Piston to bore clearance

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concours said:
comnoz said:
I fit JCC pistons at .0045 to .0048 inch. They need to be slightly looser than the old original Hepolites.
Click to expand...

Can you help us understand WHY? Cast VS Forged? Hypereutectic? The old and not the new? Inquiring minds want to know... :idea:
Click to expand...

Also,Terminology again please help show me the light !!! I'm a "boilermaker/welder" (not a Mech Engineer) as of the 60's now retired, but im yet to understand how a piston can be "forged". I understand "casting" a piston (alloy), then machining to desired size and shape... What is the difference in the piston (cast/forged) as we have been discussing ... Or is it just another "urban myth" for the plebs like me??
 
Go on to You Tube & type in Piston forging process. There is a Wiseco film among others which explains all.
Martyn.
 
Jim,

What clearance would you typically use for forged pistons like Omegas ?

sam[/quote]

Sam,
I don't have any experience with Omega forged pistons.

I have used JE, Venolia, Aries, and Wiseco forged and they all require at least .006 clearance. A little more for hard usage.

I would expect Omegas would need .0065 like a JE if they are a 2618 alloy. Jim
 
olChris said:
concours said:
comnoz said:
I fit JCC pistons at .0045 to .0048 inch. They need to be slightly looser than the old original Hepolites.
Click to expand...

Can you help us understand WHY? Cast VS Forged? Hypereutectic? The old and not the new? Inquiring minds want to know... :idea:
Click to expand...

Also,Terminology again please help show me the light !!! I'm a "boilermaker/welder" (not a Mech Engineer) as of the 60's now retired, but im yet to understand how a piston can be "forged". I understand "casting" a piston (alloy), then machining to desired size and shape... What is the difference in the piston (cast/forged) as we have been discussing ... Or is it just another "urban myth" for the plebs like me??
Click to expand...

Casting a piston requires a high silicone content to make the metal flow in the mold. High silicon content means the piston is not going to expand as much with heat. It makes the piston hard and brittle. The hardness means the ring lands and skirts are going to have good wear properties. Tensile strength is lower and heat conduction is slower. Failure from detonation is likely to show up as a hole in the top or cracked and broken ring lands or skirts.

Forged pistons are pressed into shape and can be made with low silicon alloy such as 2618 for maximum strength or a higher silicon alloy such as 4032 which is not as strong but has better wear properties.

2618 is malleable. It can bend without breaking. It has high tensile strength and conducts heat quickly. This makes it harder to melt a hole in the top with detonation but more likely to seize since the piston expands more when it is exposed to the heat of detonation and it doesn't fail in other ways first.

4032 is somewhere between a 2618 forged and a high silicon cast. I don't know of anyone making a 4032 piston for a Norton at the present time but I have been working on it. Jim
 
Are there other options for Commando pistons besides the Asian products,JE etc
 
Time Warp said:
Are there other options for Commando pistons besides the Asian products,JE etc
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JCC [sold under many names] and JE are the only two that I know of that are available off the shelf. Omega- I don't know.

Of course any of the manufacturers will make them to order, but there will be minimum quantities. Jim
 
Don't know if GPM has changed their methods, but the local Vincent bunch had a spate of problems with GPM pistons seizing about fifteen years ago. The fitter blamed it on GPM, Some people blamed the fitter. Those particular GPMs were not cam ground and he was used to fitting cam ground pistons, infact wrongly assumed all modern pistons were cam ground. If not cam ground, a greater clearance is needed. Expansion at the wrist pin area is greater than in other place around the circumference, so cam grinding allows for this. The idea is that while the piston is cam shaped when cold, it will become round when hot and the fit in the bore will be very uniform and close.

Glen
 
L.A.B. said:
Time Warp said:
Are there other options for Commando pistons besides the Asian products,JE etc
Click to expand...

GPM (Italy): http://www.andover-norton.co.uk/SI%20Pistons&Rings.htm

http://www.gandini.it/www/pistoni/?ctg=13&languageid=gb
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Yep. I forgot about them.

It has been some time since I used any GPM pistons. Last I knew their quality control was a bit iffy. Such as two matched pistons that were .002 in. different in diameter. Have they gotten better? Jim

Oh, I forgot about the ring lands that looked like they were cut with a hacksaw blade.
 
comnoz said:
Last I knew their quality control was a bit iffy. Such as two matched pistons that were .002 in. different in diameter. Have they gotten better?
Click to expand...

I hope so, (as they are sold by AN) :)?

I think that is a question only Joe/ZFD may be able to answer.
 
L.A.B. said:
comnoz said:
Last I knew their quality control was a bit iffy. Such as two matched pistons that were .002 in. different in diameter. Have they gotten better?
Click to expand...

I hope so, (as they are sold by AN) :)?

I think that is a question only Joe/ZFD may be able to answer.
Click to expand...

Maybe I will have to give them a try again.

I will have to say I have been quite happy with the JCC pistons other than the snap rings that don't fit. Jim
 
I got the standard sized GPM pistons from Norvil in 2010 and they seem to be fine so far. But I probably don't have the evil eye like Jim does.

Dave
 
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