8.1 CP piston 3.5thou , in England clearance may be less because of your cooler climate
Comet Smoking After Rebore Is the Bore Glazed ?
- Thread starter Matty
- Start date
Very interesting - I have had my Comet for 59 years and this is the best oil consumption I have ever had from it !!!
This is the 3rd rebore I have had in the last 40 years or so and I had a new liner at first with a Heplex piston because the old liner had cracked all round the top .
This new set up used huge quantities of oil - around 50 miles to the pint, so I changed the rings after measuring land clearances etc but to no avail, so then I put an Italian piston in and things improved to around 150/200 miles to the pint ( the oil feed now blocked off, was in the correct place ). I still have the unworn Heplex piston and never found out why it caused the terrible oil consumption - all the rings are in right places etc.
Ran with the Italian job for around 30 years, but then had bad piston slap so rebuilt the whole engine and had a new liner because I found this one had also begun to crack just down from the top.
Ran this with a VOC low expansion piston for 8 years or so and 35000 miles - but it always had piston slap when cold which I found rather surprising, and this also gave 150/200 miles per pint on oil. Valve guides were changed 3 years ago because the exhaust guide had come loose in the head (oversized guide had to be fitted) but this did not affect oil consumption.
Recently had another new liner and VOC 8:1 piston - 3 thou clearance - oil consumption initially 100 miles per pint but now at 850 miles down to 300 ish to the pint, which based on my previous experiences in the last 59 years is pretty good. Do not have piston slap when cold now - bike runs beautifully but with the occasional puff of smoke.
Since 1956 when I bought the Comet it has been very reliable and has always been used Summer and Winter - not even oiling the plug when doing 50 miles to the pint years ago !!
I have also had around 60 modern ex- salvage bikes I have rebuilt in the last 30 years or so, including K series BMWs, Honda Fireblades, RG250 Suzuki, Honda CBR 600s, Honda CBX 6 , Ducati Darmah, RD 350 Yamaha, several VFR 750s and many others, so I think I have some mechanical experience, even though I am in fact qualified as an Electrical/Electronic Engineer.
The Comet however still hides some of it's mysteries from me, but is much easier to run around on because of its better lock and lower seat height than most of the modern bikes .
Matty
This is the 3rd rebore I have had in the last 40 years or so and I had a new liner at first with a Heplex piston because the old liner had cracked all round the top .
This new set up used huge quantities of oil - around 50 miles to the pint, so I changed the rings after measuring land clearances etc but to no avail, so then I put an Italian piston in and things improved to around 150/200 miles to the pint ( the oil feed now blocked off, was in the correct place ). I still have the unworn Heplex piston and never found out why it caused the terrible oil consumption - all the rings are in right places etc.
Ran with the Italian job for around 30 years, but then had bad piston slap so rebuilt the whole engine and had a new liner because I found this one had also begun to crack just down from the top.
Ran this with a VOC low expansion piston for 8 years or so and 35000 miles - but it always had piston slap when cold which I found rather surprising, and this also gave 150/200 miles per pint on oil. Valve guides were changed 3 years ago because the exhaust guide had come loose in the head (oversized guide had to be fitted) but this did not affect oil consumption.
Recently had another new liner and VOC 8:1 piston - 3 thou clearance - oil consumption initially 100 miles per pint but now at 850 miles down to 300 ish to the pint, which based on my previous experiences in the last 59 years is pretty good. Do not have piston slap when cold now - bike runs beautifully but with the occasional puff of smoke.
Since 1956 when I bought the Comet it has been very reliable and has always been used Summer and Winter - not even oiling the plug when doing 50 miles to the pint years ago !!
I have also had around 60 modern ex- salvage bikes I have rebuilt in the last 30 years or so, including K series BMWs, Honda Fireblades, RG250 Suzuki, Honda CBR 600s, Honda CBX 6 , Ducati Darmah, RD 350 Yamaha, several VFR 750s and many others, so I think I have some mechanical experience, even though I am in fact qualified as an Electrical/Electronic Engineer.
The Comet however still hides some of it's mysteries from me, but is much easier to run around on because of its better lock and lower seat height than most of the modern bikes .
Matty
Matty, thats the best tip I have had on the forum this year
easy to do, testing commenced
Thanks Vibrac -my engine did not seem to smoke on a regular basis when I started and more by luck than judgement I noticed that when I checked the oil and found that sometimes the level had dropped half a pint or so due to sumping it not unnaturally smoked but I also noticed that if the engine had been left near TDC it did not seem to sump.
I found a reference on the Norton forum which said this was a known effect due to the crankpin being at its highest level at TDC which reduced the sumping greatly, because oil does not naturally travel uphill.
With our Comets the oil will drain from the oil filter as above at bottom dead centre but of course even at TDC some will drain from the filter and dribble out of the Quill directly into the sump - but hopefully not too much.
I am not sure how oil leaks past the pump into the filter but a combination of wear and the rest positions of the pump ports could account for this and its irregular happening.
