A Special Builder's Notes


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24 June 2015

What I Didn't Know.....

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

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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.

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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.

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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.

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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.

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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.

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