Comet Oil Use
- Thread starter Martyn Goodwin
- Start date
Bill,
The issue of piston clearances is a complex on and I am not sure that my two cents worth is worth two cents, but…. I am running a Nikasil aluminum barrel with a JE piston. JE has access to some very good ingots that others, like Omega, have complained that they cannot get. This was many years ago and things may have changed, but JE and Arias would ask what alloy you wished and the clearances were different. I run .003” clearance in my racer.This would be considered “loose” by most of those who run Nikasil. In fact, I am running exactly the same barrel and bore that was done in 1997. I have had three pistons in this bore since then,but the bore looks fine and I get good performance from it. It has done thousands of race miles. It has not done all the miles as I have a spare engine that I have used a few times. When the rings cracked in 2011 I used a flexi-hone to deglaze the bore and snapped the shot bellow:
I ascribe the good service to luck as I have no explanation for the long life.
With CP pistons in a stock Comet bore I would use a clearance of .0055” to .006”. I think this is also good for a racing twin like Pete Talabach’s side car machine which just returned from Barber with seized CP pistons. He used .004” in stock steel bores. Comets are running under a high load all the time and they need a bit of room for the quick heating they suffer from. I would say the same for a side car machine or racer. There was another Comet being raced at Barber and he seized his piston also. I suspect it was set to the manufacturer’s spec and that is what caused the issue, but I have not talked to him yet.
I do not think there is a good reason to go very tight on a Vincent. Carleton Palmer and I have never had a piston seizure once we followed the above specs on stock or race bikes.
David
There`s no way I would be putting carborundum on bronze, only use a scraper.