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Discussion in 'Airheads' started by Blackburn Buccaneer, Feb 27, 2019.
Yes. That's known as the cross-hatching.
And the 'filing' process described. Given the orientation of our cylinders I suspect the remnant cross hatch pattern angle is largely irrelevant to oil retention. Lubrication of the wall is dictated far more by the oil control ring rather than the hone pattern I suspect.
Some may learn something from this I found and I believe they know what they are talking about.
Cheers, I'll be back from sea in a few weeks.
Buddy of mine has some spare R80 cilinders with pistons in them and about 2000km on them since they were redone recently. They're from an 86-88 MY R80 and he says they're plug and play, would that be the case for a '78 /7 as well?
I'm doing some research online, but nothing has come up yet.
I don't think they would, but they are easy to adapt, and being nikasil is a worthwhile upgrade!
The only difference is that at the bottom of the cylinder, where it meets the crankcase, the nikasil cylinders have a small step, maybe 1X1mm, not bigger. Early /7 (I had a 78 R100S) don't have the same step machined in the cranckase, so You need to have someone removing the step from the nikasil cylindres, it's pretty easy and shouldn't cost You much.
Don't get scared, moving to Nikasil cylinders is the best upgrade You could do.
Thanks Luca for the fast response!
That makes it worthwhile, I'll have to hit up a local airhead shop tomorrow to ask them if they can machine that groove for me if I decide to go with this option.
My own cilinders (probably) have grooves worn in them, it's hard to make out on the stethoscope. There's also some metal shavings in the oil, so that would account for those as well.
Rather than a airhead shop You should contact a machine shop and show them the old and the new cylinders. The difference will be evident when You show them both. Nikasil cylindres also have a groove for an o-ring, the oring will work on your crankcase without further mods. If You take them to an airhead shop they will anyway take them to a machine shop...
I always thought seating was more due to the wear of the cylinder wall.
Essentially you take some metal away by scoring and it leaves high points and low points. The rings quickly wear off the high points and wear in themselves seating and making a better seal. With a slick smooth un-scored surface the rings would just slide around and would not wear in. Angle wise scoring straight up and down is no good because the wear in takes longer the rings do not seat evenly. Side to side is no good because they do not wear in as fast as the scoring collects oil that then acts like a mechanical equivalent of the fluid ball bearing like the effect of a dimpled golf ball.
I know that sounds esoteric but the latter just simple fluid dynamics.
Here's a golf ball going through the air.
As the air travels over the dimples the air within the dimple spins creating a little eddy reducing the balls friction through the air. It's counter intuitive but rough surfaces reduce friction through fluids due to the tiny eddies created. This is why sharks have rough skin, or tuna have small scales that change in density according to the surface pressure created by forward movement and flow patterns on their skin. Scoring perpendicular to the pistons travel would create little eddies of oil that would make the equivalent of an oil roller bearing keeping the ring off the wall, increasing oil consumption, and keeping the rings from properly seating.
So I've always thought that not enough angle produces a bad seal and increased oil consumption but the same can be said for too steep an angle. So I split the difference and did my scoring at around 45 degrees.
Since I've done it that way since I was a kid, I have no idea if it's actually the best angle, but that is how I reasoned it out back in the day.
Well that nicely confirms some of what I already thought.
"Cross Hatch Angles
The correct angle for cross hatch lines to intersect is approximately 45 degrees. Too steep an angle promotes oil migration down the cylinder resulting in a thin oil film, which can cause ring and cylinder scuffling.
Too flat a cross hatch angle can hold excess oil which conversely causes thicker oil films, which the piston rings will ride up on or hydroplane."
It's interesting that they use a 280 grit honing stone. Back in the day when I learned we used a far coarser grit emery cloth, but then again we did it by hand.
honed cylinder does not have high spots and low spots. Honing is about the most precise of the maching processes.
A nicasil cylinder is not going to just wear away. I have a set with 155k on them and they are just getting smooth near the bottom. The rings don't go down there, I think it was piston slap (and unfiltered oil) that did it.
