Omega Piston Clearance

[iandusud]

Hello. I've just bought a new Omega diecast piston for a Comet from Vincent Spares Co. The fitting instruction advise a piston to bore clearance of 2.5 - 3 thou. This seems quite a small clearance to me, but I appreciate that different pistons have different expansion rates. I would therefore like to know if others have fitted these pistons and what their experience of them is.

Many thanks, Ian

 

[A_HRD]

Ian,

When these Omega Die-Cast LOW EXPANSION pistons first came out nearly 2 decades ago, I seem to remember the advice was to fit them at about 1 thou clearance! But then a spate of seizures resulted. The 2.5 to 3 thou is fine provided the muffs are tight on the liners and the bores are parallel and square. If the mating parts are suspect, then it might be wise to go up to 4 thou - particularly so on a Comet - and especially so if you are a spirited rider! These pistons are widely used in Vincents and are generally fine. That is not to say people haven't had problems - some have - but then the pistons weren't always the cause; more likely carburation, under-oiling or muff/liner issues.

The old sand-cast Hepolite and Specialloid pistons had a much greater expansion rate which dictated high clearances. Not so these days with die-cast pistons manufactured from improved materials.

Hope this helps.

Peter B
Bristol, UK.

 

[timetraveller]

It is really important with these pistons that the base of the muff is exactly at 90 degrees to the axis of the liner. This is best checked by making accurate plugs. with centres, to fit the top and bottom of the bore and then to check/skim the base of the muff. Typically you will find that there is a several thou run out which is what will cause trouble with only about three thou clearance.

 

[iandusud]

Thanks for the replies. I think I'll go for 4 thou.

Ian

 

[mercurycrest]

I've stuck my C Comet three times riding two-up in the hills around here. The last time (a couple of years ago, by now) the clearance was .0045 and I stuffed it in the shed in disgust. I set my A Comet up with .007 and it works perfectly! They shared a set of 7-1 Omegas. Maybe, it's because what we call hills, you call mountains, but I'm thinking a hard working Comet needs the safety margin. What's the point of tight clearances anyhow? I put my D together well over 80,000 miles ago with .0055" clearance, Specialoids and Perfect Circle Rings from a Chevy LUV, it doesn't use oil..
Cheers, John
p.s. Irene just glanced at this and said; "Well, that's all you can do."

 

[roy the mechanic]

timetraveller has it right= if you have .003 piston clearance and the bore is .003 out of perpendicular -result instant seizure! it.s not the piston or machinists fault, it,s yours for not checking things out exhaustively!

 

[vibrac]

I fuss about on the racers with fancy US pistons and rings but honestly for road use 5 thou wont cause you any harm on low ex
Anyway a piston seizure on a road bike is no big deal, my trials comet did a woopsey when it was first built. I pulled it down eased the piston like it was a two stroke cleaned the bore and it was back together in a day.she smoked for a few miles then the air cleared and that was years ago.
Our engines need time to bed in then if they are not mucked about with and they dont do too many short runs they have a long plateau of oil tight running

 

[mercurycrest]

I agree, Roy plus, always bore from the bottom to keep any error in alignment at the top of the bore. But, we're talking Comets here. I built a Shadow for Bonneville that was pretty fast and reliable on petrol back in the 70's. When I took the modified parts out and put them on a Comet bottom end and tried to do the same thing as the twin would consitantly do, you couldn't. Simply because it got too hot and then the valve seats would fall out.
Cheers, John

 

[vibrac]

Yes thats the limitation with the Comet I am old enough to remember the Ray Mckay Island attempt on a Comet special with Roger Slater (Yes there was life before Egli) and that bike poped its spectacles too. It seems that the seats are the weak point when push comes to shove. it only rears its head (pun!) when the pace is kept up for a considerable time (No chance of that in English classic racing)