I haven't gotten a hold of anyone yet but they're out west a bit so I'll try later in the day. But I see where this is coming from. I looked up the linear coe for aluminum and iron. This piston theoretically expands by six thousandths over 100 C. The bore expands only three. But the bore is air-cooled, and the piston is not, so there is definitely a temperature at which the piston no longer fits in the bore, and I can imagine it being one that's attainable under normal operating conditions, especially in Florida, where I plan on moving eventually.
I learned something else as an engineer, besides how to look up and use basic physics formulas, and that is, if you want to know how to put a motor together correctly, don't ask the engineer who designed it! Ask the hot rod mechanic who fixes up the street racers' motors. I am not opposed to the idea of boring this cylinder out a little more, even if it costs me more money. More to follow after I've spoken to JE and Steve, and gotten some actual measurements.
After speaking to JE:
I talked to a motorcycle tech a few minutes ago. He confirmed that the piston is symmetrical, and staunchly maintained that .0035 is the correct clearance. He did not think that there was any risk of seizure and claimed to have set up motorcycle pistons at .0025 or even less. I appreciate the unique nature of the iron barrel motor, so I can still be skeptical. I asked him what he thought about .006 or more; he said, "you can if you want, it's just going to rattle around in there."
Do you suppose the lack of an offset has anything to do with the clearance requirements? Has anyone seen a JE piston lock up in an Enfield motor before?
The offset is mostly for reducing the noise of piston slap when cold, and is not present on all pistons. It is not a requirement, and can be made either way.
Yes, I absolutely have seen JE pistons that have locked up in an Enfield motor. I don't think there are any brand of piston used in an Enfield that I haven't seen lock up. Cast or forged
Sure, many people have installed pistons in motorcycles at smaller clearances. The OEM Enfield cast piston is set at .003" clearance from the factory. They sometimes seize during break-in, but can survive with very gentle break-in. That's a CAST piston, which has much lower expansion than a forged piston.
Believe me, there have been thousands and thousands of seized pistons in the Bullet over the years, of all brands and types.
And it's not just the piston. The bore moves around from stress risers warping the bore when it gets hot. It changes shape.
And the bore doesn't expand .003". It might expand .002" maybe, but tests in ovens showed it to barely expand more than .001" at 350*F.
If you really do your bore right, bored and honed with the barrel torqued onto torque plates, with a really good job, and a forged piston from any maker out there, you MIGHT be able to run at .0055" clearance if you're lucky. All of my proprietary brand pistons out there are clearanced at .0055" or greater, in ALLOY finned barrels with iron liners in them, and they all survived except one, which was in an all cast iron barrel. That report came in yesterday. Yes, even my piston has now seized in one application, and it previously had a perfect record of no failures. No piston is immune to what the Bullet can put it thru
If you like to listen to hot-rod mechanics who built up the street racer motors, that's me. I make some of the hottest Bullets out there. I don't just wrench on them, I design the parts.
The Bullet engine holds heat. Especially in the piston. The piston IS going to be operating at way over 100* C. In fact it is going to be over 150*C almost all the time. It's going to expand more than people think.
ALL previous experience with ANY brand of forged piston in the Bullet shows that .006" clearance is where the safe zone is, and no less without being "iffy". My piston gets away with .0055" because it is an exceptionally good design for fighting heat, and it always gets an alloy barrel to help the engine cool better. But that's still only a half-thou less than .006", and I just got a seizure in a cast iron barrel reported yesterday with an .0055" clearance, from a very capable and experienced user. My piston maker says the same thing that it can run at .0035". Not in a Bullet, it don't.
I have one customer who I visited who has a shelf where he keeps his variety of pistons that he's seized in his engine. A JE was among the line-up. Along with a Hitchcock's piston, and a OEM piston. All toast with less than .006" clearance. Okay, granted that his tuning skills weren't the best I've seen, but still, there they are.
You might be able to push it down to .0055" and cross your fingers. I wouldn't. It won't make noise at .006". I know plenty of people running them at .006" without making slap noises.
And make the ring gaps larger than book specs would say they should be. I use .018"-.020" for the top ring. It gets plenty of heat, and needs room.
The bottom line is that regardless of what an technician or spec sheet says, we have to give this engine what it requires, or sorrow will result. With these bikes and forged pistons, if there is any doubt about clearance, err toward the large side. It will run fine if it's a touch too big, but it will lock up if it's a touch too small.
Be advised.
Topic: Anyone know anything about the JE piston (part number Z91433) (Read 6409 times)