Comet Smoking After Rebore Is the Bore Glazed ?

[greg brillus]

Cut a narrow strip say 25 mm (1 inch) wide piece of alloy sheet, harder than plain soft aluminium and about 300 mm long ( Aircraft sheet alloy works really well ) .....bend one end in a vice with a right angle tang about 12 to 13 mm long. Wrap the band around a piston skirt and at the other end leave a gap of say 10 mm then mark and cut the strip off and bend the end again at right angles...so the thing looks like the letter "C" with right angle tangs at either end. The sheet needs to be around 1 mm thickness or less, say 25 to 30 thou. This is now a hand held ring compressor, that you squeeze the ends together around the piston, whilst the piston rests on a couple of steadies atop the case mouth, and gently tap the barrel down over the piston crown. Once the barrel has covered the ring area, stop and feed the alloy band out between the outer sidewall of the liner and the nearest hold down stud. I can compress and install a barrel, and remove the alloy strip faster than it took me to write this. Never broken a piston ring with this method, and you can make a size to suit whatever size of piston you have, whether it is a 125 or a 500 single. Note : if you make the "Tangs" too long, you will struggle to get it out past the gap from the liner to the hold down studs. Cheers and beers.......Greg.

 

[Nulli Secundus]

In the past I have used 25mm white PVC angle to make piston ring ‘clamps’.

In order to curve it around the piston I trimmed one side off close to the inside of the corner of the angle, but leaving enough remaining to provide a good surface for the barrel liner to rest upon. I then cut it to length and immersed it in hot water in order to shape it to the curvature of the piston and obviously keeping what remains of the trimmed edge on the outside of the curve.

I then used cable ties to carefully tighten this piston ring 'clamp' around the rings. The advantage of the returned edge on the ‘clamp’ is it cannot try to enter the chamfer at the base of the liner.

Cheap, simple and possibly materials you have lying around in the shed/garage.

 

[Nulli Secundus]

One disadvantage to the above maybe that the 'clamp' is releasing its grip on the ring as the ring is entering the chamfer, but it has worked successfully for me.

 

[Matty]

Hi Greg
I have seen a comment before about the possibility of a pool of oil round the inlet guide and valve stem and will try to have a look up the tunnel.
However the blue smoke is not consistent in its appearance, so it is bit of a problem to know when to look for the oil round the guide.
But when I take the head off I will look carefully round the valve guide area to see if oil could collect there and if so make a channel for it to drain away.

Matty

 

[Howard]

Parking it on the right hand prop stand may help, if you're not travelling far between stops - if yours leans as far as mine on the props.

H

 

[Matty]

Hi
I have bolted a second hand Kawasaki (though a Honda Fire blade/VFR one works as well) propstand onto the left hand footrest plate after removing the stand switch bits.
This works brilliantly for £15 and can be kicked down easily when parking leaving the bike at a sensible angle which you can determine when you fit it.
So my Comet is usually therefore parked to the left which means the oil possibly does not drain down the pushrod tube at rest.
However the smoking does not always happen after parking and is much worse after a run if the throttle is blipped.

Matty

 

[greg brillus]

Hi there Matty, what I was suggesting meant that you could do this without removing the head. Just remove the oil return pipe, take out the rocker locking bolt, tappet adjuster and pushrod, and slide the rocker assembly out so you can see what is going on, with a good torch. Cheers......Greg.

 

[roy the mechanic]

It is possible ,but unlikely, that the valve guide has been carelessly removed and there could be a score mark in its bore. So the oil is going down the outside of the guide. Roy.

 

[Matty]

Thanks Roy

When I take the head off and pressurise it under water or with bubbley mixture I hope this should show if there is a leak past the guide as I check for porosity .

I enjoyed the chat we had at the club on Tuesday night - it cleared up a few things I had been thinking about
.
Thanks again

Matty

 

[Matty]

This is an update now that I have done around 550 miles after the rebore.
The Comet has been run mostly at 50 to 60mph after about 100 miles to try to avoid glazing problems and not been highly stressed on our quite flat Essex roads.

The smoke became less at around 300 miles and now there is very little, even when I blip the throttle.
It used quite a lot of oil at first ( more than a pint for the first 100miles) but seems to have used less than a third of a pint for the last 100 miles.
So I must conclude that the problem was probably rings bedding in.
The oil supply to the inlet and exhaust valve areas looks OK and the bike is running very sweetly.

I may have mentioned this before, but when I rebuilt the engine 8 years ago with a standard sleeve, rebore and low expansion piston, it smoked badly.
After 200 miles or so I took the barrel off again and found that the compression rings were not touching the bore all round and only looked shiny in places where they had been touching the bore. This shiny part of the ring was only about two thirds of the outer circumference, so the seal must have been very poor even though the compression seemed good.

I sent the rings back to Vinparts who had supplied them with the new piston and they said the rings looked as if they were not ground circular and sent me two new ones which I fitted. There was still some smoke, but I had to use the bike, which finally settled down to around a pint per 200 miles oil consumption.

I am surprised therefore that more rebuilders have not had this problem after fitting a new piston and rings.

Thanks for your interest everybody.

Matty