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Initially, I hadn't decided on the future of this engine so did not spend a great deal of money on tuning it. Most Honda CB racers being 350cc, I thought it logical to save some cash in case a good 350 unit came up. I read that the only difference between the 350 and the 250 is the bits above the crankcase, i.e. pistons, barrel, and head so I may change to that later. With this in mind, I concentrated on making sure the 250 engine was rebuilt properly with just a minor amount of tuning. I worked on the principle that the original CB250 produced about 30 bhp. Compared to other likely contenders, Greeves Silverstone 30 bhp, Yamaha RD 250 35 bhp, Suzuki X7 28.5 bhp, all untuned, I considered it expedient to make reliability (and re-saleability) my primary objective until I decide where I am going with the project. Starting with the cylinder barrel and pistons - easy peasy, slides off easy, not so! The cylinder barrel fought all the way and because it is so fragile, the recommended method of 'Judicious tapping with a soft mallet' was not even attempted. I couldn't shift it by normal means so I looped a couple of strong cable ties between the two cylinders and suspended the whole engine one inch from the ground from a pair of axle stands. It's times like this when I miss my fags! In times of yore, I would have gone away, had a drag and come back to find the miracle of gravity had manifested itself, the cylinder and cases being amicably divorced. Without the nicotine distraction, I had to make do with a cup of coffee whilst gravity did it thing. Even so, the right hand piston would not exit the bore due to the build up of carbon so there was no choice but to deploy the rubber mallet. It worked, at the cost of a set of rings but as these would have to be replaced anyway, nothing lost. The whole top end was left soaking in degreaser on the bench looking very sorry for itself in its coat of corrosion and burnt on oil. have a look at the picture to get an idea of the state of things.
Everything is absolutely filthy but the crank and bearings look OK. The usual selection of rounded off nuts and bolts were in evidence, and some components were missing. Most notable was the absence of the cam guide packing rubbers which centralise the guide wheels. One look at these wheels revealed extraordinary wear on one side, a consequence of the missing rubbers so replacements were obtained from David Silver Honda. I also ordered new cam wheels and a rotor puller whilst I was at it. The rubber packing pieces were no longer available but I located a set at JTS Motorcycles who could not have been more helpful. They hold a lot of obsolete Honda stuff and were pleased to send me these small items at a discounted price and without charging for the postage. I salvaged as many of the original nuts and bolts as I could as some of them are non-standard. Specifically, the engine mounting nuts and rear damper nuts are smaller across the flats than would be normal for the shank sizes. All these were degreased, polished and then zinc plated. Anything which could be replaced with stainless allen bolts and nuts was discarded. I was also intending to send the cases away for blasting but decided not to on the basis that it would be very expensive and not really necessary - this is not intended to be a concours rebuild so clean rather than polished alloy will be perfectly OK. I stripped the cases and split them to check the crank, oil pump and gear train. Everything looked good and measured up within tolerences. The bottom end of these engines is exceptionally strong so no modifications were deemed neccessary. Unfortunately, degreaser did not clean the alloy components to the standard required. The lacquer (YUK!) had almost all but gone from these so I stripped the residue with a dilute solution of caustic soda which created a nice matt finish that I think emulates the original Honda race finish. A work of caution, caustic soda is just that, CAUSTIC. leave it on for a matter of a few moments and remove the lacquer as soon as it softens. if you leave it in the solution to soak overnight, you will have nothing left in the morning. CAUSTIC SODA EATS ALUMINIUM so be aware and test your solution on a small part before using it in earnest. I found that giving the components a light brush with acidic patio cleaner after the caustic soda treatment really enhances the finish. I used a toothbrush for this - and I suggest you don't use it to brush your teeth with afterwards! The same treatment was cautiously used on all the casings after they had been stripped of their components and internals, and worked well. Once stripped of dead lacquer and burnt oil, these were polished on a buffing wheel with industrial 'blue' wax and appearance wise, were much improved. Fortunately, I found no damage to any of the casings, nor the gearbox. This was surprising given the butchery that was evident in the screws and fixings. I also discovered that the baffle plate in the crankcase was split at the mounting holes. This plate is important as it deflects oil away from the bores. Without it, a machine will smoke like a dump and almost certainly the engine will come grief if the plate fractures. Solution - I silver soldered reinforcing washers over each mounting hole and secured it with stainless cap heads. That should fix it. Next on the list - rebuild. First thing to do was compile a list of damaged or worn parts that I would need to buy or manufacture, and this was surprisingly small. Set of Piston rings Engine Gasket Set Box of Assorted "O" rings Gearbox Sprocket Oil Filter Seal Cam Box Seal Tappet Covers Cam Chain The pistons were in surprisingly good shape although a bit of damage had been done to the crown of one in the removal process. As this was just a few thou deep, I decided to skim it out on the lathe, and did the same with the undamaged one. I looked at a set of race pistons and decided I could recycle the originals as I don't need a high compression engine at this stage. Both pistons were lathe mounted and excess skirt removed almost as far as the gudgeon pin holes. The excess casting material inside was removed, both pistons weighed, and superfluous material ground out until they were identical. The crank and gear train went back in unmodified. The bottom end is strong on these engines, and as there was no wear evident anywhere, I considered that replacement of bearings and bushes would be unnecessary. The entire set of engine bolts was replaced with stainless allen bolts apart from two of the crankcase screws which were exceptionally long and not available in that format. I also retained the kickstart mechanism rather than remove it and blank off the hole in the casing on the basis that if I decide at some time to sell the engine, any prospective buyer would have to strip the engine to replace these components and that might reduce the value somewhat. The only real issue I had with this part of the assembly was a missing washer from the oil filter. In the manual, this is described as 'special', requiring fitting a particular way round. This item was unavailable from every source I knew so I made one on the lathe. I didn't have the dimensions but the Haynes manual gave a good picture of it and the dimensions did not appear critical. The pistons went on easily (new circlips and rings of course), followed by the barrels. I retained the starter motor, considering that the additional weight it represented was a small price to pay for not having to push start a cold engine on race day. The starter motor will be powered by a separate starting battery, details of which will be posted on the Electrics page in due course. The remainder of the bottom end was subsequently assembled, including the generator rotor, but not the stator. Again, the reasoning behind this will be described later. The gearbox sprocket was replaced with a narrower section one from a CB 250N to allow the use of a 520 size chain. The original chain and sprockets are unnecessarily large for a race bike, or even a road bike, hence the change. I needed to make up a spacer for the gearbox shaft but that was a simple turning job on the lathe. An additional advantage of using the 520 size sprocket was that several different sizes are available, in particular from Worlds End Motorcycles. I chose one with one more tooth to raise the gearing slightly thus gaining some advantage from the weight stripped off the original road bike specification. Those who are familiar with race bikes will follow the logic of this, and understand the advantage of having easily changed sprockets of differing sizes..... That really was about it for the bottom end. Lingering doubts about the usefulness of a 250 led me to conclude that there was not a lot of point in tuning the engine to any extent. If properly assembled, and with balanced pistons, decent carburettors and exhaust systems, it should go well enough anyway. As a result of this philosophy, I opened the ports out to 30mm, removing all the lumps and bumps and polished to smooth. This was easily done with my Dremel and various grinding tools obtained from a hobby shop. The valves were ground and the head assembled with the original valve springs as they were well within tolerance. A new CB750 cam chain was fitted, being the same dimensions as the CB250 one. The head was torqued down with high tensile stainless cap heads screws and nuts, and the valve timing set to standard completing the engine build.
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