Piston to bore clearance for Omega's

Fast Eddie · Dec 4, 2013

Hi Gents,
I have a pair of NOS, standard bore (77mm) 1976 Omega 10.25:1 racing pistons that I am trying to ascertain whether or not I can / wish to use.
Can anyone please tell me what the ideal and / or maximum piston to bore clearance is with these pistons?
The bike is an 850, currently on a standard bore.
Thanks in advance.

Al-otment · Dec 4, 2013

Give Omega a call (UK) 01215596778.

Fast Eddie · Dec 4, 2013

Al-otment said:
Give Omega a call (UK) 01215596778.

Good point and I will do.
But I'm a little doubtful they'll be happy / able to give me info on a 37 year old product, and, to be honest, I'd actually put more trust in hard earned experience from some of the knowledgable gents who frequent this forum.
At least that's what I was hoping for!

comnoz · Dec 4, 2013

I would suspect they are a forged 2618 alloy so clearance would be around .0065 inch.

If they would end up being a forged 4032 alloy then that would be about .0015 inch more clearance than needed but you would still be safe. Jim

Fast Eddie · Dec 4, 2013

comnoz said:
I would suspect they are a forged 2618 alloy so clearance would be around .0065 inch.

If they would end up being a forged 4032 alloy then that would be about .0015 inch more clearance than needed but you would still be safe. Jim

Thanks Jim, that's exactly the kind of answer I was hoping for!
Rgds,
Nigel.

Hobot · Dec 4, 2013

commoz how much clearance would you provide for your boosted bunny?

comnoz · Dec 4, 2013

hobot said:
commoz how much clearance would you provide for your boosted bunny?

I am still negotiating pistons so depending on what I decide to use , I would still be around .005 for 4032 and .0065 for 2618.

I don't think I would attempt boost without an aluminum barrel. In an iron barrel I would need more clearance with boost. Jim

worntorn · Dec 4, 2013

Here is a link to some info on Omegas. It dates from the mid seventies, about when your pistons were made. Unfortunately, like a lot of my info, it comes from the Vincent world. When the Omega low expansion pistons became available for Vincents, they caught on rather quickly and are still a favourite for most of us.
I am running some of these in my OZ bike. They were fitted to. 0035 and have taken everything I could throw at them in fourty thousand miles, lots of WOT for miles two up with luggage climbing big mountain passes at 80-90 mph, no problems.
A friend has his fitted at 2 thou on a 10.5 to one bike and has done even more mileage but he is afraid to open the bike up on big hills for fear of squeeking a piston.
We chuck out the supplied cast iron rings and use Honda chrome rings, they seem to last forever.
With the closer tolerance oil consumption is reduced to almost nil.

The differences with the Commando would be the cast iron cylinder vs aluminium muff and liner, also bigger bore on Vincent at 84mm.

http://www.google.ca/url?sa=t&rct=j&q=o ... 5469,d.cGE

Hobot · Dec 4, 2013

Ok thanks for some more feeling on the living breathing elastic nature of seemingly solid stable engines. Oh and thanks for providing example of too much piston slap cold noises. Rallys are distracting places and wish I'd gotten a video of your Batmobile C'do.

comnoz · Dec 4, 2013

worntorn said:
Here is a link to some info on Omegas. It dates from the mid seventies, about when your pistons were made. Unfortunately, like a lot of my info, it comes from the Vincent world. When the Omega low expansion pistons became available for Vincents, they caught on rather quickly and are still a favourite for most of us.
I am running some of these in my OZ bike. They were fitted to. 0035 and have taken everything I could throw at them in fourty thousand miles, lots of WOT for miles two up with luggage climbing big mountain passes at 80-90 mph, no problems.
A friend has his fitted at 2 thou on a 10.5 to one bike and has done even more mileage but he is afraid to open the bike up on big hills for fear of squeeking a piston.
We chuck out the supplied cast iron rings and use Honda chrome rings, they seem to last forever.
With the closer tolerance oil consumption is reduced to almost nil.

The differences with the Commando would be the cast iron bore vs aluminium muff and liner, also bigger bore on Vincent at 84mm.

http://www.google.ca/url?sa=t&rct=j&q=o ... 5469,d.cGE

The pistons in your link are hyper-eutectic cast pistons. They would run at tight clearances in an aluminum muff.

