TUBE TROUBLE
Two questions from Trevor Dawkins. He’s building a ’62 Bonneville engine from bits and his first problem is that the pushrod tubes are too short to fit between barrel and head. “Which tubes do I need?” he asks. “It’s not easy to measure the gap.”
This is a minefield. For the original eight-stud alloy head, I think there’s only one type, but if Trevor has the later ninestud top end, as fitted to 1963-on unitconstruction bikes, Triumph kept altering them trying to cure leaks, to which the later engine seems more prone.
Unfortunately, not only the tubes but also the tappet blocks pressed in the cylinder can vary, which creates even more room for error. What I’d do is loosely assemble the tubes and seals, then lift the tube as high as it will go and use drill bits as feeler gauges to measure the gap – that will reveal how long the tubes should be. But if Trevor can’t find any suitable drill bits, I’d make up spacer washers to sit on the tappet block.
Trevor’s also asks, since he has a nonstandard timing cover breather fitted, should he block the standard, timed camshaft breather to avoid a negative sump pressure?
Of the two, I’d block the timing case breather. Breathers exist to prevent build-up of combustion gases that leak past the pistons. While race bikes often have extra breathers, they run looser clearances and work hard. If a road bike breathes heavily it needs a rebore, not another breather. Most old engines were designed to run a slight vacuum; it helps to keep the oil in, and that’s why the breather is timed – it prevents it breathing in as well as out.