Skimming lesson
Clive Fletcher emails from New Zealand to ask a few questions about his 1961 T120 Bonneville restoration. The cylinder head had been repaired and skimmed, and Clive wonders how to work out what’s been removed. what’s been removed, ‘So,’ as he says, ‘I can install the correct-thickness copper gasket to maintain original rocker geometry – I know the problem these engines can be if there is any appreciable deviation from original specification.’
The only spare head I have to measure is a later unit 650 item – that’s 1.125in from the centre of the dome to the gasket face and since the same pistons produce the same compression ratio for both heads I’d guess the combustion chambers are the same.
This got me thinking about compression ratio. If the head has been skimmed, say 1mm, what difference will that make? CR is the swept volume (‘cc’) of the cylinder plus the combustion chamber volume, divided by the combustion chamber volume. If you had a 900cc single with a 100cc chamber, that would be 900+100/100 = 10:1. There’s a natty formula to work out chamber volume: divide cylinder capacity by one unit less than the compression ratio – 900/9 = 100. A 650 Triumph twin has a cylinder capacity of 325cc. With a 9:1 compression ratio, we divide 325 by eight, giving 40.6cc chamber volume. The Triumph’s bore is 71mm; Pi x R2 x H tells us that a 71 x1mm cylinder is about 4cc, so a 1mm skim will reduce chamber volume to 35.5cc, lifting compression up to an alarming 10:1.
Curiosity aroused, I wondered what difference increasing capacity with a 1mm rebore would make; 1mm oversize takes capacity up to 668cc, but that only raises compression to 9.25:1. Maybe I learned something in school after all!