some weeks ago I bought my first Scott, a 1929 600cc squirrel. Yesterday I dismantled the engine and realized there is a brass bushing in the piston as you can see in the attached picture. Is this original? I think they did this because the piston bore was worn. Why didn’t they just use a thicker gudgeon pin?
Just in case I have to renew the gudgeon pins, would you advise to use some with clips or brass pads?
Thank you very much in advance for your help!
Gudgeon pins on new pistons were an interference fit and were only supposed to rotate in the small end bush. Usually, through lack of adequate lubrication, the gudgeon pin would partially seize in the small end bush and start to rotate in the piston causing wear in the gudgeon pin holes. In order to reclaim the piston and carry on using it, the pistons were bushed. It is possible to get oversize pins but it is better to try to keep every thing as standard as possible. Also the original gudgeon pins would have had brass pads on the ends of the pins and would not have been retained in the pistons by circlips
Pistons were alos bushed to alter them from long stroke to short stroke and vice versa. Putting larger pins in is an option but there is the issue of supply, and going too far over makes the pin heavier which is bad.
Generally if the pistons have clean clip grooves I use clips as a preference over brass pads. Brass pads are okay if the pins are a nice tight fit in the piston but they can walk around in use and wear a groove in the cylinder wall.
If you are doing a full engine strip just be careful how you knock the cranks out of the flywheel as eay to strip theads or bring the cups out too…
That makes sense to me. I think I am going to leave it the way it is, because the gudgeon pins and bushes are in really good condition.
I just read at Roger Moss’ Homepage that the following piston to bore tolerances are advised:
For racing 0.0125” at ring land and 0.0045” at skirt.
Sorry but my technical english is not the best. Would please somebody explain to me where exactly the ring land and the skirt is in the cylinder bore?
Thanks in advance.
He is talking about measuring the piston, which is slightly conical. It should be 0,0125″ smaller than the cylinder bore at the top (rings), and 0,0045″ smaller at the bottom (skirt).
]Just be careful with the stated measurements, it does not give piston material, heat treat, bore material and finish. The “Racing” note could refer to Roger and his alloy barrels etc. That clearance is tighter than I would run a road engine on iron bores, and the taper is more. for into I attach a Scott piston drawing that shows much bigger clearances…
[attachment=0]Scott-L52M piston cropped.jpg[/attachment
I forgot to add, there are 4 piston dia measurements to machine to when requalifying or machining new ones:
Under ring land dia (taper)
Ring land dia
How are the end bushes removed, are they pressed out via the centre of the piston, one of my top end has a slight “knock”
thank you very much for your information. This weekend I opened up the gearbox and everything looked fine exept one bolt, but I already fixed that.
Everything seems to be almost ready to be put back togehter again.
I just wonder if it is original that my Scott has aluminium fenders?!
When we have pistons with worn or damaged gudgeon pin bores, it has long been standard practice to jigbore the holes oversize in one pass, so the two sides are in good alignment.
Next fit bushes in SAE 660 10% leaded bronze with a slight press fit that have been produced with outside diameter and bore concentric. The outside diameter is a light press fit and the bore is 0.6235″ / 0.6240″ Next drill and tap 2BA through the gudgeon pin boss and in situ bush. Next put slave4 undersize pin in the bore and fit brass 2BA security screws with loctite. The slave pin enables you to screw the brass screw in just flush with the bore. Next remove excess screw length. Now use a good long 5/8″ hone and finish hone the two bushes together to ensure alignment to give a free slide fit on the gudgeon pin. If circlips are to be used, the piston is mounted on a 5/8″ expanding mandrill and the grooves put in. Obviously this all takes time, but the final job is much more resistant to wear than the original aluminium bores. For this reason, I finish the bushes to the same sliding fit as the rod little end, so we have a fully floating pin which shares out the wear nicely. When you have finished, put the piston on a surface plate and clock the height of the bores both sides to confirm they are in line. Max error would be about 0.0015″ but with all the tooling sorted out, we would expect to do better than this. Hope this helps Roger
Thank you all for the detailed information.
I am putting it all back together, engine and gearbox are already in the frame. Hope to show you a picture of my Scott soon.
Does someone of you have a used exhaust pipe for sale or may tell me where I could find one?