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Home-Made Engine Inside Story: Part III

As regular readers of the Message Board will know, Humbernut has been teasing people with tales of his 350cc Humber engine rebuild. We thought you'd enjoy a sneak preview of the work-in-progress...

I have kept pictorial evidence of my restoration of the 350cc OHC Humber engine and will write a full article about it once I have finished it. It will probably fill a book, I have so much to write about. In the meanwhile, here are the photos of the home-made crankshaft, alignment jig and crankcase to keep everyone going!

Above is the home-made crankshaft showing drive side main journal, left-hand flywheel and crankpin retaining nut. It's difficult to photograph bare metal due to the reflection from the flash. It also makes the flywheels appear as if they're rough-finished but they aren't.

On the left is the crankshaft assembly. The only commercial components are the conrod, the -inch rollers and outer sleeve for the big end bearing. The pic on the right reveals the completed crankshaft, and shows the locking tab on the crankpin retaining nut.

The next item is the crankshaft alignment jig (see pic 1) . A word or two on its use may help readers understand how it works:

A true round bar is placed in the V-grooves and the clamp plates tightened, with the blocks separated sufficiently to permit the flywheels to fit between them. The blocks are then clamped to any flat surface (the milling machine is ideal for this).

The round bar is then removed (see pic 2).

A pair of dummy bearings are made up to fit accurately over each mainshaft. The bores must be concentric and the outside diameter must be identical on both bearings. The dummy bearings must be made strong enough to withstand the clamping forces so that they do not distort and squeeze onto the shaft (see pic 3).

1: The crankshaft in place in the alignment jig with clamping plates secured. 2: Alignment jig showing Vee groove in which true round bar is clamped to set up jig. 3:Two robust dummy bearings. Both have the same outside diameter.

The dummy bearings are placed on their respective main journals of the crankshaft and the whole assembly placed in the jig (see pic 4). Note: the flywheel halves can be checked for true running between centres in the lathe or by checking in the jig. Then the crankpin can be inserted by using a dial indicator and gently tapping the high spot until the flywheels run true, while slowly in stages tightening the journal retaining nut.

the crankshaft assembly in jig prior to fitting clamping plates

Once the flywheels are running true and the crankpin is inserted, the clamping plates are then placed over the dummy bearings and nipped up (see pic 5).

showing clamping plates in position and assembly all nipped up

Finally, slowly tighten up the crankpin retaining nuts, occasionally rotating the assembly to check for tightness.

If the two blocks are machined on their inner faces perfectly square to the V-groove and truly vertical, then the jig could be used to align the flywheels with the journals. This would be done by clamping the plate over a dummy bearing with the shaft inserted, and lightly clamping the flywheel to the block, then drawing in the shaft on the retaining nut until it is tight. Release the clamp on the flywheel and you should find it is running true.

The final group of pictures are of the crankshaft / crankcase assembly. I will add at this point that the crankcase was machined from solid billet and is not a casting. The finish was achieved by sand-blasting to give it that just-cast look. The only commercial components are the conrod and the rollers for the big end bearing.

Dummy bearing is just visible on main shaft. View on right shows yet-to-be-completed magneto chain case inner cover. View from top - the camshaft drive is taken via spiral bevel gear on mainshaft.

What will Humbernut build next?


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