Blow-by and your Bullet

[ace.cafe]

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.

[Slider]

Blow-by and your Bullet

I thought this was going to be about when you're riding your Bullet and a guy on a sportbike blows by.

[Foggy_Auggie]

You the man Ace! 

Back in the '60's I remember a buddy's BSA 650 twin when he was installing new pistons.  I inspected the iron cylinder barrel and to my surprise the twin bores looked like they were WAY off center to the crankshaft throws.  On one side the cylinder wall looked like it was about 200 thousands thick - the other side looked about 60 thousands thick.

The clincher was that with the base spigot alignment and stud hole placement the bores were aligned with the crankshaft throws.  The whole iron barrel rough casting was what was off center.  The casting mold was crap as far as exterior parameters of the cylinder to the base spigot.

Good old English 1960's practicality - instead of tossing the cylinder they just bored it and used it with the uneven cylinder wall thicknesses. 

[Phlakaton]

haha.  that seems to happen on a daily basis for all us I guess.

[The Garbone]

Good stuff Ace, posts like yours make this forum better than most motorcycle magazines these days...

 

[Slarsonroy]

So what is the correct head nut torque? I just guessed at 20 ft/lbs on all of them. I imagine I need to retorque them after 500 miles or so. The motor only has 1700 miles on it, but the previous owner dismantled the top end anticipating a crankshaft failure, which turned out to be a sprag clutch failure.

[ace.cafe]

So what is the correct head nut torque? I just guessed at 20 ft/lbs on all of them. I imagine I need to retorque them after 500 miles or so. The motor only has 1700 miles on it, but the previous owner dismantled the top end anticipating a crankshaft failure, which turned out to be a sprag clutch failure.

The specs call for 24 foot-pounds of torque on the head nuts.
I typically down-rate that to 22 foot-pounds on my bike, and 20 might be ok too. Especially if you have an alloy cylinder barrel fitted, because they have more vertical expansion when hot, and stress the studs more because of that.

All that's necessary is to get the sealing to work.
They changed the threads to metric threads in 2002, and the metric thread pitch that they used is not as strong as the original thread, and sometimes results in pulling the studs out of the engine cases. I think they should have down-rated the torque spec a that time, but they didn't.
So, I don't use the full torque, and  use about 10% less.
This will cause less bore distortion too.
You can torque as you like. I'm just saying what I do with my own bike.

[Slarsonroy]

Sounds good. You dont retorque after a few hundred miles?

[ace.cafe]

Sounds good. You dont retorque after a few hundred miles?

I'll usually retorque once after a few rides.
But, if you are continually taking up any stud stretch, you'll keep retorquing until the studs break, or you pull the threads.
Generally if it's holding and not showing any signs of problems, I'll leave it be.

[jonapplegate]

Very nice post Ace. Let us not forget that the intial break-in is also "final machining" for the rest of the moving internals. I know Ace knows this but there is always this debate it seems about alternative ways to break in your engine for proper ring seal. I won't get into specifics since you are all probably familiar with them. The problem with these other techniques is that they don't take into account the archaic nature of Bullets. Sure, you may get a nice tight seal on your pistons but the rest of your engine and transmission will suffer for it. On new tech engines this is not a great problem but we aren,t talking new tech engines our we? So just remember that while there are different break-in procedures out there, they are not geared to the needs of your new Bullet. Maybe the new, new, design but even then why chance it?
  Ace, in your opinion, would polishing the fins (not too a sparkle, but knocking down roughness)  and running a file over sharp machined edges to reduce heat and stress points be advantageous for getting a true cylinder on a new barrel or is this overkill?
 Thanks Ace, your posts are always the first thing I read.     

[ace.cafe]

  Ace, in your opinion, would polishing the fins (not too a sparkle, but knocking down roughness)  and running a file over sharp machined edges to reduce heat and stress points be advantageous for getting a true cylinder on a new barrel or is this overkill?
 Thanks Ace, your posts are always the first thing I read.     

Jon,
On cylinder fins or head fins, no I don't think that's going to be of any benefit, except maybe visual appeal.
The stuff we're concerned with for barrel true-ness are very deep stress-risers that are in the metal "grain" itself, inside the actual iron liner, and to a somewhat lesser degree, the casting around the liner. The fins are far enough away from the liner, to not be of much consequence, and they can move vertically and twist a bit to give way to any stresses in them.

