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Honda CB350 with PVL Ignition Dimensioned Drawing
Drawing not to scale. This is just a quick sketch to show various dimensions related to the installation of a PVL magneto ignition on my Honda CB350. I stumbled across a 5 year old discussion on Caferacer.net regarding PVL ignition timing. Folks lamented how the same questions come up every time someone installs a PVL because the generic instructions lack certain details. Tex Mawby recommended Honda CB350 owners shorten the stock crankshaft bolt because, "the stock bolt is too long and will eventually break off in the crank when using an impact to take it on and off."
When I read this I had already installed my PVL rotor. Fortunately, I had not snapped the bolt off inside the crank yet! The crank snout is tapered and so is the inside of the rotor. As the bolt is tightened, the rotor is drawn onto the crankshaft until it can go no more at the recommended bolt torque (Honda service manual says 17 ft-lbs). I shortened my stock crank bolt a few threads and reinstalled the PVL rotor. To my surprise, the rotor was drawn further up onto the crank snout than previously. This meant the first time I had installed the PVL rotor my stock crank bolt had bottomed out the threads before the rotor was fully seated on the crankshaft. I couldn't be certain that my newly shortened bolt wasn't experiencing the same problem. So I took everything apart and measured using my trusty digital calipers.
I assumed that if the rotor was pushed all the way onto the crank shaft until it was flush than the stock bolt could be no longer than 25.8mm. So I took that figure and subtracted a thread or two and arrived at approximately 24mm thread length. This would ensure that no matter how far the rotor was drawn onto the crank snout the bolt would not bottom out on the threads. In reality, the PVL rotor stops at some point before becoming flush with the end of the crank shaft. With my new shortened bolt I reinstalled the PVL rotor. Using the highest point of the stator as a datum point I discovered that the PVL rotor was approximately 1mm further onto the crankshaft than previously. I was now satisfied that my rotor was fully seated on the crankshaft and the threads were not bottomed out.
I can't tell you how much you should shorten your bolt as this is a tolerance issue and there's no telling exactly how much your bolt needs to be shortened without having your exact crankshaft and PVL rotor. Ideally, you want to shorten it as little as possible. craig641 suggested a 1/4" or so, which is about what I ended up taking off. I probably could have gotten away with a little bit less than that. I hope this info proves useful to someone else out there following in my footsteps!
Honda CB350 with PVL Ignition Dimensioned Drawing
Drawing not to scale. This is just a quick sketch to show various dimensions related to the installation of a PVL magneto ignition on my Honda CB350. I stumbled across a 5 year old discussion on Caferacer.net regarding PVL ignition timing. Folks lamented how the same questions come up every time someone installs a PVL because the generic instructions lack certain details. Tex Mawby recommended Honda CB350 owners shorten the stock crankshaft bolt because, "the stock bolt is too long and will eventually break off in the crank when using an impact to take it on and off."
When I read this I had already installed my PVL rotor. Fortunately, I had not snapped the bolt off inside the crank yet! The crank snout is tapered and so is the inside of the rotor. As the bolt is tightened, the rotor is drawn onto the crankshaft until it can go no more at the recommended bolt torque (Honda service manual says 17 ft-lbs). I shortened my stock crank bolt a few threads and reinstalled the PVL rotor. To my surprise, the rotor was drawn further up onto the crank snout than previously. This meant the first time I had installed the PVL rotor my stock crank bolt had bottomed out the threads before the rotor was fully seated on the crankshaft. I couldn't be certain that my newly shortened bolt wasn't experiencing the same problem. So I took everything apart and measured using my trusty digital calipers.
I assumed that if the rotor was pushed all the way onto the crank shaft until it was flush than the stock bolt could be no longer than 25.8mm. So I took that figure and subtracted a thread or two and arrived at approximately 24mm thread length. This would ensure that no matter how far the rotor was drawn onto the crank snout the bolt would not bottom out on the threads. In reality, the PVL rotor stops at some point before becoming flush with the end of the crank shaft. With my new shortened bolt I reinstalled the PVL rotor. Using the highest point of the stator as a datum point I discovered that the PVL rotor was approximately 1mm further onto the crankshaft than previously. I was now satisfied that my rotor was fully seated on the crankshaft and the threads were not bottomed out.
I can't tell you how much you should shorten your bolt as this is a tolerance issue and there's no telling exactly how much your bolt needs to be shortened without having your exact crankshaft and PVL rotor. Ideally, you want to shorten it as little as possible. craig641 suggested a 1/4" or so, which is about what I ended up taking off. I probably could have gotten away with a little bit less than that. I hope this info proves useful to someone else out there following in my footsteps!