In at the deep end
Poor Phil Martin; apart from a slight worry about getting it all back together again, he felt quite pleased at getting the engine out of his first project – a Yamaha RXS100 he’s aiming to tidy up – but then he found something more worrying. The threaded boss for the gearbox drain plug is cracked and he asks how best to repair it.
Rotten luck, this. You want your first rebuild to be simple, building confidence and inspiring you to get involved in further projects. The obvious answer is that it needs to be aluminium welded, but that would mean stripping the engine completely.
After welding, some machining would probably be needed and the job isn’t fun any more. So what can Phil do that’s a bit more approachable for someone with limited experience?
Well, since the engine is already out, I’d put it on the bench and carefully tighten the bolt to see if it can still be nipped up without excessively opening up the crack. It’s probably been carelessly overtightened rather than being a weak design and if so, it should still be possible to tighten the plug carefully without making things any worse. I would then clean it scrupulously using cellulose thinners or acetone – anything that leaves no residue – and once dry, try building up around the crack with a chemical metal product. This should seal the leak and hopefully support the boss enough to prevent the crack getting worse. Gearbox oil doesn’t need changing that often and I would suggest getting a Dowty washer – the type containing a neoprene seal, as these only need to be nipped lightly to seal.
A ‘proper’ repair is very satisfying, but sometimes simplest is best. What’s the worst that can happen? If the chemical metal falls off, Phil has to take the engine out again – he’s already mastered that!