My first mod along these lines, an additional banjo 'let' into the A.T.D. cover way back in 1954 seemed to help, certainly didn't make it worse.

With the advent of the Series 'D' breather cap (ET 24/6) many 'C' models were retrofitted with this new straight-through vent located above the forward spring cap, actually to little effect in most cases. Two reasons occurred to me, simple in detail yet subtle in their relationship. First, the original design is based on sound engineering, as was everything Phil Irving penned, yet it was made less than fully effective by presence of a slug of oil ever about the vent hose or banjo bolt. Use of the tank breather adapter (T29), suitably opened out, as a direct connection to a rising and rewards flowing length of hose as used on Factory Lightnings aided breathing efficiency as it eliminated the banjo arrangement's restriction. However--and this brings me to my second point--there remains real illogic in the pairing together of a 'C' and 'D' breather as the one destroys the effects of the other.

The original breather was designed to promote a depression--or a partial vacuum--by the timed nature of its opening and closing. This aim was doomed to poor efficiency by the weak and erratic release of its pulse being interfered with by said banjo set-up. But this effectiveness, no matter how partial, is immediately negated when one also mounts a 'D' breather cap! Adding the 'D' breather with its perfect (i.e. continuous) leak destroys the vacuum the timed 'C' breather is attempting to sustain! Flies in the face of all logic to me.

Real results, and they are valuable, will only be achieved where the timed affair is simply blocked off using a spare crankcase drain plug (A 26) and washer, and the 'D' breather solely employed. When mounting the 'D' you should make sure the milled away area to the right of the top guide is deepened and somewhat widened, and all edges rounded as was done in factory 'D's to enhance air flow, otherwise the modification is useless. Actually it is beneficial to so treat all four top breather seatings as this better lubricates and cools the valve springs, stems and top guides, resulting in longer life and less friction.

The final trick is to place in the hose near its forward end--and laying horizontal--a large P.C.V. valve (one way flow). Having a large bore and easy internal action, this valve, when mounted, allows outward flow but no return passage of air, thus restoring Irving's original intent. In this way a really effective negative pressure will be felt throughout the lower cases with all its attendant benefits, described shortly. I prefer to 'ease' all internal passages of banjo and bolt to lessen resistance. A double layer of fine brass screen secured high up within the breather cap serves to catch oil droplets. I also find this same set up works flawlessly when positioned above the front cylinder inlet rocker arm, where the 'D' top guide releaving has not been done and actually gives more effective flow there without loss of oil. This is all really easy and simple to grasp, and the addition of the P.C.V. to an already fitted "elephant trunk" unit does the same thing providing the timed breather is blocked off as already described.

The results will be several, as follows: far less or no joint seepage, less loss out of the decompressor opening, less oil down the lower guides and better oil ring function resulting in drier combustion chambers thus more power, and--one more-- the motor will feel freer and rev quicker due to less pumping losses beneath the pistons (i.e.--faster!).

All of the above works equally well on a Comet or Meteor.

A very sensitive double scaled aircraft gauge attached to the breather vent hose set up (as described on both singles and twins) reveals a few pounds of positive pressure being held immediately upon start-up, which, if released after about 30 seconds running will be replaced by a fluctuating reading of less than one pound, this centering upon the zero reading, or below, according to how well sealed your crankcase happens to be. Factors bearing directly will be the efficiency of rubbers (ET 187) and the gap between spacer ET 19 and the distance piece (scroll) ET 77 which can be reduced to no more than 1 thou., as well as by fitting a thin Viton 'O' ring into a grove cut into the O.D. of said spacer (ET 19), this again to improve sealing.

However, even a fully original motor is made far sweeter to live with if this simple external modification is performed as described. Observations of at least six examples all bear out my findings and when combined with modern pistons and sealed guides, the motor runs sweet and clean, having close to zero oil consumption (very much like a modern power unit). Really, simply by improving lower guide drainage via careful grinding and use of this breather system, you will achieve satisfying results, as my own hard run '52 Shadow demonstrates. And finally, Max Lambky's liner will run with elephant trunks and auto P.C.V. units as described, on both motors, his originals plugged off.