Sunday, June 22, 2014

BSA Bantam - Rebuilding the Ignition Timing Plate

The last major repair for the engine was to fix the worn-out points and points plate bushing. The reproduction points cam that I purchased had not been correctly finished, and had a sharp edge at the beginning of the cam lobe instead of a smooth curve. 

The points arm, literally eaten away by the sharp edge.

The points timing cam. The sharp transition is clearly visible.
A points cam should have a much smoother transition, such as the one seen here on a CB450 cam:

Smooth with no sudden transitions
I had a machine shop carefully smooth out the edge with a fine tool grinder.

Refinished cam lobe

The sharp transition is completed smoothed out
After fixing the cam, I had to replace the bronze bushing in the points plate. This bushing supports the end of the crankshaft and keeps it dead center relative to the points arm. It also prevent the crankshaft end from wobbling, which has been known to cause metal fatigue failures and ruin the crankshaft. I pressed out the oil bushing with a large screw and hammer:

Successfully removed

Brand-new bushing

Bushing installed.
After pressing out the bushing, I installed the new bushing in its place, using a large amount of Loctite 680 to lock it into place. I reassembled the points, re-timed the engine, and now it starts on the second kick every time.

Tuesday, June 3, 2014

Steam Engine Restoration Part 8 - Assembling The Piston and Packing the Seals

The first step of the mechanical restoration was to replace the piston rod that had rotted away from rust. I purchased a piece of steel rod from McMaster with the same outer dimensions, and since I cannot cut threads on my lathe, I had a machinist friend make an exact copy 

Raw material next to the original

Finished copy
Installing the rod was the next step. I pulled the head off the cylinder, installed the rod through the hole in the bottom, installed a lock nut, and screwed it into the crosshead. 

Screwing into the Crosshead

Locked in place
 I dropped the refinished piston into place, locked it down with a nut, and reattached the cylinder head.

 To keep compressed gas from leaking out of the cylinder and valve chest around the piston and valve rods, you have to install shaft packing. Shaft packing is a treated rope that is wound around the shaft and pressed into place to provide a tight seal. In this engine, the seal is pressed between screws with the shaft passing through them, and a threaded port in the cylinder or chest. The face of the screws and the bottom of the threaded port are tapered, so when the screw is tightened, the packing is pressed inward against the shaft. When installed correctly, very little gas can leak out.

The packing screws have been opened and cleaned.

Graphite-impregnated packing rope

Wrapped around the shaft...

...and pressed into place. The screw is then inserted and tightened to finish.

Same process for the piston rod.