How To: 1913 Veloce motor bottom end build
While rebuilding the 1913 Veloce motor, I wanted to make sure that the heart of the motor was strong and would last for a long time. The first step was to layout all the parts in Solidworks. Then I could measure and modify things if needed quickly, and without cutting metal. The only fundamental change was to eliminate the bronze bushing in the big end of the connecting rod, and to insert an INA needle roller bearing. But of course, that wouldn’t fit into the rod, so a new one was designed then cut from 4140 chrome moly steel. It mostly matches the size and shape of the old one, but the big end is larger and the section thickness is a bit more. Also it is a bit more squared off as it is machined instead of being forged. It should be plenty strong for the 5 horsepower motor, or 10 times that amount. A new crankpin of 8620 steel was needed too, to match the INA inner roller bearing race. It is oversized compared to the original and the crankpin OD now matches the OD of the(redundant) inner race, and the rollers ride directly on the pin surface. This was done so that the crank pin could be as large of a diameter as possible to retain strength. It was heat treated and hardened to Rockwell C 62 to .050” depth. Edwards Heat Treating did the case hardening and I highly recommend them.
Section view of the crank assembly. On the far right is a representation of the inner wall of the crankcase, to verify that the crank pin nuts wouldn’t foul the boss for the main bearing. No crankshafts or main bearings are shown in this view.
By using CAD, I was able to compare the old and new crankpins on top of each other. Things like the taper fit vs a .002 interference press fit, or the new vs old OD were easy to check. I owe Greg Summerton for his advice on the best size of the pin OD, the root radii and the selection of the better INAs from their big catalogs. He has used similar designs while rebuilding big JAP racing V twins in Australia. When the layout was done, I created some fabrication drawings:
The crank pin after heat treat, but before grinding.
The crankpin after grinding at Harbur Grinding. They kept the two outer surfaces round and concentric to .0001”. Crank pin nuts are new items, made to fit a flathead HD. They were bought aftermarket, for about $10-15 with keepers, and were already heat treated and ground perpendicular on the mating surface. I set the threads on the crankpin to match the available nuts, instead of making nuts, or trying to buy a set from England. So there is one American thread set on the bike, but it is buried very deep inside the machine.
Here is the rod with the INA bearing installed. There is a grooved machined into the face of the big end of the rod. This should allow oil mist to get into the needles. There is also .028” of end float for the same purpose. Early motors like this do not have positive oil flow to the crankpin,and rely on the mist that flies around inside the motor from a hand pump that puts a squirt through the crankcase wall every few miles. Primitive, yes, but it works for 4-6hp. Needle rollers don’t require a lot of oil if they aren’t heavily loaded, so this setup should last longer than the old plain bronze bushing. A few guys have run these bearings on their early Triumphs, Premiers, etc. and report that the motor revs a bit more freely, and have lasted for several years thus far. The INA rollers and outer race were used, but the inner was discarded. The rod also has a new bushing in the little end that I made from bearing bronze to fit the piston wrist pin.
Ready to assemble.
Here it is with the VW bug 1600cc piston in place. This piston is a good fit for early Triumph and other motors. It is just about .015” over the nominal size of the old cast iron piston, so you don’t need to remove too much material from the bore to fit it. The early barrels are pretty thin down by the mounting flanges, and mine had already been broken and repaired at least once before I got it. Fingers crossed that it will last for a while….