tech50

                                                       WHAT THE MANUALS DON'T TELL YOU
                                                          BSA A50/A65 UNIT TWIN ENGINES

So, it's a long winter and you decided to take that old Lightning/Thunderbolt 650 engine apart to have a look-see and freshen it up for the Spring. Armed with the factory manual and parts book, along with a great deal of enthusiasm, you're on your way. The unit twins are relatively easy to work on, with the vast majority of components being removable with the engine still in the frame (top end,clutch, gearbox, etc.). The factory manual is quite well detailed. Here are some notes I've added to mine over the years to supplement factory explanations. Hope they help!

1) When disassembling the primary chaincase, a clutch puller is almost always necessary. Screw the puller into the clutch center, tighten the puller bolt and then give the bolt a sharp rap with a hammer. If the clutch doesn't come off its taper, tighten bolt a bit more.

2) If the engine sprocket seems stuck on the splines of the crankshaft, you will notice that there are 2 tapped holes in the sprocket. If you don't have the BSA tool, just screw in a couple of screws/bolts through the holes, which will push against the crankcase and thus force the sprocket outward. Note the importance of placing some scrap metal between the screws and the case to prevent damage.

3) To remove the ignition auto-advance mechanism from its taper, fit a long bolt of proper size into the mechanism (after first removing the center fixing bolt which goes right through it). By giving the head of the bolt some light, sideways taps with a hammer, the mechanism will easily be jarred from its taper.

4) To easily remove the gearbox cluster, after clutch and gearbox cover nuts have been removed, tapping on the end of the mainshaft (with a piece of wood against it to prevent damage) will drive the entire assembly out of the right side case. I suggest that the drive sprocket nut be loosened beforehand to facilitate removal of top gear and access to its bearing.

5) Loosening that drive sprocket nut can be difficult, especially if you don't have the proper size tool. You can lock the sprocket using a length of chain either bunched up against the case, clamped in a vice, or pinched tight with vice grips. Then, after bending back the tabs on the washer, some good taps with a punch against the leading edge of the flats of the nut should loosen it off for you. I know this is a bit crude, but it does the job and has enabled me to remove a variety of butchered nuts (no this is not a gourmet delicacy!).

6) If you need to remove any gear bushings, carefully cutting a slot across the bushing with a hacksaw will relieve the pressure of the press fit and allow easy removal. Installation can be facilitated by freezing the bushing and heating up the gear. Of course, a proper press helps here, but not everyone has one.

7) When preparing to split engine cases, it is usual for the camshaft pinion to be quite tight on its shaft. It doesn't have to be removed to split the cases, especially if the cam is in good shape. Once the cases are apart, tapping on the end of the shaft with a plastic mallet will free the cam from the pinion, if further inspection is required. (I seem to be using hammers a lot!)

8) When preparing to split engine cases, be sure to loosen off all retaining nuts for gears, clutch, rotor, etc. on both sides of the engine before removing the primary drive assembly, as this job is much easier with the engine locked up, as shown in manuals. Be sure to slip some wood strips between the bar that goes through the connecting rod small ends and the crankcases.

9) After removing the oil pump, if you find it very stiff to turn or it seems to jam at certain points, take it apart and look for a small sliver of metal lodged between gear teeth.

10) If you don't have bore gauges and want to determine if a cylinder is worn, place a piston ring in the lower, unworn part of the cylinder bore and measure the end gap of the ring with feeler gauges. Then, using the piston, move the ring up the bore and measure end gap at several places up to about ½ in. from the top. Take the largest end gap measurement and subtract the measurement from the lower part of the cylinder. This difference is the circumference wear amount. To find the approximate diameter wear amount, divide this figure by 3. Most resources I have read indicate a rebore is needed when wear exceeds .007in., but you should consider the overall condition of the bores and pistons as well.

11) If you want to get a general idea of the condition of your crankshaft bearings without totally splitting the cases, remove the primary drive system and the oil pump and idler gear from the timing side. This allows the crank to sit freely and allows you to determine if there is any up and down play at the bearings by grasping the ends of the crank. The timing side bush is especially subject to wear and any more than slight up and down play means replacement(stock clearance is .002in.).

12) This is also a good time to measure crankshaft end float. There should be no more than .003 in. of side to side movement. If you don't have a dial gauge, you can determine this measurement by placing feeler gauges between the left crankshaft flywheel cheek and the inside of the crankcase.(I'm assuming the top end is off the engine). Push the crank in one direction, take a measurement, then push in the other direction and measure again. The difference in the two measurements is the end float.(not as accurate as a dial gauge, but close). The amount of end float is adjusted by shims placed behind the left crank bearing. This bearing is sometimes easily removed, but can be a real bear to extract. If anyone has a simple method of removing this bearing, please e-mail it to me! (jbakker21@hotmail.com)

13) When replacing barrels over pistons, support the pistons on a couple of pieces of wood strips. Effective ring clamps can be made using hose clamps. Dimple the top edge of the clamp outward in 4 places using side cutters to prevent the clamps from going in the cylinders. Tighten them up just to the point that the ring end gap closes and no more. This makes the job easy. Of course it also helps to have a friend supporting the cylinders as you lower them over the pistons.

14) If you need to heat cases to remove bearings, or heads to remove guides, clean parts well with solvent followed by a wash with detergent and water. Then place in an oven at 350 F. Jarring the case on a block of wood will usually have the bearing drop right out and guides can easily be drifted out of the head. Best do this job when you- know- who is out for the day!

I hope these tips will be of some use to you BSA Twin owners and please send in your own tips to add to the list so I can scribble more notes in my manuals.

Keep those bikes running!

John Bakker