What I Didn’t Know…..

…. was that unlike any other distributor I’d come across, Rolls-Royce had done things a little bit differently.

RR book

This slim volume that anyone who contemplates the maintenance of the Rolls-Royce 20/25 engine must add to their library, was particularly useful to me because it had quite a lot to say about the removal of the timing gear case – as described in ‘A Mystery’, back in May – and subsequently the removal of the crankshaft vibration damper for servicing and also the idler gear between the cam gear and the distributor drive gear.

Timing gears

As it turned out, the damper was deemed serviceable and the only remedial work was to renew the bearings in the idler gear. The problems began when I realised that I hadn’t marked the gears before disassembly and the valve timing et al was lost. Consulting with Very Learned Counsel, I was assured that this was not a problem and the procedure for re-timing everything was mere child’s play. I also looked in the book – everything did seem pretty straightforward – and cross-referenced the text with the excellent and illustrated diary of Stephe Boddice who happily for me, documented every step of his own 20/25 rebuild project.

IO mark

The crankshaft-to-camshaft timing on Rolls-Royce engines is done with reference only to the marks on the flywheel. The crankshaft must be turned only from the flywheel end, never from the front of the engine. So the first step was to set the mark IO (inlet open) to the pointer set into the bell-housing. Then No.1 pushrod is adjusted to allow .020 thou clearance with both valves on No.1 cylinder closed.  The camshaft gear is then turned anti-clockwise until the tappet is coming off the heel of the cam onto the ramp and the pushrod can just be rotated by hand. A mark is made on the rim of the camshaft gear that corresponds with the gear case stud conveniently placed at 12 o’clock to the cam gear. The cam gear is then rotated 2 teeth clockwise to allow the helical gear on the crankshaft damper to mesh correctly and, as it is tapped home, the cam gear repositions itself so that the mark regains its position at 12 o’clock. Simple (ish) so far.


And here’s the bit that foxed me. How do I get the rotor arm in the right place? By adding the idler gear to the mix, the distributor drive would turn (all the gears are helical) so where would I set the gear to achieve the correct ignition timing? With No.1 cylinder just over TDC on the inlet stroke, maybe the rotor should be pointing somewhere around N0.5 (the bang previous to No.1) – surely RR wouldn’t encourage that sort of slap-dashery? I couldn’t work it out and nowhere could I find any information on what to do. I went home and slept on it. That didn’t help.

Distributor cam

But what did help was another chat with Very Learned Counsel who informed me that the distributor cam, unlike any other distributor cam I’ve come across, was on a taper – not keyed. So it didn’t matter what was going on with the drive, the distributor cam could be set wherever you wanted it. What I don’t know is always worth knowing.


4 comments on “What I Didn’t Know…..

  1. Tim Lamrock says:

    I am curious to know if it was the idler gear causing the noise in the timing cover. My 1931 20/25 has developed a loud rattle at certain revs which I initially thought was the dynamo drive brakes and bearing. Having replaced all these, the noise is still there. So did replacing the idler bearing fix your problem? Maybe that is my problem too. Thanks. Tim

    • Hello Tim,
      In short, no. But, the bonus of having stripped the timing gear out and re-timed everything is that the car has never gone better! The rattle still pops up at certain revs and I think I know what it is – the fan spindle bush. If you tighten up the fan belt, you may, as I found, cure the rattle. I just haven’t got round to doing the job yet so it still rattles on occasion.
      Hope this helps.

      • Tim Lamrock says:

        I just thought that I would let you know what happened in my case with this rattle.

        After reassembling the damper and putting it all back together, the engine ran very smoothly, but the loud rattling noise was still there when the revs increased to about 1500 rpm and then stopped above this. By chance, I was inside the car doing this test and noticed that the ammeter would not immediately show a charge when the charging circuit was switched on, but increased slowly while the rattling noise continued and reached full charge when the rattling noise stopped. When I removed the dynamo cover and did the test again, all became very clear.

        One of the brushes was frozen and of course was arcing and sparking to the commutator, and it was this that was causing vibration right back to the drive and the timing wheels. So whilst the noise was coming from the front of the engine, the problem was nowhere near there. A clean up of the commutator and the brushes has fixed the problem totally.

        So if your rattle continues, you may wish to check out the dynamo to see if all is well.


        Tim Lamrock.

  2. Thank you Tim, that’s this coming weekend’s job then. I’ll let you know if we have the same symptoms.

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