Cylinder Oil Feed on Comet

[Matty]

I am about to have my Series C Comet rebored from standard out to +20 thou. The standard resleeved barrel was fitted about 8 years ago with a VOC low expansion piston and after 35,000 miles the bike was using about a pint of oil in 150 miles and oiling up plugs, so I thought I had better fix it.

I can get a 10 thou feller gage between the piston and liner half way up the bore so things are pretty worn !!

The barrell has an oil hole which is drilled at an angle to be below the scraper ring, but the groove in the outside of the cylinder which connects to the hole in the crankcase mouth and in fact also the hole in the cylinder, are below the lower edge of the crankcase mouth - so the oil will run down the groove, past and over the hole to the cylinder and then into the crankcase !!

I had previously attached a piece of very thin and carefully shaped shim to the cylinder, this came below the crankcase mouth so that the oil did not run straight into the crankcase, but I have been advised by various experts to simply blank off the oil feed in the timing cover to the cylinder, because with modern oils the feed was not necessary. I hoped this would reduce the oil consumption, but for the couple of years I tried this and it made no difference.

So when I put the engine back together next week - is it OK to allow oil into the cylinder oiling system again and just let it run into the crankcase, should I put the shim in to ensure the oil does not go directly to the crankcase, or should I put a plastic washer behind the oil feed in the timing cover to stop any oil going to the cylinder oiler at all?

Any advice would be welcome please, but I understand that the feeds in the crankcase mouths of twins are not positioned in the same place as that of the Comet and that the above problem does not arise with them and should my cylinder in fact be the same or different to those used on twins?

Matty

 

[vibrac]

this forum is full of items on this eg: http://www.vincentownersclub.co.uk/index.php?threads/cylinder-lubrication.6944/#post-54793 this deals in reducing flow (something I have never needed to do in many engines)

further info just enter 'cylinder lubrication' in the search box

 

[Matty]

Thanks - having had a look on the site I find that I had asked this question some years ago and concluded that blocking the feed to the cam follower spindle which feeds the cylinder oiler had little effect on oil consumption. It's me age you know!!
I shall therefore remove the restriction and restore the system to its original state when I put the rebored cylinder and new piston together next week.

However I still do not quite understand how the oil is intended to get positivly onto the cylinder wall, because the flow seems to be from the feed in the crankcase mouth, into the groove in the outside of the cylinder, after which it will run down the groove and mostly go into the crankcase and then with a bit of luck some of it may go through the small hole down onto the cylinder wall.

This state of affairs will happen for my Comet because the scraper ring at bottom dead centre is only just above the lubricating hole in the cylinder and the groove with the hole drilled at a downward angle at the end, finishes below the level of the crankcase mouth - so most of the oil will go straight into the crankcase.

Any comments please or is my Comet very non-standard - which I doubt because it had only been used for two years when I bought it in 1956 !!

 

[nkt267]

I have the same as your Comet barrel..The liner and muff are new but are drilled as per standard ,even worse is the fact that the new liners are even looser than the originals in the crankcase throat..I have not had any problem with the bike seizing or nipping up..Don't forget that you will be getting some plash into the barrel from the crank..John

 

[robin]

The only time I built a comet with the oil feed to the cylinder wall blocked off, the engine tightened up at 70 mph. The upper edge of the feed hole should be approx, 1 16th inch (11cm)below the top of the liner flange. This puts the hole just below the oil scraper ring. As the front cylinder leans forward, the oil reaches the piston face.
Please note that this applies to the front cylinder of twins as well The crankcase for a twin rear cylinder has considerably more metal in this area. It is possible on most b/c motors to place the oil feed hole as low as 1 1/4 inches below the muff lower face.
As f0r D crankcases- revert to comet dimensions on both front and rear cylinders.
The golden is to measure and re-measure before drillins.
\\\\\\\\robin

 

[Matty]

Thanks, but afraid the back of my crankcase mouth is only 1 and one eighth inches deep so the hole through to the cylinder wall is below this level and at the bottom of a groove about a quarter of an inch wide in the outside of the cylinder liner. The bottom of the groove is at least 1 and a quarter inches down from the muffler flange
To stop the oil going virtually straight back into the crankcase with only a fraction going through the hole onto the piston, on a previous rebuild I cut a piece of 4thou shim and araldited it to the outside of the barrel wall over the groove, but leaving a small gap at the top to allow oil from the feed in the crankcase into the top of the groove. There was enough clearance between the liner and the crankcase mouth for the piece of shim.

I find it difficult to believe that otherwise there is no positive feed of oil to the cylinder and that the bike was designed like this - though I accept that with modern oils and pistons the oil feed may not be necessary.

My puzzle is how was the system ever intended to work and would like to know before I rebuild the rebored engine next week?

Maybe I'm just a nickpicky old Engineer but I do like to understand how things were supposed to work.

Matty

 

[Pushrod Twin]

I once honed a Comet bore for a mate & when it was assembled it turned out that the sleeve had been assembled 180 degrees out, from new! Yes, the oil hole was on the wrong side & it had run like that for over 40 years. I would block the hole & use the oil to better effect somewhere else in the motor.

 

[davidd]

If you have the early MPH issues there is an article by Bruce Main-Smith on the oil system. MPH 129 and following. I have never drilled an oil cylinder feed hole. I always thought it was redundant, but it would take some good reasoning at this point to convince me that the holes were necessary. This diagram was printed in MPH and seems to be a good way to visualize the system:

David

 

[greg brillus]

Geez.....no wonder the oiling system has always been a bit suspect .......All those square bits trying to fit through round holes......

 

[Oldhaven]

... I have never drilled an oil cylinder feed hole. I always thought it was redundant, but it would take some good reasoning at this point to convince me that the holes were necessary.
David

I find this reassuring. You are the second successful racer I have heard say this. Does this include both your road bikes and racers? (Different amounts of oil being splashed about.) Then there is robin's experience above that seems to contradict that, plus anecdotal evidence and the wisdom of the collective and even warnings from the designers over the years about blocked oil feeds. It seems as though if done right the extra oil does no harm, but done wrong all kinds of oiling problems can happen. I think, especially with the early two piece head bolts like I have, that getting oil tightness is difficult, even if the oil ends up correctly inserted below the rings. How do you seal the joint between the outer ET 55 and the crankcase insert ET158 when the slots to allow installation and removal of the insert appear to give oil in the feed gallery a direct path right up the inside of the ET55 to the top of the head? One piece bolts are sealed above the groove in the threads when they are installed to prevent this. I will be using restrictor discs with no holes because of the potential head bolt problem, but probably would have followed conventional wisdom and practice otherwise. I guess we will see what happens.

Ron