I am building the 7BIM double piston engine by PM Research. The two cylinders are not yet connected with a manifold ; I am running each cylinder on air to test them. I can get them running nicely for about 15 minutes but then they start binding up…usually at BDC.

**LINK**

I think the clue lies in step 4 of the instructions :

''Adjust piston rods in crossheads until piston strokes are centred in cylinders. Torque screws evenly to prevent pistons from binding.''

How does the reciprocating motion of the piston cause a torsional force ? If I can understand what is happening then I can work out a way to prevent it.

NOTE : I have already built the single cylinder 3BIM but this must have been a fluke as it runs perfectly !

No, it is mechanical binding. When I release the nuts at either end of the piston rods then things start moving again. I do not understand this at all. There is enough lubrication.

I might try loosening the piston rod nuts and holding them in place with Loctite and see how this goes ie. I will not tighten the nuts against the crosshead and the piston. I will leave some play there.

NOTE : these piston rod nuts control the stroke of the piston. It is how the piston is centred in the cylinder.

Edited By Brian John on 03/09/2014 14:29:22

UPDATE : using Loctite on the piston nut did not help but it does not hurt either. Now I will not have to worry about this nut coming loose. I found that the binding only occurred under pressure and when I disconnected the air hose and turn the flywheel by hand the engine gradually loosened up. This indicated a problem with my valve settings. I found that the valve on the RH cylinder was not centred so I unscrewed it from the eccentric one full turn and this seems to have solved the major problem.

1. The RH cylinder is still giving a few problems : it requires about twice as much pressure (about 5 PSI) as the LH cylinder and even then it runs much slower. There is also a knock which I cannot locate. Any suggestions there ?

2. I also removed the gland packing as a possible source of friction and I have found that it makes no difference to the running of the engine. What is the point of it ?

Edited By Brian John on 06/09/2014 05:29:51

Edited By Brian John on 06/09/2014 05:31:28

I am still having problems with the RH cylinder : it runs for about 10 minutes then gets slower and slower and stops. Yet both the left and right side have been set up the same in terms of valve and piston settings. I am really puzzled at this.

Edited By Brian John on 08/09/2014 06:50:39