Well, I guess A74XS would be closer. I was looking at ways to make a 90 degree crank. Carving one from solid takes a whole lot of metal and most ends up on the floor. I rejected the idea of bolting anything together and welding takes a lot of straightening afterward. In addition, the fundamental problem with the British twins is the two bearing crank letting the centre flex all over at high RPM. The XS650 has the same bore/stroke as the A65. There have been a few XS650s converted to 76 and 90 degrees. The former by just rotating the flywheel on the centre spline, the latter by making a new splined shaft and using two female flywheels.
Great except the XS crank is too wide with the cam drive and two bearings in the middle. Making a few measurements it was clear that it would fit if there were only one centre bearing. Parts are not too hard to come by, a good crank can be had for about $50, though of course you need two. Rods, pins and bearings are relatively cheap and Wiseco makes 80mm pistons to make it a 750cc and cheaper than BSA pistons. I needed a shaft splined 90 degrees off on each end. I talked to a gear company and they were willing to cut the spines. I made blank shafts, they cut the splines and I ground the bearing surface. The spuds on the female flywheels had to be narrowed so they would meet in the middle of the bearing.
The timing side flywheel had to be recut to take the A65 gear and pump worm drive. These cranks are hardened so most of the roughing was done with a body grinder in the lathe.
The intermediate plate was a chore, cut on a mill and rotary table. Next time I would have the CNC shop do it. The bearing cap had to be rotated backward to give the most support on the front bolts. The left case was relieved to make room for the plate.
I decided to make the centre bearing a ball bearing to locate the crank. The XS has the drive side a ball with the others rollers. A roller on the drive side would be better for load plus there is no way to retain the drive bearing toward the crank. The ball bearing has a circlip to locate it in the plate. The plate is bolted around the crank, inserted into the right case then the left case is added.
The XS crank has odd size bearings with 32mm I.D. Fortunately I found a roller bearing for the drive side and needle bearing for the timing side that are only about 0.040" larger than the A65 bearings. The XS crank is oiled by a spray bar. I drilled through the OPRV cavity and put a matching hole in the intermediate plate to hold a spray bar. The 80mm pistons will not work with the stock cylinder. I plan to make a new cylinder and head that will work with both A65 case cylinder bolt patterns. There is no simple way to use the XS crank starter gear. I have another way to add a starter.
I wish, unfortunately the day job chews up a lot of my time. The intermediate plate took the longest, a week of evenings carving the reliefs, making the bearing cap and studs then boring for the bearing. I had to borrow a 12" rotary table to do it. Finding the bearings took a while because the I.D. is odd (32mm), the key was finding the timing side bearing. I did not want to grind the crank down to 30mm.
Well a lot of chips have passed under the mill since I last posted. The engine has become a little more inter-racial. Now have BSA A65 cases, Yamaha XS650 crank, H*nda CR500 rods, Suzuki oil pump gerotors and lastly (?) BMW 325i valves and guides. When will it ever end?
Stop teasing us, need more pics, if using 80 mm pistons with the stock barrel centre are you staggering the bores? So many questions , cant wait to see the oil pump set up. The flywheels look a good bit smaller than the A65 centre gyro, the motor will feel V different. Losing that Inertia should make it spin up a lot quicker. I wonder if the "on rails " feeling will be lessened? probably a rural myth, I recall once reading somewhere that XS 650 flat trackers were not competitive untill the fly wheels got added mass, possibly bollocks, internet nonsense. Very interesting work. thanks for putting it up. G
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
Good stuff David, always enjoy seen the work of a free mind, as Gavin said more photos please. perhaps if you used an intermediate gear and mounted a starter motor on top of the front mounting you might manage to utilise the original starter ring! just a thought.
Before making the 750 head and cylinder I am going to use +1mm XS650 pistons (76mm) to test out the crank arrangement. The bore centres are 3.5" with the XS crank but 3.5625" for a stock A65. Not a problem moving bores with the aluminum cylinder as I plan to use sleeves rather than Nikasil. Aluminum sleeves in the cases would have to be very thin. Yes, the crank looks a bit lonely in there. Plenty of room for a stroked crank. 80 x 80 anyone (804cc)? I posted the oil pump before, uses Suzuki gerotors with spur gear drive from the crank. The starter mounts behind the cylinders and engages the clutch. Trying to mesh with the web gear would require cutting a window in the cases. Not something most people want to do even for the attraction of mounting a starter.
Having gone to all that trouble i would think you are after a high revving motor (circa 10k+) increasing the stroke may not be good for piston speed. As you are making a barrel why not mod the cases and try for the biggest bore you can get. I ran an SRM iron barrel at 80.5 with no bore related problems, a man of your calibre could do much better. Having gone to a centre main rather than than a new EN40 crank you must want a significant advantage to justify it, i don't think the benefit of all that work is being exploited unless you are aiming for 90+BHP.
After all, the old mod of using t140 pistons and norton crank gives you 808cc and you can go +40 on that = about 828cc. Now decent t140 pistons are generally available it would be a good motor. So you have to do better than that.
From memory the Bore on this motor is around 82mm the cyl are made with the bolts moved further out, the head modified to suit. The crank is standard but welded on the pins and reground to give 86.5mm stroke giving 914cc. Compression is over 11-1,maybe 11.4? The rocker castings on the head are machined off and a new section bolted on to support roller rockers which have greater offset, so the std cam gives .5" valve lift to the oversize valves. Valve lengths are longer and use beehive springs. Carbs are 36mm Mikunis. It's a lovely looking cyl casting and sounds very crisp.