Alot of talk seen about break-in.
What does it all mean?
The Bullet uses an old but reasonably effective way to seal the rings in the bore.
They use a high-peak cylinder honing method, and let the rings break-in the bore over time.
This has some good points, and some bad points.
The Bullet uses fairly crude manufacturing technology in the engine.
This means the bores aren't always perfectly round.
And in addition, even if they were bored round, they might not be round anymore when you torque-up your cylinder head studs, and cause distortion in the bore.
On top of that, since we deal with high heat generation and air cooling, any stress-risers in the castings can cause more bore distortion as it gets hot. I don't imagine they subject the barrels to much stress-relieving processes, so they just go on as they are manufactured.
How does this high-peak honing help?
Well, it starts out with a relatively tight piston to wall clearance when new.
And then the rings wear the peaks off the hone in the bore as they break-in. And it does this over time, with the varying engine conditions presented to it during break-in.
What this does is "compensate" for the changing bore dimensions over the different heating and cooling cycles that the engine goes thru, and it kind of "evens out" the bore for these overall conditions that are encountered.
So, it might not wind up being perfectly round when cold, if you measure it very accurately after break-in. But, it will likely present a "reasonable facsimile" of "round" at running temperatures. If we're lucky.
So, now we have a better idea about what all this "break-in" activity might be all about. They are letting the first thousand miles of running, take care of the stuff that they didn't really handle during the manufacturing process. And it works to a degree, but it's inexact, and it makes it a bit touchy for the user, if he decides to push the engine more than it's really happy to do in the early going.
I've inspected some pistons that have suffered seizing in some Bullet bores, and they show classic signs of seizure from bore distortion. We know the bore is very likely to have distortion, from the things I previously mentioned. And we know that they are expecting this high-peak honing to be able to somewhat accommodate the issues over time, as long as we follow the instructions for break-in. The reason that these pistons seize in the bores during break-in is because they were overheated or pushed beyond expectations during the running-in period, and the high-peak honing hadn't been fully worn-in to provide the dimensions and ring-sealing necessary yet.
So, that's why we need to follow this break-in process with stock Bullets.
Now, what can we do if we are replacing a piston, or rebuilding, so that we can have some better results with our job?
Well, first we should pick a good quality piston.
And we should bore and hone the cylinder to fit this piston properly, in a way that will take bore distortion and ring sealing into account.
The way to do this is to bore and hone the cylinder on a set of "torque plates".
Torque plates are simply metal plates that are bolted onto the top and bottom of the cylinder barrel, and torqued to our proper specs, so that it simulates the pressure on the cylinder barrel that it sees when the head is torqued onto it. Then we have the boring and honing done like that, so when we install it on the engine and torque the head on, the bore distortion is already accounted for, and it is more round when installed, than we would previously been able to have otherwise.
Okay, that's pretty cool.
Can we do more?
Yes, we could run the cylinder barrel thru some stress-relieving processes, such as cryo-cycling, heat cycling, and shaker table de-stressing methods.
This will relieve the stress-risers that occurred during the casting, manufacturing, and machining processes. The stress risers are areas which have "built-up" stress inherently in the metal structures, and warp or undergo changes with various temperatures. So, by using these stress-relieving methods, we can effectively stop these things from happening. At least to a pretty good degree.
All right. So now we know how to get a good round bore with no stress-riser problems, and we should have a pretty round cylinder now, which can give a good environment for our rings to work in.
Now, we can "plateau hone" the cylinder in the torque plates, at the correct cylinder to piston clearance that you'll want to run, to give a much faster break-in of the bore, and be more effective at ring sealing in a shorter period of time, because we've already done the things that Enfield didn't do, and had to use a high hone to try to "skate by" with a long break-in.
Lastly, avoid the temptation to overtorque your head nuts. It doesn't do any good. If you have an oil leak at the head, cure it the right way by dressing the spigot to get proper compression of the gasket. Don't distort the hell out of the cylinder bore trying to cure an oil leak in the wrong manner. Torque your head down at exactly the torque that you used on the torque plates when you bored and honed your cylinder.
Now, the rings.
The rings are the other half of the system of sealing the compression and controlling the oil in the cylinder.
We are going to use whatever rings come with our new piston, and generally they will be pretty good rings, unless we buy a stock piston and ring set, which I wouldn't recommend. At least you should have a 3-piece oil ring, anyway.
So, now it comes down to fitting and gapping the rings.
Well, there are some things that have come to light over the years, which allow us to do a little better than the "old school" method of just gapping all the rings the same.
I know that is something that most people do, and it works, but we can do better.
In a high heat application like our Bullets, we need to have sufficient ring gap, and this is often a little more gap than a water cooled car might need. So, I think we need to have at least .016" as a minimum ring gap for our top ring. And if you did .017", it wouldn't be bad either. That's a bit more than recommended in the manual, but it helps to deal with the heat.
Now, the 2nd ring is where the controversy starts. Mostly all "old schoolers" say to gap the 2nd ring the same as the top ring. I don't, and neither do alot of racers.
I think that the 2nd ring needs to have a bit more gap than the top ring.
So, I'm going to recommend about .020" gap on the 2nd ring gap.
Why?
Well, there is going to be some blow-by past the top ring, and this is evident by the fact that we have a 2nd compression ring. If there was no blow-by past the top ring, we wouldn't need a 2nd compression ring, right?
Okay, so why does it need to have a bigger gap? Because when pressure is forced down into that area above the 2nd ring, it is trapped in a very narrow area between those 2 compression rings. And if the pressure gets high, then it unseats the seal of the top ring. And then alot more blow-by happens, and we get "ring flutter" and bad sealing. And when that happens, alot of very hot combustion gases flow down along the sides of the piston, overheating the oil down there. And then that burned oil causes the rings to stick, and then we end up with a piston seizure. See how that works?
So, we're actually better off to let that excess pressure bleed-off down past the 2nd ring, than to build up above the 2nd ring where it could cause a problem with the top ring's job. But let it flow down in a controlled way, by having a slightly larger ring gap in that 2nd compression ring. It's a matter of dealing with the issue in the least detrimental manner.
Capice?
Gap your oil ring at .015", or whatever the book says.
Do these things, and use the proper piston to wall clearance that is correct for the expansion ratio of that piston you bought, and you'll never seize a piston. And you'll have great power because you aren't leaking compression.
Your engine will thank you.