I feel that some oil will inevitably end up in the bottom of the crankcase however, because some will come from the oil filter and leak out of the quill - this I think would be about half the filter capacity - my Comet seems to collect around 150cc overnight but the loss from the oil tank does not get more in the short term if the engine is left at TDC. The 150cc does not seem to cause much if any smoke on startup and is probably just oil lurking in dark corners after running.
I feel the jury is still out on the TDC trick and I would be very interested if anybody else has found that this helps, or if not enough data has been collected to prove anything yet.
PS In most cases the engine will naturally stop near compression on a single, so perhaps that is also why sumping is a comparativly irregular event !!
Matty
I found a reference on the Norton forum which said this was a known effect due to the crankpin being at its highest level at TDC which reduced the sumping greatly, because oil does not naturally travel uphill.
With our Comets the oil will drain from the oil filter as above at bottom dead centre but of course even at TDC some will drain from the filter and dribble out of the Quill directly into the sump - but hopefully not too much.
I am not sure how oil leaks past the pump into the filter but a combination of wear and the rest positions of the pump ports could account for this and its irregular happening.
I feel that some oil will inevitably end up in the bottom of the crankcase however, because some will come from the oil filter and leak out of the quill - this I think would be about half the filter capacity - my Comet seems to collect around 150cc overnight but the loss from the oil tank does not get more in the short term if the engine is left at TDC. The 150cc does not seem to cause much if any smoke on startup and is probably just oil lurking in dark corners after running.
I feel the jury is still out on the TDC trick and I would be very interested if anybody else has found that this helps, or if not enough data has been collected to prove anything yet.
PS In most cases the engine will naturally stop near compression on a single, so perhaps that is also why sumping is a comparativly irregular event !!
Matty
Below is a copy of some text I put onto another thread in late December 2016 on the subject of excessive oil consumption.
I had taken my Comet head off had new valve guides with a seal put in by Conway and then treated the casting with Loctite 290 (could not find any 294),
Below is the result so far.
Now replaced the head after pressurising it (off the bike) via a modified push rod tube. It was difficult to really check for bubbles when I put water into the inlet port because of the angles involved, but I could not see any.
However I cleaned off the inlet port and rocker area above the inlet valve, heated the head to above boiling (no signs of oil from cracks or porosity) and painted the Loctite on as it cooled according to Loctite instructions.
Also ground a channel about 1/16 " deep and 3/16 wide down from top of the inlet valve guide to the rocker tunnel in case oil was settling round the valve guide. ( though I doubt if this was the cause of my consumption.
Deglazed the bore with emery cloth and after much cleaning put the piston back in without moving the rings much to minimise running in.
Then on the advice of a Mechanical Engineering friend, I did not oil the bore except for a little oil in the ring grooves. On the first runs I ran the engine hard under load so as to force the rings out onto the cylinder and avoid glazing.
Because of a bad cold and nasty weather I have only done 150 miles so far and friends say there is no blue smoke and after a check I do not seem to have used any oil at all and would normally have expected to use at least half a pint in that distance.
As a research Engineer who always had to identify the real cause of any problem so that it did not come up in production or later I have not obeyed the rules by doing several things at once.
However I am much too old now to keep pulling the bike apart just to prove a point, so have done a belt braces and elastic waistband job on the bike and hopefully fixed a very long standing irritation which coked up the head and caused lots of pinking and loss of power up hills.
Matty, A moment ago
e subject.
I had taken my Comet head off had new valve guides with a seal put in by Conway and then treated the casting with Loctite 290 (could not find any 294),
Below is the result so far.
Now replaced the head after pressurising it (off the bike) via a modified push rod tube. It was difficult to really check for bubbles when I put water into the inlet port because of the angles involved, but I could not see any.
However I cleaned off the inlet port and rocker area above the inlet valve, heated the head to above boiling (no signs of oil from cracks or porosity) and painted the Loctite on as it cooled according to Loctite instructions.
Also ground a channel about 1/16 " deep and 3/16 wide down from top of the inlet valve guide to the rocker tunnel in case oil was settling round the valve guide. ( though I doubt if this was the cause of my consumption.
Deglazed the bore with emery cloth and after much cleaning put the piston back in without moving the rings much to minimise running in.
Then on the advice of a Mechanical Engineering friend, I did not oil the bore except for a little oil in the ring grooves. On the first runs I ran the engine hard under load so as to force the rings out onto the cylinder and avoid glazing.
Because of a bad cold and nasty weather I have only done 150 miles so far and friends say there is no blue smoke and after a check I do not seem to have used any oil at all and would normally have expected to use at least half a pint in that distance.
As a research Engineer who always had to identify the real cause of any problem so that it did not come up in production or later I have not obeyed the rules by doing several things at once.
However I am much too old now to keep pulling the bike apart just to prove a point, so have done a belt braces and elastic waistband job on the bike and hopefully fixed a very long standing irritation which coked up the head and caused lots of pinking and loss of power up hills.
Matty, A moment ago
e subject.
Matty,
I am going to try the tdc trick myself. The logic seems reasonable, so let's see if it makes a difference.
I am going to try the tdc trick myself. The logic seems reasonable, so let's see if it makes a difference.