Re-bores need the right tools and expert knowledge!
3/ And swear… loudly! The bore is already at second oversize i.e. 0.5mm and as we know the damage goes deeper. There really is no point in re-boring unless there’s a piston in stock to suit. 4/ In an ideal world these third oversize pistons would be perfect but our barrels are so badly worn in places they really need to be machined to fourth oversize or 1.0mm…so more expense! 5/ Always check the dimensions of replacement pistons; never assume anything. It’s not uncommon to find the wrong sized pistons in correctly marked boxes. And very occasionally the pistons are actually stamped up wrong; measure twice, cut once etc. Always take the measurements 10mm from the bottom of the piston front to back, not side to side. 6/ The cylinder is positioned on the bed of the boring bar with short, oversized, steel cylinders top and bottom as supports. SP A bore cylinders from bottom to top; this keeps the line of the bore perpendicular to the base gasket/crack case interface ensuring correct alignment to the crank. 7/ The boring head needs to be centralised in the cylinder or we’ll just be causing carnage here. The three pins, located at 12 0 degrees from each other, facilitate precise centring.
8/ With the boring head set and the tool in place the cylinder is carefully bored and generally the cuts are made incrementally if there’s a lot of metal to take out. ob emberton bores to one tenth of a thou whereas the factory specs set the limit at two thou! This is just one of the differences between a quick rebore and a proper job. 9/ ost boring bar the freshly machined liners will need honing; if this isn’t done the rings will get a really rough time. Hones come in a range of various sizes to suit different bores. They also run different configurations of stones or cutters. One crucial element of two-stroke re-boring that has been omitted is port chamfering. Every specialist has their own tricks of the trade and ve been asked not to photograph Rob' s techniq ues but trust me when say they are done and done properly. f the ports aren' t chamfered then the piston rings will make dramatic, sudden and hard contact with the port edges which is guaranteed to do damage. With decent two-stroke oil in the tank, allowing the engine to warm up when cold and a modicum of throttle restraint as the motor gets to operating temperature, the rebored barrels of Project amaha CS3 C should last for ages. ou' d doubtless nd some engineering shops offering 15 rebores as opposed to the 5 VAT SPA Motorcycles charge but get thisº a uick re-bore and a casual hone are often only one step above the knackered cylinder you just handed over. Surely your motor is worth more than that? 10/ We’re using a four stone hone on the two-stroke barrel to aid the cut on the base or spigot area indicated. With a two stone hone there’s a distinct possibility the forces involved might deform the unsupported liner which is surprisingly easily distorted. 11/ After the barrels have settled overnight from the re-bore they’re honed close to size, then allowed to sit again to recover. e-honed a second time, they may get another session if necessary. Overkill possibly? Not when you realise ob builds top line racing two-stroke engines! 12/ rior to fitting the barrels, get an in-depth wash in hot soapy water which might sound like madness but it’s not. very last fragment of re-bore and honing detritus needs to be evicted or there will be ring/ piston/bore wear: post dewatering with some isopropanol, the bores are coated with ACF -50 to keep corrosion at bay.