Need advice with diesel engine problem.

[Dave1964]

Hi all, I am after a bit of advice, I am pretty sure I know what the issue is but wanted to ask in any case.

I bought a Davies Charlton Sabre diesel on Ebay that was said to be in excellent condition, turned out to be "not as described".

The engine is free to turn but the piston gets stuck in bore just after it passes exhaust port (travelling up). It seems imposible to turn over, but I know better than to force it, I do not want to damage rod or even break crank pin off which can happen. On inspection the cylinder head was loose (so definately been tampered with) and the engine shows signs of wear on all parts eccept the piston and conrod. I believe that the bore on these engines is tapered and someone has fitted a new piston which is ever so slightly to large and needs lapping to fit bore.

What do you think?

I have been building and flying models for 40 or so years, so am experienced with these ehgines. Have worked on many car and bike engines and have a good mechanical knowledge. I have also now built a handful of live steam engines, so know enough about fitting pistons. I also once made a piston for a Nova Rossi car engine. Attached is a picture of my last project, designed and scratch built by me, to give you an idea of my ability and knowlegde.

TIA
Dave

 

[Mechanicboy]

I can think, it's a bad job to fix the engine of former owner. Lap the old cylinder to true bore and make new piston and lap the piston. Then the piston must be lapped into the cylinder to create tapered cylinder with chrome polishing paste or ultra fine lapping paste. + new contrapiston.

 

[Dave1964]

Thanks for reply.

I have read some of the small diesels were made new with a tapered bore.

I also understand that back in the day when these were made the piston and liner would have been a matched pair.

 

[JCSteam]

Hi, i cant comment on the issue at hand, but just wanted to say that the little traction engine looks very reminisent to LS Loc offerings, its a fine piece of work and looks like a fine example of miniature engineering

 

[Dave1964]

Thanks JC

Yes I was inspired by LS Loc traction engine. Have a Rocket on the drawing board.

cheers
Dave

 

[JCSteam]

I've sent you a PM regarding the LS Loc subject so as to not divert the thread.

 

[xpylonracer]

Hi Dave, have you checked the contra-piston hasn't been screwed down too much which prevents the engine turning over TDC ?

xpylonracer

 

[Dave1964]

Hi xpylonracer,

Yes had checked the contra-piston. It was actually pretty well stuck in liner, took quite alot of force to remove it. While it was out was a good opportunity to see what is going on. No question piston is getting stuck in liner as in nears TDC. With head off and contra out it pushes liner up out of case. Would definitely damage something if forced.

I got a DC Sabre new in 1974 for my 10th birthday, it was my first ever engine and got a Dart the following year.

Dave

 

[WOB]

Make sure the piston and bore are perfectly clean and well oiled before trying to turn it over. If no go, it is easiest to lap the piston to fit the cylinder assuming the cylinder bore is not excessively tapered. The problem is to determine where the taper is and how much correction is required. Measuring the piston is easy, measuring the bore is not. For a one off, buying precision ground gauge pins is the easiest way to measure the bore. They are not expensive and are available in tenths sizes. The piston should start to get tight at about the exhaust port and be very tight beyond that point. It will loosen up rapidly once it starts the first time.
1200 grit diamond compound is ideal to lap whichever part needs it. See this for a step by step procedure: http://www.adriansmodelaeroengines.com/catalog/main.php?cat_id=178

WOB