Saturday, 26 June 2021

The Lubrication System

Motorcycle lubrication systems underwent a major transformation in the 1920s from total loss, with either a hand or mechanical pump (or both!), to recirculating dry-sump systems in which oil was contained in an oil tank and pumped into and out of the engine.

As far as I am aware the OHC AJSs of 1928 – the 350cc K7 and 500cc K10 were the first production motorcycles from AJS that had dry sump lubrication and this was a huge technical step compared to the previous total loss systems. Admittedly, Velocette had a recirculating system in production from 1925 in their OHC Model K.  The most significant difference with a dry sump system is that oil is delivered strategically to critical parts of the engine rather than just having fresh oil dribbled into the engine and hoping that it reaches the right places.

One aspect of the relatively (by previous standards) sophisticated lubrication system on early AJSs is that there are pretty well no internal passages for moving the oil from one part of the engine to another and this results in a lot of external oil pipes around the engine. My recreation is no exception and this does endear it with the appearance of a Victorian steam engine.

In this V-Twin engine I have incorporated what I believe to be the better features of the original AJS engine whilst learning from the previous experience of the AJcette single cylinder engine, particularly with regard to cambox lubrication. One point to bear in mind is that the reciprocating oil pump, of AJS manufacture, fitted to the original OHC AJSs and which looks like this:

(apologies for the blurry picture)

has a very low flow rate and delivers oil at a much lower pressure than the Velocette gear pump that I have fitted.

As discussed in a previous blog this engine has been built with 3 oil pumps, one to deliver oil to the engine and return it to the tank and 2 additional pumps to scavenge the cam boxes. It is interesting that the first incarnation of the original AJS V-Twin used the AJS reciprocating pump (as above and the same as the single cylinder engines) and there were no cambox scavenge pumps but this evolved into the same configuration that I am using here with 3 gear pumps, seen in the picture of the bike as it is today (in supercharged form) in the National Motorcycle Museum.

 The oil circuit that I have adopted is as follows:

1)  Oil is fed by gravity from the oil tank to the upstream side of the oil pump. A brass petrol tap is incorporated into the oil line to avoid wet sumping. The tap has been bored to ¼” ID to reduce the flow restriction. The main oil pump, which contains 2 pumps - the feed from the tank into the engine and the return from the crankcase to the tank, is driven at half engine speed from the camshaft drive and has inlets/outlets as illustrated below:


A suction filter has been incorporated into the oil return pipe from the crankcase – more later.

 2)  Oil is fed from the downstream side of the feed pump to the left side of the engine crankcase where it is then distributed to other parts of the engine.

A pressure gauge has been incorporated so that some quantitative measure is available of the oil pressure being delivered to the camboxes and this is controlled by a valve that diverts excess oil to the centre of the crankcases, immediately above the connecting rods. There is also an oil drilling in the casting below the main connections that delivers oil to an annular phosphor bronze distributor ring sandwiched between the drive-side main bearings, shown below

and then via a drilling in the mainshaft to the big end. This is the only internal oil passage in the entire engine.

Incidentally, the copper tubes need repeated annealing to allow bending and I always make a pattern using galvanised fencing wire before bending each copper pipe.

 All connections are silver soldered – there are no soft soldered joints.

3)  The oil pipe to the camboxes is split at a T-junction and the flow rate is then controlled by forcing the oil through an orifice contained in a brass housing. There are 2 of these, one for each cambox, as shown below in assembled and disassembled forms.

Inside this fairly compact oil restrictor is an orifice with a 0.6mm diameter hole and 2 filters, a 100 mesh filter immediately upstream of the orifice and then a coarser filter upstream of that. This arrangement worked well on the AJcette engine but will almost certainly require some calibration when the V-Twin is eventually fired up.

The restrictors are located on the sides of feed pipes that are screwed directly into each cam chamber. On the top of each feed is a drilling, blanked off with a small knurled screw, to enable the cam chamber to be primed with oil if the engine has not been started for some time, for example, over the winter months.


 4)  Oil enters each cam chamber from the downstream side of the restrictor, lubricates the cam and rocker skid and can then “escape” via 2 possible routes. The first is via the large bearing at the drive end of the camshaft and then into the top of the timing case; the second is via the 2 holes in the casting through which the rockers emerge immediately above the cams. The oil can then find its way into either the primary or secondary gutters on both inlet and exhaust sides or it escapes from where the rockers exit the cambox and then covers the cylinder head and everything else in oil!

 Oil from the primary gutters is collected at the end of the camshaft and fed into one outlet whilst oil from the secondary gutter on the exhaust side is fed into another outlet. Oil from the secondary gutter on the inlet side is fed via a 2.5mm diameter pipe directly into the inlet valve guide where (maybe?) it lubricates the inlet valve and guide. The picture below shows the arrangement on the front cylinder.


The oil drains from the gutters on each cambox are combined using a Y-junction (see above picture) and then fed into the upstream side of the cambox scavenge pump at the top of each timing case. An oil pipe connected to the downstream side of each scavenge pump deposits the oil into the right (slack chain) side of each timing case from where it can dribble down and lubricate the chain and gears.

Oil is transferred, in either direction, between the crankcase and the timing case through the timing side main bearing.

 7)   Eventually all the oil finds its way to the bottom of the crankcases (at least, the oil that hasn’t leaked out or escaped by some other means) and is collected through a pipe connected to a small oil collection chamber. The picture below shows this just after the crankcases were machined.

It is beneficial to include some kind of filter before the oil is sucked into the scavenge pump and returned to the tank to try and prevent any debris causing damage to the gears in the pump and, indeed, to prevent any debris from circulating further in the engine. There was not sufficient space to incorporate an adequately sized filter into the crankcase itself and an external filter was therefore made, housed in a cylindrical brass casing, to have an in-line filter between the crankcase and the oil pump.

The picture below shows this plumbed into the oil return line and contained in an aluminium housing. 


This will be secured to the bottom of the engine plate using 2x 3/16 studs the next time the engine is removed from the frame.

As I mentioned at the beginning, the combination of a fairly sophisticated lubrication system and a lack of internal oil passages does result in a lot of oil pipes around the engine.


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