil droplets which then drip down into the rocker bushes. The rest of the 'upstairs' bits get by with just the mist.

My own Morini has now done 98000 miles and 3 of the 4 rockers are original (cam belt failure took care of the 4th one) and there is nothing wrong with them.

In the car world, Austin-Morris A and B series engines,(designed around the same era of the original Dominator) while having a positive oil supply to the rockers, certainly didn't flood the top end with oil and were no worse for it.

My personal opinion is the plain spindles were simply a Plumpstead cost saving idea, and that necessitated all the other changes.

Regards,

George

 

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You could well be right George, a cost saving,  just like the famed slimline frame "to ease the riding position".   In reality to make possible the fitting of the rear bathtub of the Jubillee without a new expensive tool for pressing, and to use up a pile of bathtubs.

 

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