YAMAHA DT175 MX
Ralph Ferrand is on with part eight: the top-end.
LOur Ralph sets to work painting the motor and sorting the top-end of the dirty DT.
ast month I rebuilt the bottom-end of the DT, encompassing the tiny six-speed cog changer and clutch. I’ll now be continuing to build up the mighty powerplant of the mud muncher. The first job was to sort out the electronic ignition pick-ups and generator coils on their stator plate. As a result of the disaster with the flywheel, alluded to in an earlier edition, I had bought another ‘working’ engine (it wasn’t), for its crankshaft and flywheel. Unfortunately, it was from an earlier model and the stator and wiring were as rough as a badger’s botty and also different, meaning I also had to buy another CDI box to match the earlier flywheel and stator. I stripped it down and bead-blasted the bare plate. I took plenty of photos with my smartphone to remind me later how the wiring was, before cutting all the existing sub loom off, ready for rejuvenation with fresh wire. Because space was tight, I cut my usual butt connectors in half and fitted the wires into the connector in an overlap fashion, to keep the splice short (see diagram). I then used my little heat shrink kit, with the little butane gas powered hot air gun to insulate the connectors using coloured heat shrink for a tidy job. Some of the windings on the coils had been bashed over the years and the enamelled wire had lost some of its insulation and in places the side cheeks had broken away. I thoroughly degreased the coils and then applied some fast
setting two-part epoxy glue to repair the damage. It is very important, with a job like this, to ensure that it is neat and that the wiring is routed carefully to prevent the inevitable vibration from damaging the integrity of the insulation. To keep water and other crap out, as the wires exit through the casing, a multi holed and shaped grommet is used. Sadly these aren’t available anymore, so I had to be careful to save the original. Once I had all the new wires pushed through the grommet and in the correct place, I filled the recess with RTV silicone, having degreased it all first with brake cleaner. Once the RTV was set, I added the neutral switch wire to the little loom and insulated it with heat shrink. Next, I had to paint the black engine covers, head and barrel. This meant more time with the paint stripper and more bead blasting. Once all the parts had been divested of what was left of their previous coating and all beads removed with the compressed air blow gun etc. I got down to the serious excitement of masking up.
For jobs like this I use a light weight packaging board. I still have plenty of unprinted sheets lying around from my years making props for television. Printed board like cereal packets is fine if you don’t have easy access to unprinted material; it’s just easier to see markings on white. With the barrel, I started by pressing the board against the liner bottom with my thumbs to mark the shape. I then cut the two half-round slots out with a scalpel, so that the board would sit on the bottom of the barrel casting. I then rubbed the impression of the contour of the bottom of the casting with my fingers into the card; always reminds me of the brass rubbing that my primary school introduced us to. I then cut about a quarter of an inch (that’s about 6mm for the yoofs) inside this indentation to allow for the masking tape to stick against the machined face. I then fitted it to the barrel underside and secured it with masking tape. Through bitter experience, I would caution against uber-cheap masking tape, as with most things, you get what you pay for and the extra pound a roll or whatever is well worth paying in the long run. Using a nice sharp scalpel blade, I then trimmed the excess tape off. I repeated this procedure for all the other parts before moving outside to the garden. I usually hang parts from the wife’s washing line with welding wire for painting; she only gets the hump when I forget to wipe the overspray off with gun wash and it gets on her white blouses etc. The last thing to do before painting anything, is to thoroughly degrease every surface that is to have paint adhered to it with panel wipe. Paint will not stick to your greasy paw prints, however expensive it is. I used PJ1 Fast Black for this project, which is the best rattle can engine enamel I have found in the UK. I use two pack paint on big four-strokes, but because of the high temperatures involved with the ring dinger, I didn’t feel that this would be suitable. Because this barrel has such deep fins, it is quite hard to get the paint all the way in. With a good quality air-fed spray gun, you have a huge amount of control over the spray pattern and can adjust the amount of paint carried in the fan; you have no such luxury with a rattle can. You have to spray into the fins from all angles and ensure that you frequently turn it over so that you can see the paint
coverage on both sides of the fins; I have found that if I spray from above or below, I am always surprised how much gets missed – everything must be viewed from all angles. There’s nothing as annoying as washing up a spray gun, binning the excess two-pack paint, only to discover a previously unnoticed bald patch (no comments please Bertie). Start with very light dust coats and gradually increase the amount of paint applied. Let each coat go off a bit before the next, but you should be painting what is known as ‘wet-on-wet.’ The surface should be tacky, but not wet for the next coat. It is always a balancing act; too thick a coat and it will run, not thick enough and you will get an orange peel finish. If you’re unsure how much it has cured, check by touching the overspray on the masked sections. Because strokers aren’t my usual fayre, I had to take some advice from friends in the trade when buying a piston. The world and his brother seem to offer pistons for tiddlers, but not all are great and some are terrible, usually indicated by a not realistically low price. Stock Yamaha wasn’t an option as the ring dinger was already on +1.00mm which is as big as OEM (Original Equipment of Manufacture) goes. I was recommended Mitaka pistons which are supplied by Grampian Motors. While they are a wholesale supplier to the trade, they will supply Diyers and are very helpful. I had a local firm bore the barrel to fit the specification of the piston. I think that it is worth noting here, that different makes of piston require a different piston-to-bore clearance when doing a re-bore. Most good auto-engineering shops will know the clearances for very common pistons like Wiseco, but should ask you what clearance you require for less common makes. If they don’t, you should be concerned. I was building a Laverda SF2 last year and purchased some pistons for it from a very well-known supplier of parts for this particular marque of Italian stallion.
When I rang up to ask the piston-to-bore clearance he did not know and suggested that any engineering company should know the clearance for an unknown make of piston. Suffice it to say any future Laverda pistons will be bought from north of the border or from the Germans. If you are considering buying a piston, ask the supplier what piston-to-bore clearance is required and if they don’t know move on to a professional supplier. Once I had demasked the barrel after paint, I was still a bit concerned about how sharp the edges of the ports were following the re-bore. A little research soon told me that my worry was not misplaced and that the edges of the ports needed to be relieved, to prevent the piston rings snagging on them. I used a high speed pencil die grinder with an industrial diamond impregnated grinding bit, to gently chamfer all the sides of all the port windows in the barrel. The secret is a steady hand and to keep the grinder moving at a constant speed so that you get an even chamfer. I fitted new oil pipes inside the right-hand engine cover and fitted it to the engine unit with new stainless cap screws and a new gasket buttered with Wellseal. I fitted the rings to the piston and then attached the piston to the con-rod using new circlips. No you can’t ever reuse them. On this little stroker the rings can only fit one way and have pegs to stop them turning. ‘She who must be obeyed’ held the barrel in place I pushed the piston in and took a photo. Easiest piston I have ever fitted. I bolted down the barrel with the aluminium sleeve bolts, tightening them to the prescribed torque setting. Next I fitted the copper head gasket followed by the head and tightened that to setting decreed by Mr Yamaha. Easy peezy, no cam-chains, shims or valves! Next month I’ll be investigating how the Yamaha’s CDI works.