MIND THE GAP!
One thing I’ve learned from experience is never to assume a new part can’t be faulty – dud bulbs and spark plugs are classic examples. Simon Clark emails with his cautionary tale.
He rebuilt the top end of this Triumph TR5T, fitting new valve guides and pistons after de-glazing the barrel. On firing up, the garage filled with smoke, which he put down to the assembly oil. Outside it was still as bad, although it diminished after the first ride. But starting up a few days later it was as bad as ever.
Simon suspected the issue was oil draining down the valve guides while standing, so after using it for a while with the problem coming and going, he stripped the top end and, finding the valves had a bit of play in the guides, fitted another set. It still smoked, so he stripped again to re-check the bores
– but they were still fine. Says Simon: “Luckily Pete from Britsteel Classics suggested checking the ring gaps. He measured them at 37 thou instead of the usual 10 to 15! Being new with the pistons, I hadn’t thought to check them – they were obviously under size from the start. Pete fitted new rings, gapped correctly, and cured the problem.” Thanks for the tip, Simon. It’s always worth checking gaps on new rings as they can also be too tight, leading to rapid bore wear – even seizure. I’ve heard that some new piston kits come with dodgy rings and American ‘Goetze’ rings seem to be a good choice for quality. Sometimes compression rings are tapered or stepped; the lettering usually faces upward, but check the instructions in the box. Getting them upside-down can cause smoking, because the ring has a scraper action – you want it scraping oil downwards not up!