BUILD THE LOBO PUP TWIN-5

By Gail Graham

At this point I test fitted the centre crankshafts into the centre bearing housing. The points to look for were that everything turned smoothly and that the rear centre crankshaft did not protrude through the front centre crankshaft. It should be slightly recessed or worst case be flush.

Since all looked OK I inserted the mess into the crankcase from the rear and installed a connecting rod on the front, then slid the assembly forward and put on the other connecting rod. I approximately centred the assembly and rotated the rear crankshaft to top dead centre. Then I engaged the crankshaft installation tool with the bearing housing and rotated it until the screw hole in the housing lined up with the hole in the crankcase. A visual inspection showed that the rods were centred in the cylinder bores and that at top dead centre there was enough clearance to insert the piston wrist pin. So at least I don't have any major busts on dimensioning. No photos of all this as most everything takes place inside the crankcase where it is hard to see and near impossible to photograph.

One small problem did show up. The crankshaft installation tool worked, but it would have been much easier if the slot for the connecting rod were wider and on both sides. So the slot was changed from 3/16 to 7/16 wide and a second slot put on the other side. I also milled a small flat on the handle so I had a reference as to where the rotation was without having to look down into the cylinder bore. A swipe with a permanent marker would have worked as well, but I was already working on the tool in the mill.

This is how the business end of the tool looks now.
















Assembly of the centre crankshaft assembly

First a word about the use of Loctite to hold things together. Generally, I make it a rule not to Loctite any parts together that might have to be disassembled later. This part only half violates this rule. The assembly can be disassembled using heat as the Loctite gives up it's strength at 350 degrees F. This much heat will probably destroy the seals and/or grease in
the bearings. In any event it won't do them any good. But, I consider this OK as the only reason that I can think of that this assembly would ever need to be disassembled is to replace the bearings. So if I am going to replace them anyway I am not going to worry about damaging them.

First clean the inside of the front half and the shaft of the rear half with solvent. Acetone or alcohol work fine. Notice the cotton swab in the second photo. That’s what it looked like after cleaning the bore of the front half of the crankshaft.

The front half of the crankshaft is inserted into the bearing assembly. I put it in the side of the bearing assembly that does not have the notches, but it really does not make any difference.

Apply a high strength Loctite, I use 680, to the inside of the front half, being careful not to get any on outside or the bearing. I wiped a small amount of Loctite on shaft of the rear half and wiped all of it off that I could. This is to prime the shaft. I don't know if this does any good, but I do it anyway. The rear half is inserted into the front half with a twisting motion to
distribute the Loctite. I hold a paper towel over the far end of the front half to soak up any excess Loctite as it is forced out. Makes it easer to clean up. Before the rear half is inserted all the way I look to make sure there is no excess Loctite on the shaft that might get squeezed out and into the bearing. If there is any, wipe it off with a swab before seating the two crankshaft halves together.

Now place the assembly into the crankshaft assembly jig and press the crankpins down on the flanges while pressing the bearing housing down against the bottom of the notch. At the same time make sure the crankshafts are fully inserted in each other. The crankpins are now at 180 degrees to each other. Take a break while the Loctite cures. After removing from the jig, it will look like the photo.


Making the front crankshaft

I made the front crankshaft next. If I were building a plain bearing version, I would have made the front bearing housing first and then turned the crankshaft to fit the reamed hole in it.

The front crankshaft starts off as a standard turning operation from 5/8 diameter steel bar. The only close tolerance section is the 0.236 diameter that fits the ID of the MR106-2TS bearing. That section should be polished to a close fit on the bearing. The transitions between the 0.203 to the 0.236 diameters and the 0.236 to 0.266 diameters has to
be be squared up so the prop driver and a bearing will seat. I used a parting tool with a square tip the same way I did on the centre crankshafts. A small centre drill is used to create a centre in the end of the shaft to facilitate removal of the thrust washer with a gear puller should it ever be needed. A bevel is machined on to the end of the shaft for starting the
die to cut the threads.
A straight knurl is used to cut a spline on the 0.203 diameter, raising the diameter to between 0.206 and 0.209. It will depend on what your knurling tool is like. The knurl does not have to reach the shoulder of the 0.236 transition. The 10-32 thread is cut with a die for 14 full turns. With the jaws of my tailstock drill chuck retracted, I use
the face of my drill chuck in the tailstock to keep the die square, advancing the drill chuck as I cut the thread.

After parting off with an allowance left to clean up the crank disc to thickness, the crankshaft is reversed and the crank disc is faced off to 0.094 thickness.

Moving to the milling machine, I put the crankshaft in the vice
using a small aluminium V-block that I have made up with different size Vs to for just such a purpose. Its height is the same as my vice jaw height. The crankshaft needs to be clamped firmly, but not so tight that the hardened vice jaw might put a flat on the crankshaft. I have never had a problem doing this, but if it is a concern, but a piece of aluminium between the
hard jaw and the crankshaft.
After indicating the crankshaft to locate the centre, the crankshaft slot is milled to be a close fit on the centre front half crankshaft crankpin. I used a 5/32 end mill and found that the slot was too tight so I offset the y-axis by 0.001 inch and took a second pass. The crankpin then fit the slot

smoothly.















After deburring, the front crankshaft looked like this.














Part 1 here  Part 2 here Part 3 here Part 4 here  Part 5 here  Part 6 here 

Part 7 here    Part 8 here    Part 9 here    Part 10 here    Part 11 here

Part 12 here

 

MEWS IS SPONSORED BY -  HARROGATE EXHIBITION - TEE PUBLISHING - MERIDIENNE EXHIBITIONS - LYNX MODELS - CAMDEN MINIATURE STEAM - TRANSWAVE CONVERTERS - MESSE SINSHEIM - CHRONOS -   PAULTHECAD.CO.UK -  ECCENTRIC ENGINEERING 

- WARCO - BRISTOL EXHIBITION