OS 70FL

[Martin Harris - Moderator]

I was asked to take a look at an OS 70FL for a clubmate. For those unfamiliar with them, they are a lightweight budget offering with, amongst other features unusual in the range, ABN ringless construction.

This engine has done very little running and was bought second hand with an unknown history. On removal of the plastic backplate, the bearings were obviously rusted, probably during extended storage. On stripping the engine in order to remove the bearings I was surprised to find the cylinder liner looking as though the plating was almost non-existent!

 

There's some discoloration of the inside of the piston with an even gold/brass colour as there is on the outside of the skirt - which doesn't wipe off...possibly oil related deposits?...but little sign of wear. There is no sign of the flaking as often seen in the OS 2 stroke ABN engines - it just looks like you can see through the nickel plating!

This impression is reinforced by seeing bright brass edges when viewing the cross hatched honing marks closely in bright sunlight. These honing marks are more pronounced than when looking at a well used Irvine liner, which supports other evidence that the engine has had little running.

I don't suppose anyone has seen such thin plating on one of these engines or on some of the OS 2 strokes? Could the appearance of worn plating be simply a feature of the technology used by OS - in the absence of any other information I'm only assuming it is supposed to be ABN...I've not stripped my Laser V twin to see what Nikasil looks like, for example!

Edited By Martin Harris on 26/02/2020 15:54:48

[Nigel R]

It looks very different to the pictures of flaking I have seen posted.

What is/was compression like?

Did OS ever make another four stroke without a ring?

[Martin Harris - Moderator]

Agreed. Flaking tends to result in localised pits.

Compression seemed fine although it didn't want to run or fire under normal conditions - suspected carburation problem which is what I took it home to look at. It did run for very short periods after priming.

Not to my knowledge.

[Jon H]

It looks fine from here Martin. I used to have one of those and never got it running right. Admittedly it was a very long time ago so its possible i did something wrong back then.

[Martin Harris - Moderator]

Even though the brass appears to be visible? Or do you think it could relate to the brass coloured deposits on and inside the piston?

It certainly seems to be an engine with more than its fair share of bad experiences - many people seem to have given up on them... With its simple air bleed carb, there's little opportunity to mess up the tuning!

[Jon H]

That colour is not the brass, its just the marble effect in the plating. When the plating peels the brass is really obvious and its possible the brassy colour on the piston is light castor residue, rust, or staining from another fuel. Engines run on bekra (i think, dont hold me to it) fuel seem to turn orange inside in my experience

[Denis Watkins]

Even a new liner has some heat patination Martin

And the inner liner could be the same

Edited By Denis Watkins on 26/02/2020 18:34:08

[Martin Harris - Moderator]

Thanks Gents,

Jon, I'd wondered if it was something along those lines - the piston staining is almost like anodising but the metallic brass visible in the honing in bright sunlight made me wonder about the plating thickness.

It's less than a tenner for the bearings - it even uses sintered bronze bushes for the camshaft which won't need replacing, so I'll recommend that I just bung in a pair and see if it will run with a cleaned out carb...although the various reports around the net don't hold out much hope of it being a reliable long term power plant.

Edited By Martin Harris on 26/02/2020 18:47:10