However, polishing the fins and removing rough casting marks actually will reduce the cooling efficiency of the fins by reducing surface area of the fins to work, and be reducing turbulence over the surface, which help in the cooling process.
So, I personally think that polishing the fins is not a particularly good idea, but it can look good.
If you filed them down smooth and nice, contoured the shapes of the fins the way you like them, and clean up the casting marks, AND THEN gave them a nice even bead-blast or sand-blast finish to restore the surface area and still look good, that would get a "thumbs-up" from me.
If you have the iron cylinder, it will need some paint on it to prevent rust, so I'd use a VERY THIN coating of paint, of the color you desire  The paint does hinder the cooling by a few percent, so don't make it thick.
Black is good for IR heat emissivity, but it also absorbs heat rapidly.
A good option might be the "thermal dispersant" coatings that are available from the performance coating companies. They would give you the best performance for cooling, that could could get in any spray on type covering, or paint.
For a regular paint, some engine paint that is loaded with aluminum dust, like silver paint or aluminum paint that uses aluminum for pigment, will help the heat to get out by following pathways created by all the aluminum particles in the paint touching each other, and giving some improved pathways for thermal transfer of the heat to the air.

[Kevin Mahoney]

There is more science involved in the design of fins than you may know. For example in the new UCE engine a ton of thermal analysis was done in the initial design. The fins are designed very purposefully to maximize cooling. On that engine I wouldn't touch them. I am less sure about the design of the cast iron engine. I totally agree with every last word that ACE says about modifying them. One thing I have always found curious is the fins at the rear of the cylinder. They are much shorter than the rest. This is the result of making room for the magneto when that was the source of ignition. I always wondered why RE didn't change this. On  the other hand with that engine they worked on the principle of "If it ain't broke don't fix it", which is something I have never been able to control in myself.

[t120rbullet]

One thing I have always found curious is the fins at the rear of the cylinder. They are much shorter than the rest. This is the result of making room for the magneto when that was the source of ignition.

Years ago Terry Smith had some aftermarket aluminum cylinders that did not have the flat in the back like all the other cylinders today. Made the engine look bigger.
Got some pics of it somewhere, I'll post em if I can find em.
Just one more in the long list of things I should of got my hands on.
CJ
 

[ace.cafe]

One thing I have always found curious is the fins at the rear of the cylinder. They are much shorter than the rest. This is the result of making room for the magneto when that was the source of ignition.

Years ago Terry Smith had some aftermarket aluminum cylinders that did not have the flat in the back like all the other cylinders today. Made the engine look bigger.
Got some pics of it somewhere, I'll post em if I can find em.
Just one more in the long list of things I should of got my hands on.
CJ
 

I spent some time looking at the finning on the iron Bullet, and did a little bit of initial analysis.

I don't think that the shorter fins in the rear are a major culprit. I think it is a matter of air circulation on the fins, which is the bigger factor.

The fins on the drive side are pretty clear all the way to the rear, and that side gets some pretty good cooling, because the air can enter at the front of the fins, get in nice and deep, and flow all the way to the rear on that side.
However, the air doesn't turn behind the cylinder, so the back fins get next to nothing, in terms of air flow.
But on the timing side, there are real problems.
The pushrod tubes don't have any airways behind them. So the air that enters the fins at the front, gets deflected outboard of all the finning behind the front pushrod tubes, and it never comes back inboard again. At least at any decent road speeds, it doesn't. So, that means that the finning area from the front pushrod tube, all the way back around the rear of the cylinder, is starved for cooling air flow.
In additon, the area between the pushrod tubes is very thin in cast iron, and is almost down to the cylinder liner at that point. Very little mass to handle the high heat there. I've heard a report of someone who could see "glowing red iron" at that point between the pushrod tubes on the iron barrel, when he pulled up next to his friend's Bullet at a stoplight, after a good hard run. There's very little air at all, that makes it into that little area between the pushrod tubes. If that area, or any area, is "glowing red hot", then it's going to burn that oil in the bore, and piston seizure is only seconds away after that.

I have some ideas that I might get into developing, which would incorporate some suitable air deflectors that are not too conspicuous, that might be able to address these issues with airflow around the barrel on the iron Bullet.

[mrunderhill1975a]

 the fins at the rear of the cylinder. They are much shorter than the rest. This is the result of making room for the magneto when that was the source of ignition

I had always assumed the fins were shorter on the rear because there wouldn't be much airflow behind the engine and thus longer fins wouldn't help. Thanks Ace for the info.