Omega Pistons - Clearance at the bottom of the skirt

[pete4000]

Hello everyone,

Just a few questions concerning Omega pistons.

I have ordered as set of them in 9:1 + 0.40 and would like to know what would be the clearance at the bottom of the skirt . I'm going to rebore my liner and put those pistons.
The Richardson tells 0.15 mm /0.16 mm, but it is for specialoid pistons I guess.
Should I put 0.15 mm or less ??? It seems to me quite lot.
Any idea ?
What would be also the closed gap to respect for the rings ? (0. 50 mm / 0.60 mm / 0.70 mm ? )
Many thanks for your answers,
Pete.

 

[Comet Rider]

Hi Pete,
Run the clearance on the loose side, rather than tight, after all oil is cheaper than a siezed engine.
For racing I used to use 0.0005" per inch of bore plus an extra 0.0005". (Half a thou')
Therefore for 85mm (84 + 0.4) nominal 3.3" gives between 0.0015" and 0.0020" (2 thou') which is a max of 0.05mm.
Remember that the old specialoid pistons where cast, and the new omega's are forged, so different rates of expansion.

Neil

 

[timetraveller]

Assuming that you have just bought the pistons and not had them on the shelf for years why not ask the person you purchased them from? Assuming that they are for road, and not racing, use then a figure typically given for the low expansion pistons is three and a half thou! Several local people have used this and some have had problems. However, what is often not realised is that with this very low clearance it is vital that the bore of the cylinder is exactly perpendicular to the base of the muff. Roy Robertson set up a jig to measure this and typically the base runs out by about five thou. There goes your clearance and here comes the seizure. If this angle only meant that the cylinder was tilted for and aft then it would not be a problem but if it is side to side you will see that there are going to be problems. People who have ensured that the base of the muff is at 90 degrees to the axis have not had problems with their bikes and if the low clearance pistons are fitted at the same time as sealed lower valve guides many of them have found no need to top up the oil between oil changes. Good luck

 

[Howard]

Hi Pete,
Run the clearance on the loose side, rather than tight, after all oil is cheaper than a siezed engine.
For racing I used to use 0.0005" per inch of bore plus an extra 0.0005". (Half a thou')
Therefore for 85mm (84 + 0.4) nominal 3.3" gives between 0.0015" and 0.0020" (2 thou') which is a max of 0.05mm.
Remember that the old specialoid pistons where cast, and the new omega's are forged, so different rates of expansion.

Neil

Hi Neil

Are you sure about 2 thou? It seems very close for racing.

I seem to remember my Comet racer was bored out with much more clearance, in fact when I took it to be bored and told them the size I wanted, the machinist said "You mean you want it half worn out?"

I think one of "the" books Richardson, Know thy Beast, Tuning for Speed gives the clearance for pistons and rings, but it's a long time since I had a rebore (haven't rebored the bike either) so I'm not sure which one. The other problem, all these books are getting on a bit, like the owners, so the figures may not be as rellevent with modern pistons.

H

 

[nkt267]

If I remember correctly the clearance specified in the leaflet that came with my omega stated 3 thou clearance..The 2 different companys that I have used recommended 6 thou and I have not had any problems, and no piston slap..John

 

[mercurycrest]

I stuck my C Comet twice with an Omega piston! Once with .0035 and lastly with .0045 clearance! Granted, they happened on hot days (90F) and two up. My A Comet has the same type piston, but with .006 clearance, the iron barrell feels like it runs twice as hot as the alloy one if you touch it and yet has no problems with seizing. It's .0055-.006 from now on for me.
Cheers, John

 

[pete4000]

Hi everyone,
Many thanks for your answers, I appreciate. I notice that some of members put a tight clearance and others greater. I will probably go on a 0.005 clearance, seems for me to be good and acceptable.
Pete

 

[Glyn Baxter]

I think that you will find that the pistons are supplied with the correct clearance for the specified bore so, if they are for 84mm + 0.040" then you should bore to that size. The maker sets the clearance during manufacture of the pistons.

Glyn Baxter