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70 BSA A 65 Thunderbolt discussion forum:BSA Mechanic |
| BSA Mechanic |
| David said 2002-10-31 02:52 |
| The BSA Thunder Bolt is a very competent motorcycle as far as the old British Twins are concerened. The Thunderbolt engine goes a little flat around 5,000rpm´s but can be made to rev to 8,000 if y ou insist. A twin carb Lightening head is the solution if you want more top end power. The frame is quite stong and can be raced with little modification. The shift action is much better than all Triumphs, especially 4-speed Bonnevilles. Front fork action is better damped than on contemporary Bonnevilles and power is about the same. If you are looking to build an A-65 be warned that one side of the crankshaft rides in a pressed in cylindrical bronze bushing. Once the bushing is pressed into the case, it must be bored to fit the crank with .0008" to .001" clearance; pressing the bushing deformes it slightly, necessitating the boring procedure. Since alignment with the other main bearing (a barrel roller bearing) is critical, very few machinists are able to or are willing to do the setup to make the precedure work out right. Once the crank is fit to bushing, it still reguires much precision machine work. The rod journals must often be ground to an undersize dimension. If the journals were as big as auto crank journals, it would be simple. They are small. If you find a man with a crank lathe, you have to convince him to break his machine down to install his smaller grinding stone to grind your one little crank with two journals. Is he does it, he might have still fouled the job. A good machinist is gold. If you get the undersized journals to fit the bearing shells correctly, then you must have the pistons, rings, circlips, rods and pins installed on the crankshaft to use for the next procedure: The balance factor on this crank is 55%. When the motor is revved over 3,000rpm then it vibrates a lot. It needs to be rebalanced to a factor of 70%. Again, finding a machinist to do this job on a little crank can be difficult. It takes a special setup. If you get the crankshaft set up correctly and the main bushing installed properly, then you are ready to bolt the cases together. And take them back apart. Be prepared to do this for about 4 hours. It is hard work. You see, when there is a bushing on one end of the crank and a barrel roller bearing on the other end, there is nothing to locate the crankshaft like a ball bearing main would. You have to add shims until the end float is between 0 and .003". If you get the bottom end right, then the rest of the motor is not too bad. If you get the bottom end wrong, expect the engine to last a few miles and them bolw up. If you get it right, the bike will probably outlast you. The crank and its bearings are why BSA´s are so rare. If you want a project bike, complete engine or parts, give us a call at 850-575-6889. Ask for Jon. He knows way more than I do and is willing to answer all your questions. We are currently restoring a Thunderbolt to sell. |
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More 1970 BSA A 65 Thunderbolt informationOur discussion forums are intended for short messages. We also welcome more extensive motorcycle reviews.Bikez also recommends that you check out the technical data and the riders' rating for the 70 BSA A 65 Thunderbolt. You can also rate it yourself. Use our efficient motorcycle classifieds to sell or buy this bike. Your motorbike gets a high quality presentation. Ads are free. |