‘using ingenuity to give mathematics a swerve is an ability I developed at school’
I knew that writing about the method of converting piston travel to crankshaft degrees last month would come back to haunt me. First I put the wrong date for the back issue that contained the answer and wasn’t able to correct it before it went to press; so if you’ve been vainly scouring the 1992 back issues, try Febuary 2005 instead. Sorry. Then, out of the blue when I walked into the office, Mark Graham from Bike magazine casually asked me if I knew how to perform the conversion. I directed him to the 2005 issue and he retired a broken man. Then, to cap it all, I receive an email from Bob Ashton saying that he sent in a correction at the time the 2005 article was published and forwarded me his version of how the job should be done, confirming my suspicion that it just isn’t that easy. Bob’s version is too complicated to enumerate here, so if you want a copy I’ll send you one. Ian Brooks also dropped me a line suggesting the following web link: fastfromthepast.com/PDF/static_ angle.pdf as a source of inspiration. Thanks both; but to be frank, I can’t help feeling that this solution is slightly hypothetical in that the reason most people want to convert crank degrees to piston travel is to find a quick and easy way to time their engine. But it turns out that you need to know the centre-to-centre length of the conrod to perform the calculations, so in reality it’s going to be simpler just to fit a degree disc. I’ve employed various methods to mount them, ranging from using a flywheel puller as a screw-in adapter for a rotor to bolting the disc to a socket, blutacked to the crankshaft nut; mind you, using ingenuity to give mathematics a swerve is an ability I developed at school... Once you have fitted a degree disc, of course, you can set the crank in position and take a piston-travel measurement without any of that sine and cosine unpleasantness, so I reckon that’s the way to go. Hey, at last I get to say it... correspondence on this subject is now closed!
make clock cables
Ever wonder how they make the squared end on a speedo or rev counter cable? It’s a vice-mounted die stamp with a range of removable dies to square different diameters of cable. You grip the bottom in a vice, the stepped portion keeps it from slipping, and then insert the cable into the veegrooves machined in the dies and tighten down the knurled ring that clamps the dies together.
The knob at the top is an anvil on a floating spindle that fits through the knurled ring, so a wallop on the anvil crushes the cable between the dies. Take up the slack with the knurled ring and hit it again until the correct shape is formed.
jetting it right
Paul Bonatti says since fitting Norton Commando ‘peashooter’ silencers to his 1976 Bonneville, one plug is sooty while the other is black with a white electrode. The main jets are currently different sizes anyway, so can I recommend a starting point for jetting? Well Paul, 190 is the standard main jet size and I think it was usual to increase to 200 when fitting the less restrictive Norton silencers, but it’s worth bearing in mind that although the plugs indicate the mixture is already too rich, modern fuel burns differently, making it hard to achieve the ideal ‘biscuit brown’ plug colour. Also remember that although the main jet is the obvious tuning adjustment, it is the least used setting for road use. It’s sized to supply the correct amount of fuel for a fully open throttle; the slide needle/jet and throttle cutaway control most everyday running, therefore these may require changing. The throttle slide should be number 3 – the cutaway tends to affect starting and pulling away or dawdling through traffic, producing a slight misfire or cough that disappears once up and going. For general riding, the needle position and jet will be the most effective adjustments. The jet should be 106 and the needle on the middle position.
Ben Lang got in touch via my internet blog (rickparkington. co.uk) to ask what he should do about a loose main bearing on his 1950 Royal Enfield Bullet. He’s heard Loctite Bearing Fit shouldn’t be used in an engine, but although he has a milling machine he’s not confident it’d be accurate enough to try boring out the case and shrinking in a sleeve to restore fit. No, I wouldn’t go that way either. I have enlarged a main bearing bore (on the project Tribsa scrambler a few years ago) but that was simply to accommodate a larger bearing. Making a perfectly concentric sleeve that is a suitable interference fit in the case and will create a similar fit for a bearing is a more complex job that, as an amateur, I’d avoid unless there was no option. I don’t know why Ben has been told to avoid Loctite; obviously you mustn’t leave any at large in the engine and although heat releases it, an engine should never get near the necessary temperature. But it may be worth using something stronger than simple Bearing Fit. Asking around, it seems that Loctite 638 is probably a good choice, but it is worth reading the informative application list on Loctite’s website. Loctite works by expanding as it cures so what goes on as a thin smear dries as a much thicker resin sleeve between bearing and housing. The danger lies in using a formula that swells enough that the bearing is impossible to remove; so it’s worth considering the bearing type. A ball race can be firmly pressed out by its inner race, but a roller, with a separate outer ring, that relies purely on heat and gravity for removal may warrant something lighter duty, like Bearing Fit, after all.
Jonathan Johnstone emails from South Africa for advice on his Kawasaki Z750. Initially, he planned to build it as a café racer – but as it’s complete and very few Z750s were imported there, he’s unsure. “Does the bike have any collectible value; is it worth restoring?” he asks. Good question. The Z750 twin wasn’t popular over here – and the danger with less popular bikes is that you spend far more doing it up than you’ll ever get back. Building a café racer enables you to design your way around parts supply problems and expensive potholes, but it’s a shame to destroy a rare bike to do it. I think the bike’s condition will be the arbiter. It looks good in the picture and if it’s a mint, low mileage, original-paint bike, I’d say leave it alone. But if it’s just OK and needing things like a new exhaust and mudguards, it’s probably going to be expensive and there is likely to be a better one out there somewhere. If the bike was a Z900, the money you save by building a café racer would be reflected in how much less the finished bike was worth. But when the bike is not a sought-after model, customising can be a worthwhile option.
Pete Bryson asks whether he should stick with the standard NGK B8ES plugs in his Suzuki GT 380 or lash out on a set of fancy Iridium ones. Well, there have been various versions of the performance plug over the years. The plug books used to list them alongside the stock grade, creating the impression that these were the plugs of choice for competition or hard road use, but all the benefits I can find relate to the central electrode being thinner. I note with interest that my 1982 Champion book says that performance is improved because the thin electrode allows a smaller plug gap than a fat electrode, which would mask the spark; yet current advertising suggests that the material improves performance because it allows a bigger gap... All of which suggests to me that it is an idea that might have some effect in very highly-tuned engines, where tiny things add up, but it will make little difference in an old engine. Assuming the GT runs happily as it is, I’d stick with the B8ES plugs. I remember getting pukka surface discharge plugs for a Kawasaki H1 I rebuilt in CB years ago. It misfired like hell until I fitted the manky old L82 Champions it came with, after which it ran perfectly.