Marine Compound Engine

January 2011.

John R. Bentley 2011.

Machining the
Low Pressure and High Pressure Pistons
-  for the Stuart Compound Launch Engine  -

The two finished pistons waiting to be surrounded by cylinders

The pistons were turned from cast iron and made to the same diameter as the cylinder bore. Unlike aluminum pistons, the thermal expansion rate is identical to that of the cylinder walls. The combination of cast iron against cast iron forms one of the best known sliding bearing surfaces among common materials. The low pressure piston is 1 1/4" diameter and the high pressure piston is 3/4" diameter. Each piston carries two identical cast iron rings which are located 1/32" apart.

First the rods were threaded 5BA on both ends
I center-drilled one end of each rod to enable tailstock support while turning the pistons

Here the two piston rods are temporarily in position

The raw casting for the LP piston

Initial turning shows some pitting in the cast surface
This is not uncommon in this type of sand casting

Setting the Taig compound to 45 degrees to finish the piston's bumps and recesses

Tapping the casting 5BA to match the piston rod

I have flipped the casting around in order to face turn the bottom

Now the piston is screwed on to the rod and held in a collet
so that it can be finish-turned concentrically to the rod

Cutting the 1/16" grooves for the rings

Note that the tool chosen is slightly narrower than the finished groove
- it is always a good practice

The completed piston
(not yet fully screwed into the crosshead)

These are Stuart rings - still black from heat treatment
Their edges will become polished when they are seated in the cylinders

I very gently worked the rings over the piston with my fingers and into the grooves
Since I only have three hands I don't have any pictures of the actual process
- but really, there was nothing to it.

To give you a break I have included the pictures of the HP piston below almost without comment
- it's just the same only different!

After this recess was made, the sides were tapered to 45 degrees as in the LP piston

The two finished pistons awaiting installation

Ready for the mounting of the cylinder block

Obviously that is more than perspective distortion...
- the the two bottom cylinder covers were leaning apart in this temporary setup.
The straight edges of the covers are supposed to be touching.

Yep! The pistons fit the big holes!

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

                 Pistons (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.