Eccentrics and Straps

These components have often been referred to as “monkey motion” because of the unique way they wobble around when the engine is running. Properly setup eccentrics are key to efficient valve timing and result in a good running and powerful steam engine.

The four eccentric straps are bronze castings that were ganged together for initial machining to ensure uniformity.

Then they were individually faced to the required thickness.

I drilled and tapped a hole on both sides of each eccentric strap, one half was tapped, the other half a through hole for 5-40 bolts. After the straps are split, these bolts will hold the strap halves together while the center is bored.

Using a slitting saw, each strap was cut down the centerline.

Once the saw cut through one side, I placed a clamp on it to reduce harmonic vibrations as the cut continued on to the second half.

All four eccentrics ready to be bolted back together for boring.

I fabricated a jig to hold the straps in place during the boring process which ensured the exact same final dimensions for all the eccentric straps.

Eccentric rod recesses were milled and mounting holes drilled and tapped.

Dual oil holes were drilled in the top of each eccentric strap for easy-access lubrication.

The eccentrics were turned from cast iron round bar. I turned the final diameter on a good portion of the bar and then machined each eccentric one at a time, individually match-fitting each eccentric strap.

Once fitted, I oiled the parts and broke them in for about 5 minutes while running the lathe.

Each eccentric was then parted off and the cast iron bar was refaced to start the next eccentric.

I made a clamping fixture by squaring up a block of aluminum and turning a clearance-fit recess for the eccentrics to be mounted when drilling and reaming the offset hole.

Scrap angle iron was used to create three simple hold down clamps which bolted to the fixture.

I scribed a lateral line and spot drilled two marks to help with future positioning when mounting on the crankshaft as the eccentrics will need to be angled 19° from horizontal at top dead center.