Speed 25 distributor

I have found that there is about 5 to 10 degrees of free rotational movement of my rotor arm before the timing springs come in to play. Surely this can't be correct as it will cause random changes in the ignition timing.  Testing with a timing light does indeed seem to show such changes. Attempting to investigate I tried to remove the bakelite base of the distributor from the metal pedestal beneath but in spite of removing all visible fastening screws (2 on opposite sides at the outer edge, two retaining the spring arm for the points, and one retaining the condenser) I couldn't move it at all.  I was very fearful of cracking it so the force applied was limited.  Can anybody shed any light on what might be causing the free play and how I might correct it.    Also, two devices protrude from the pedestal whose function I don't understand, a knurled knob protruding towards the offside of the car, and a longish 3/16 whitworth bolt head protruding forwards.  These are in addition to the locking nut which protrudes towards the nearside.  Can anybody shed any light on what these are for ?  Delighted that my post about pressure relief valve in the radiator has prompted such an interesting discussion ! Posted by NeilWillatt 15/05/2019 18:10:57

Will have a butcher's tomorrow. Does anyone other than Brits still use offside and nearside? Confusing to this long-transplanted Brit!  Left and right seems so much easier. Is your car offside drive or nearside drive? Posted by johnlayzell 16/05/2019 22:03:12

Same, or at least some, free play in my rotor arm. Here's what the base of my dizzy looks like:  [attachment=0:2y7ppvc1]<!-- ia0 -->SP25 distributor base.jpg<!-- ia0 -->[/attachment:2y7ppvc1] Posted by johnlayzell 17/05/2019 18:13:42

Thanks John,  I don't think I have the same distributor as you.  I have attached  picture of mine, taken from directly in front and showing the various parts mentioned in my original post. [attachment=0:s47w1itd]<!-- ia0 -->Distributor.jpg<!-- ia0 -->[/attachment:s47w1itd] Posted by NeilWillatt 18/05/2019 14:32:19

Yes, quite different.  [attachment=0:3v59t372]<!-- ia0 -->Alvis SP25 JD6 distributor.jpg<!-- ia0 -->[/attachment:3v59t372] Posted by johnlayzell 18/05/2019 15:45:21

Neil, it appears that you have the &quot;coil only&quot; ignition of the later Speed 25, not the combined magneto/coil system of the earlier version. The &quot;Mystery knurled nut&quot; is in fact the dust cover on the lubricator tube for the distributor shaft bearings. It has a cut-out at the base of its circumference which, when it is rotated, reveals a corresponding cut-out in the tube leading to the bearings. Use an oil can to squirt lubricant in. This saves wear in the shaft and its bushes, another cause of wandering timing. The &quot;mystery 3/16&quot; Whit. bolt&quot; is the screw which retains the distributor in its mounting. It should be 1/4&quot; BSF with a spigot end to engage in the groove machined in the distributor body shank, allowing the distributor to rotate when the hand advance lever is moved. Removing this bolt allows the distributor to be lifted out of its mounting. It is frequently lost and replaced by any handy 1/4&quot; BSF screw with the end filed down. 1/4&quot; BSF has the same measurement across the flats as 3/16&quot; &quot;old Whitworth&quot; and old spanners marked 3/16 Whit. will fit. &quot;Modern Whit.&quot; has the same A/F measurements as the hexagons on the corresponding BSF thread diameter.  Lastly, the &quot;locking nut&quot; tightens the clamp holding the manual advance/retard plate in position. This must be loosened to set the timing or remove the distributor. Many, but not all, of these plates have two slots through which project bolts holding springs and washers arranged to push the whole assembly downwards to promote consistent timing. Posted by DeaneHarding 18/05/2019 21:44:15

Robin, many thanks for your comprehensive reply.  Can you shed any light on the 5 - 10 degrees of free play ?   Should it be there, and if not is there anything I can do to fix it ? Posted by NeilWillatt 25/05/2019 17:04:28

Neil, there will always be some free play in any mechanism, but yours sounds excessive. I'd go back to basics. Does your distributor rotate in the correct direction? This may sound daft, but I have encountered a situation where one with the wrong direction was fitted, with the result that the ignition retarded as the engine speed increased. The rotation direction should be indicated with an arrow somewhere on the data plate on the distributor base. Do not trust the arrow, if any, on the rotor arm! Turn the engine over with the distributor cap off to see that the rotor arm revolves in the correct direction. That on my early SC Speed 25 goes clockwise, viewed from above. Next twist the rotor arm in the direction of rotation. It should move but you should feel resistance from the inertia advance mechanism springs. The rotor arm should spring back when the pressure is released. From the amount of play you describe, I would be suspicious of a derangement of the springs, weights and levers of the advance mechanism. Backlash in the worm gear driving the distributor will be present, but frankly it would have stripped by the time it reached the level mentioned. To access the advance mechanism, remove the cap and rotor arm, then undo the two 6 BA screws at the periphery of the base plate and remove the base plate. Some gentle persuasion will likely be needed. It is not necessary to remove the contact breaker or the condenser, nor to undo the screw under the rotor arm which holds the cam in place.  Once this is done it is obvious how it works. Photos attached of how it should look, with authentic patina. Complete disassembly involves removing the drive gear which is usually rivetted. Posted by DeaneHarding 26/05/2019 13:45:46

Neil,   Also check for any sidewards movement of the distributor shaft as this will indicate bearing wear. If there is movement check the inside of the distributor cap for signs of the rotor butter carving a path in the cap electrodes.   Richard Melbourne Posted by RichardWallach 27/05/2019 21:50:21

Many thanks Robin and Richard for your help with this. A couple of days ago, at home in Sheffield, I made another attempt at lifting the distributor baseplate and failed. However, while the cap was off I decided to lubricate the rotor arm cam, and may have done so a little too liberally. I then drove down to Malvern where I stayed with friends and on to the steam rally at Abergavenny.  The car performed faultlessly throughout.  This morning, still in Malvern, I had another attempt, and the base plate came away cleanly as soon as I removed the two screws, I suspect as a result of a soaking in the excess oil mentioned above. This revealed that all was well underneath with the two springs intact and the two bob weights moving freely. I have now cleaned everything up re-assembled and test run.  All is well. I suspect the free play I mentioned is normal, especially after reading John Layzell's comment that his car is the same.  Again, many thanks to all who have replied. I find this bulletin board an invaluable resource.  This particular thread has elicited useful information from members in Scotland, Florida and Australia ! Posted by NeilWillatt 28/05/2019 15:15:47