Cylinder Boring / Honing 40 years later - a KawBoy perspective

Stolen from kz1300.com 

Because info like this can't exist on just one forum that might go poof..

 thought it was time we had a topic / discussion about piston cylinder maintenance, machining practices and fits and tolerances due to the fact that we have 40-year-old motorcycles with Operating / Service Manuals created 40 years ago and machining practices have changed but there have been no updates to our provided information for operating / maintaining our bikes. 

Back in the 1970’s and early 80’s, cylinders were bored with a boring bar then finished honed to size with a “precision hone” which typically had two opposing 280 grit honing stones and two opposing guide bars. It got the job done leaving a reasonably true cylindrical finish. The cross hatching was on a 45 degree pattern and the surface finish was consistent with an obvious cross hatching pattern.

Along came the 1980’s and demands got placed on manufacturers to improve emissions. The manufacturers put on their thinking caps to try to come up with improvements to meet the new standards.First, they needed to understand what was the root cause of poor emissions and in the forefront was crankcase ventilation. Crankcases had to be vented of the blow by gases that escaped the piston / cylinder sealing, allowing unburnt gases into the crankcase which mixed with the hot oil fumes. Positive Crankcase Ventilation rerouted those gases back in to the intake to be burned in the combustion process and like a 2 stoke gas mix, the exhaust emission was higher than what was produced from burning just gasoline. How could they improve the piston / cylinder sealing?At the time, cylinder finishes were left moderately rough and needed to be “broken in”. Because of the roughness left by the honing process, the cylinder / piston clearance was left tight and instructions in the operating manual restricted engine operating rpm’s until the break in period was completed. During break in, crankcase gases developed was moderate and as the break in got completed, the gases developed got less.

There is a wealth of varying information available on many internet sites, magazines and aftermarket piston ring suppliers regarding break in practices but what if we could bypass all of those requirements and produce and engine you could run right off of the assembly line as if all that break in period stuff was done?So that was the goal and then they needed to figure out how to get there. The standard practice was to leave the cylinder with a moderately rough surface and then get the piston rings to break in the surface finish of the cylinder to leave a smooth surface for sealing of the piston rings to the cylinder. The end goal also needed to leave clearance room for an oil film to lubricate the sliding parts of the piston and piston rings on the cylinder wall without leaving excessive clearance.

So just what would a broken in cylinder look like?If you have ever tore apart one of these KZ1300 engines that had been broken in and looked at the cylinders, you would have seen a smooth bore with a shiny finish and what appears to be some cross hatching sort of underneath the smooth finish. What you wouldn’t have seen would have been the out of round dimensions caused by the major and minor thrust of the piston on the front and the back sides of the cylinder, otherwise the cylinder would be considered to be in great shape. A smooth shiny finish for the piston to thrust upon and a smooth finish for the rings to slide / seal on with grooves to hold an oil film in. Perfect.Well, if that’s “perfect” after break in, how can we achieve that before break in? We need To be able to produce that finish and leave enough clearance for the required oil film and that’s how Plateau Cylinder Honing came to be the standard practice so let’s dig in to Plateau Honing.

If you have ever prepped a previously painted surface for repainting and have done it right, you would have started with a rough grit sandpaper to flatten the surface and then progressed through finer grades of paper to produce a smooth finish to be painted. If you didn’t do it properly, your finished painted surface looked terrible as if it had sanding grooves underneath that freshly painted surface. Hold that thought for a moment.When prepping for paint, you would have sanded with maybe 100 grit until you had a even scratched surface that was flat. Next, you grab some 180 grit and remove all those deep scratches leaving a consistent flat surface with finer scratches and then again, found maybe some 400 grit to remove the 180 grit scratches leaving 400 grit, even finer scratches. Some would leave it at that and then paint, some would go even further and head to 600 or 800 grit before hitting the surface with paint. The goal here would be to have a super smooth finish with no evidence of any sanding marks at all.Now go back to the finish we had when we left some deep scratches in the metal and then we painted the surface and had a shiny painted surface with what appeared to be scratches underneath. Isn’t that what we said we wanted the freshly broke in cylinder to look like? Yes? So how can we achieve that in a machining process.

This is how Plateau Honing came to be. Plateau honing starts with an 80-grit stones to “flatten” the surface and leave 80 grit cross hatching marks in the surface. That’s followed up with a 280 grit honing stones to take off the highest peaks off of the 80 grit cross hatching, flattening those peaks. This second stage could take a few as 10 strokes of the hone in the cylinder. This is then followed up with 400 or 500 grit to make those flattened peaks super smooth for the piston and piston rings to ride on. Possibly 30 to 40 strokes for this process. Whatever it takes to remove the 280 grit cross hatching of the flattened peaks. What’s left to be seen is the deep 80 grit cross hatching with a super smooth almost mirror like finish for 90 % of the surface. What’s great about this process is we have an engine that can be run as if it was already broke in so no break in period required. The piston rings provided today have a lapped surface finish and are round to marry up with a mirror finished surface of a cylinder that’s round and an almost perfect seal between the cylinder and piston rings is obtained right from the fresh rebuild. 

One thing we need to understand is the finished piston clearance. If we reference the Service Manual for “specs” we find some interesting information. First, all through the manual you’ll find many references to “Service Limits” and seldom any reference to “Clearance”. In this one particular reference to piston cylinder clearance, we find both service limits and clearances.
The cylinder has a service limit of 62.10mm maximum 
The piston diameter has a service limit of 61.80mm
The piston to cylinder clearance is quoted at .031 - .058 mm (.0012” - .0022”)
 If the piston is at service limit and the cylinder is at service limit we have a difference of 62.10 – 61.80 = .3mm (.012”) 

We need to rethink the piston cylinder clearance quoted in the Service Manual. We have to remember that back when the manual was written, the cylinder machining practices left the cylinder with a rough finish and the rings had to “hone” that finish which removes material. So the clearance was purposely left tight. Now, if we Plateau Hone the cylinders, we have to leave a clearance for the oil film and expansion of the piston in the cylinder. Remember when I previously said “I know a few machine shops that won’t finish a cylinder with a .001-,002” clearance “? So they would finish to  a .003" finish and then allow the piston rings to hone the final which probably removed another .002” off of the cylinder.
That would be a final clearance of about .005”. 

So I would suggest if you plan on having a Plateau Hone job done, you should aim for .004- .005” of final piston cylinder clearance and call it a day. 

I hope this info is useful to those looking at a rebuild and look forward to comments here. I wish we had the ability to update the Service Manual but in lieu of that, we need to be able to find info like this for reference and that’s why I write this. 

KB