On the other hand when you put the rings in they are not round. their gap varies and so does the ring expansion. So each ring gets worn to match the cylinder and it's gap.
Pour a little oil on a piece of smooth aluminum and hold it vertically so the oil runs off. Now scuff the aluminum and repeat. Notice anything? use very thin oil or heat the plate so don't have to wait all day for all the oil that is going to run off to finish doing it.
The hatch puts a side load on the piston. 45 degrees going 2 ways creates two loads that cancel each other. This is good.
A scratch is a low spot, a series of directional scratches next to one another creates highs and lows in a pattern, these patterns of highs and lows create the conditions that allow the ring to seat.
Both the surface of the wall and ring wear to match each other that wear is instigated and influenced by the pattern of highs and lows created by the scratches you put into the cylinder wall.
This is so obvious it's not worth arguing. Back in the day we did this with emery cloth in the back yard sitting in a lawn chair but the principle is the same if you do it by hand or with a high tech perfectly precise computerized machine.
Say you have a pitting in the chrome on your fork stanchions. They might be tiny to the naked eye but put those suckers under a microscope and they are craters. The good chrome is high, the pitting is low and over time that high to low variation will create wear and damage.
This is why I love you Plaka
I came up with my above theory on how this all works when I was a teen and rebuilding a VW with my dad. He used a drill, a piece of wood dowel equipped with flaps of emery cloth and an old all metal one speed drill to do the job, and I had to clean it up by hand, because my hands were smaller. Like a lot of shit I thought up as a kid I never checked the ideas, but they stuck with me, and thanks to you I just did.
When you hone the cylinder you produce a surface covered with scratches which create high and lows. in the surface some of the scratches are deeper then others and they are not perfectly uniform. Once your running the motor the rings wear down the high points creating a series of flat plateaus but the deeper scratches remain.
Here are fresh honing scratches under a microscope.
Here's one that's worn in with the plateaus or flat areas between the deeper scratches.
Here's one with more magnification where you can not only see the plateaus but also clearly make out the wear marks from the rings that created them by eroding the high points between the deeper scratches
I always thought that had to be how it worked but I've never seen any actual evidence of it until now.
Even though today's processes are more precise, and the materials have improved, the basic principles remain the same. So basically got it right 43 years ago, sitting in a lawn chair sipping A&W root beer, in our backyard, while doing mechanical work with my dad under our ash tree
Alright, so we ordered some Siebenrock 1000cc power kit jugs for 820 euros (good prices when you're close to Teutonic lands, ha) and they have arrived! Our local Siebenrock dealer said they were last pair available in Europe for months to come, something to do with the production run ending. Will pull the heads come Friday and ship those off to a BMW shop for a checkup. For now I'm just prepping for the job.
These babies came off without a hitch!
Left side thread is perfect, right side only has a small blemish on the top but otherwise it's perfecto:
After this is all done we'll continue with the whining trans... bearings starting to go.
...and so it goes... Dig in, just DO it, and keep on making these ole machines work. Today's engines are such precision/precision in comparison to these air-cooled beasts,but on the other hand, these beasts can take alot of use and abuse! I love that there is nothing electronic to randomly go bad, just good ole mechanical reliability.
Yep, just keep going because the riding is too fun. The electrics are playing up though; the lights don't work. That's for another day to figure out I guess.
They're worth it!
Hey guys, I'm doing the prep work for pulling the heads at the moment and I'm almost ready to start removing the valve covers, but I think I might have a problem.
I've got it set to the OT mark in the peep hole in the casing:
But it looks like the intake valve on the left side isn't completely closed?
The exhaust valve on the left is properly closed:
Is this a problem? Can I pull the heads like this? Might give me problems with the rockers, correct? Do I have to turn the engine over 180 degrees?
Turn the engine over 360 degrees, or start at the other side.
On these engines, in OT, one cylinder is at it's power stroke, just after ignition. (both valves closed).
The other cylinder is between exhaust (valve nearly closed) and intake stroke (valve just open).
Sweet, thanks Paul! Just as I suspected. Cheers
besides, You can just back off completely valve play on the rockers and go ahead and remove the heads. You'll need to reset it anyway.