If the Omegas for the Norton are cast then they would be run a bit tighter also. Probably around .0045 unless they are a T-slot design where they could be .0035 to .004. Jim

lcrken · Dec 4, 2013

The short stroke 750 engines came from the factory with custom Omega pistons. They were cast, not forged, but I don't know the alloy. I don't know what clearance they started out with, but when I tore down the engine from one of the factory flat track engines, the clearance was .005". The engine had some racing time on it, but the bore was still perfect, with no measurable wear. I think Jim is right on the mark at .0045 - .005" for your 850.

Ken

comnoz · Dec 4, 2013

lcrken said:
The short stroke 750 engines came from the factory with custom Omega pistons. They were cast, not forged, but I don't know the alloy. I don't know what clearance they started out with, but when I tore down the engine from one of the factory flat track engines, the clearance was .005". The engine had some racing time on it, but the bore was still perfect, with no measurable wear. I think Jim is right on the mark at .0045 - .005" for your 850.

Ken

Yeah, I have heard of cast Omega Norton pistons but never seen one.

The only Omega Norton pistons I have seen were forged with a raised dome for high compression. I believe they were customs.

Maybe Nigel better post a picture so we can see them from the bottom. Jim

lcrken · Dec 4, 2013

These are original Omega short stroke 750 cast pistons, except I reshaped the dome to give me some squish in the full hemi combustion chamber. From the factory the domes had a straight 45 degree (or thereabouts) taper for the dome, and I contoured it for a better fit. As you can see from the lack of combustion deposits, the fit was pretty good. The valve cutouts have also been enlarged for the larger valves and high lift cam the race team had put in this particular engine.

Ken

Fast Eddie · Dec 4, 2013

comnoz said:
lcrken said:

The short stroke 750 engines came from the factory with custom Omega pistons. They were cast, not forged, but I don't know the alloy. I don't know what clearance they started out with, but when I tore down the engine from one of the factory flat track engines, the clearance was .005". The engine had some racing time on it, but the bore was still perfect, with no measurable wear. I think Jim is right on the mark at .0045 - .005" for your 850.

Ken

Click to expand...

Yeah, I have heard of cast Omega Norton pistons but never seen one.

The only Omega Norton pistons I have seen were forged with a raised dome for high compression. I believe they were customs.

Maybe Nigel better post a picture so we can see them from the bottom. Jim

Here are some pictures guys, they look pretty much like cast pistons to me. There are no product codes or numbers on them or the box, hence I doubt Omega will help much...

comnoz · Dec 4, 2013

Yep, Definitely cast, no slot. I would set them at .0045 to .0047 for the street or .005 for racing with a short break-in. Jim

Fast Eddie · Dec 5, 2013

comnoz said:
Yep, Definitely cast, no slot. I would set them at .0045 to .0047 for the street or .005 for racing with a short break-in. Jim

Thanks Jim. I'll go with .005 me thinks. A bit of slap is better than a seizure...!
Out of interest, what clearance would a stock 850 have had from the factory?

Al-otment · Dec 5, 2013

Fast Eddie said:
comnoz said:

Yep, Definitely cast, no slot. I would set them at .0045 to .0047 for the street or .005 for racing with a short break-in. Jim

Click to expand...

Thanks Jim. I'll go with .005 me thinks. A bit of slap is better than a seizure...!
Out of interest, what clearance would a stock 850 have had from the factory?

According to the manual between 0.0035" and 0.0055" depending on grade A or B cylinder bore and dia. of pistons which apparently varied by 0.001". I understand the smaller clearances were possible because of higher silicon content in the pistons. Not questioning Jim's experience but no harm in getting a second opinion from Omega.

comnoz · Dec 5, 2013

Fast Eddie said:
comnoz said:

Yep, Definitely cast, no slot. I would set them at .0045 to .0047 for the street or .005 for racing with a short break-in. Jim

Click to expand...

Thanks Jim. I'll go with .005 me thinks. A bit of slap is better than a seizure...!
Out of interest, what clearance would a stock 850 have had from the factory?

Somewhere between .004 and .0045 for a stock 850 with Hepolites. Jim