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Assembly of Bottom Half |
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on a picture for enlargement |
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Again, the bevel shimming process is not rocket science, but can be a bit fiddly and relies mainly on ‘feel’ rather than any special measuring device. When correctly shimmed there should be no noticeable tight spots at any point, but with only the slightest trace of backlash. Added to this, when looking into the bevel box, it is preferable to see that the horizontal and vertical gears have full contact, rather than any overhang by one gear over the other. To get to this point, it is simply a case of obtaining a selection of shims for both the timing side mainshaft and those for the base of the lower bevel box. Then much trial and error fitting ensues, along with associated swearing, until satisfied that all is ok. Admittedly this was not a quick process and can be quite expensive on shims, but is satisfying when you feel you have it right. Having Assembled the bottom half this far, other than ensuring everything is spotless on assembly, that is about it. I will cover the timing cover and oil pump drive gear in a later section. |
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Head and Barrel As I mentioned earlier, one of the most attractive features of the engine when first offered, was that it had a very desirable bronze skulled alloy head and alloy barrel. On close inspection the head looked very nice, with very little noticeably amiss. Unfortunately, both valve seats looked slightly pocketed, which is not uncommon with these bronze heads (seats are not very hard), but other than that, pretty good. At some time in its past the exhaust port thread had obviously been damaged, as this had had a steel insert put in, but whoever did it had made a very professional job of it as it looked very neat. Actually, when looking at the head closely with Stu Rogers, a couple of months later, we noticed that a very small locating screw had been used to hold the threaded ring in place, this being very subtlety added from above, between cooling fins. We will see how this works in practice – hopefully it wont come loose under working conditions. |
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Valves I talked to Stu Rogers about the pocketed valves, and also about the specification of head he uses on his own (very fast!) racing engines. He told me that his heads use a larger inlet valve than standard, with a inlet tract bored to suit. He also suggested this works best with a smaller exhaust valve (which I assume, is useful in that it means less chance of valves clashing with the larger inlet valve. I decided that as I was going to have to have the seats looked at anyway, I would entrust the work to Stu, and have the mods he suggested done as well. What I liked about the idea of the bigger inlet valve, was that it meant I might get away with not needing a new valve seat, as the seat was going to be opened up anyway. Anyway, a few weeks later I went up to Stu’s (sideways actually, Stu is near Wisbech and I am close to Leicester, and picked the finished job up. As well as fitting a larger inlet valve and a new exhaust valve seat, he had also opened up the inlet tract to a whopping 1.250". In the meantime, I had managed to get hold of an alloy 10TT carburettor and had machined it out to the same (very large) bore. It will be interesting to see how well this works, but I am assured by Stu that a similar arrangement in his own engines gives very satisfactory results. Final job on this was for me to reduce and shape the inlet guide (I cant help myself, I always do this) and at the same time take the opportunity to do some extra detail work on flowing the inlet port and final polishing. I think you will agree from the photographs, that if nothing else, the finished job in bronze certainly looks luvvly! |
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Assembling Head |
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Hairpin Valve Springs
and How to Compress Them The Hairpin valve springs used on cammy Nortons are one of the most distinctive parts of the engine and are synonymous to me of exotic 1930's engines. The only problem with using them (apart from the fact that they are exposed and allow Castrol R to be flung out all over the rest of the bike!) is that you cannot fit them using a standard valve spring compression tool. This provided an interesting diversion from assembling the engine, as I decided what to do about a special tool for the job. Obviously, hairpin valve spring compressor's are not the sort of tool you can buy from Machine Mart, so the first thing to do was to try and dig out all those old line drawings seen in 1930's instruction manuals, that would at least give me a reminder what they look like. Looking at a pair made by Stu Rogers when I was in his workshop a few days later, was also very useful to give me a good idea how they should be made. What I really needed as a starting point was an old pair of some sort of blacksmiths tongues, that I could then convert. Having hunted my own garage to no avail, help came in the form of a pair of old tin snips in Titch's workshop which he remembered as 'never being any good . . .' (to be fair, as Titch is in his late '80's he doesn't even use his better tools quite as much these days!). Once I had the tin snips home I set to with them, first heating one of the cutting blades with an oxy-acetylene torch, so I could bend it through 90 degrees, where it could form a platform for a fabricated lip that would seat around the base of the hairpin spring. The other blade was then ground down for much of its length and I brazed to this a specially shaped tool that would press down on the top tongue of the spring. Finally, I then heated and re-formed the handle positions, so that the two handles could be easily held with one hand, once the spring was compressed. I made a little steel ring, attached by a cord, that could be placed over the handle so it could hold be left compressed, freeing both hands. Finally, having cleaned them up, I painted them a very pleasant shade of British Racing Green, just to add a bit of BS! The finished result was surprisingly pleasing to the eye and has made fitting the springs quite a pleasure, particularly knowing they have been made specially for the job. |
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