Marine Compound Engine

January 2011.

John R. Bentley 2011.

Boring the
Main Bearings
-  for the Stuart Compound Launch Engine  -

This is the first work of 2011 on the engine after I put the project aside for a trip to Britain and Ireland beginning in April of 2010. When I returned I found plenty of things to do during the summer and autumn (the weather was unusually pleasant) and consequently I only returned to the project in January of 2011.

This was the state of the soleplate and bearing caps at that time


The first step was to mill the sides of the caps parallel

The milling described above allows holding the caps in the vise
so that the bottoms can be milled at 90 degrees to the sides

Marking out

Drilling the fastening holes

A test fit

Spotfacing for the nuts

Using lathe bits for packing underneath to ensure the bottom is milled parallel to the two spotfaced surfaces

A quick test to check after the caps are milled to final height

Beginning the main bearing groove with the first pass of a 1/4" ball endmill

Using the same endmill to make the matching grooves in the caps

Just a rough check (while the bore is still undersize) to make sure nothing is obviously off
(the final diameter will be 5/16")

A piece of 5/16" silver steel
- what a lovely name!

( in North America the nearest equivalent to this metal is called "precision-ground water-hardening drill rod" )

Here I have tapered the silver steel from 5/16" to 1/4" at one end and ground a flat so it forms a special reamer
(the lower piece is a 1/4" pilot which screws in the end of the reamer)

Testing the assembled reamer before hardening

The 1/4" pilot section guides the larger reamer section by utilizing the other two bearing holes

The reamer was designed to guarantee the alignment of the three bearings in this manner:

The 1/4" pilot section of the reamer enters through the first hole, then continues through the other two holes before the 3/8" diameter part of the reamer begins to enter the first hole. This ensures that the first hole will be reamed to size along the axis of the other two holes. When cutting hole two, hole one and hole three will support the tool. When reaming the last hole, holes one and two are now 5/16" in diameter so the 5/16" shank at the back of the reamer maintains the alignment as the last hole is reamed out to 5/16".

Note the original 1/4" holes were lined up correctly as an accurate guide groove was first cut in the soleplate using milling machine.

The reamer section after initial hardening

After this photo was taken the reamer was cleaned and tempered to a light straw colour

The new reamer in use, being powered by a 40-year old Black &Decker hand drill

To ease the strain on the drill (and the old guy holding it) I left the bearing caps raised somewhat at first

There is a full 1/32" of metal to be cut away from around the bore...
For extra help I also used the tapered end of a standard 5/16" straight reamer to start each hole, turning it by hand in the chuck

Approaching final size, I put paper strips under the caps to avoid
making the bore too large to precisely fit around the crankshaft

The inner entrance to each hole will need to have a radius cut to provide clearance for the crankshaft.

I started with a countersink and rounded the resulting chamfers with a fine half-round file

Bedding in the new bearings under power from the Taig lathe

Although this looks fast, the effect is caused by a slow camera shutter
- actually I used a varying range of speeds - most of them below 800 rpm

The mating of the "two great halves" of this engine is looming ever closer
...but I will save that for another page...

Back to

Compound Launch main page


Castings, Materials and Fastenings


Cylinder Block

Top Covers

Bottom Cylinder Covers

Steam Chests

Crosshead Guides and Bracket




Connecting Rods and Crossheads

                 Main Bearings (this page)


Fittings: Oil Cups

Fittings: Drain Cocks

Fittings: Exchange Pipe, Flanges and Glands

Stephenson Link Reversing Gear (5 pages)

Completing and Erecting the Compound Launch Engine


Return to main website home page

(c) John R. Bentley 2011.