- Cylinder Specifications
- Zeroing the Micrometer
- Assembling the Bore Gauge
- Calibrating the Bore Gauge
- Marking Cylinders For Measurements
- Using A Spreadsheet To Record Cylinder Information & Measurements
- Honing The Cylinders With Brush Research Flex-Hone®
This bike has the Nikasil plated aluminum cylinders. Nikasil is a nickel matrix of silicon carbide that is extremely hard, wears very little and can be platted onto aluminum. That said, I want to measure the cylinders to determine if they are in good condition since the bike sat for about 25 years and had 64,000 miles on it.
I previously measured the cylinders on my 1973 R75/5 and you can see that procedure here.
However, the R75/5 cylinders have a cast iron liner embedded in an aluminum outer layer that has the cooling fins. This configuration is called ALFIN by BMW. You can read more about these two types of cylinders, and much more, here.
- Robert Fleischer: Cylinders (iron, steel, Nikasil, Galnikal), Boring, honing, cylinder shims, plates, gaskets, o-rings. Fitting to other years. Removing cylinder shims, ridge removal, measuring cylinder taper, installing new rings, etc.
On this bike, I will measure the bore, ovality and taper to see if these cylinders are still serviceable. They should be, but I want to be sure before assembling the top end.
I use the following precision measuring tools.
- a bore gauge, 2-6 inch range and micrometer with a 0.0005 inch graduation,
- a 3-4 inch digital micrometer with 0.00005 inch digital readout
- Brush Research standard duty Flex-Hone® ball hone with the following specifications they recommend for honing Nikasil cylinders:
- 3-1/2 inch (89 mm)
- Aluminum Oxide abrasive
- 240 Grit
- Brush Research honing oil.
Here is the digital micrometer I use. It can measure from 3 to 4 inches and the digital display can be set to inches (shown) or millimeters. It comes with a 3 inch gauge rod to zero the micrometer at exactly 3 inches.
Here is the a micrometer bore gauge kit I use. It can measure openings from 2 to 6 inches and has 0.0005 inch, or 1/2 thousandth, graduations. It is easy to estimate down to 0.00025 inch which is half way between the smallest graduations.
This is the Brush Research ball hone and honing oil. They offer the honing oil in a smaller size, but I bought way too much honing oil. 🙂 I ordered the 3-1/2 inch diameter hone with 240 grit aluminum-oxide (ALO) balls.
Here is a video showing how I do the work.
VIDEO: 1983 BMW R80ST Cylinder Measurements & Honing
I had the cylinders vapor honed and the markings on the base are quite visible.
The “08” indicates these are 800 cc cylinders.
The “B” indicates these are class B cylinders. BMW grades cylinders into three classes, A, B and C, based on the range of the bore size after the Nikasil plating has been applied.
I used the engine specifications from Duane Ausherman’s web site. I use the B class range for the bore, and the ovality and the taper values for this engine. I verified that the engine has B class pistons and I compute the piston clearance based on the average diameter of the cylinders.
Zeroing the Micrometer
I use a precision micrometer to calibrate the bore gauge to a known length. The micrometer comes with a reference standard 3 inch gauge rod used to zero the micrometer. I put the rod between the anvils and turned the smaller knob (not shown) on the end that has a built in clutch until it clicked indicating it was slipping.
Then I “zero” the micrometer to show 3 inches.
Assembling the Bore Gauge
I setup the bore gauge with the 3.2 inch rod and all the spacers (shown below). I tested the gauge inside the cylinder to be sure the dial gauge indicator moves, which it does. That confirms the length of the rod is long enough to measure the bore.
A screw cap locks the spacers and rod onto the end of the bore gauge foot.
Calibrating the Bore Gauge
I need to set the zero of the bore gauge dial indicator to a known distance that’s about where I expect the cylinder diameter to be. I use 3.3390 inches as that’s in the middle of the “B” size range; 3.3388-3.3392 inches. I set the micrometer to 3.3390 inches and lock the micrometer anvil in place with the small black locking handle above the digital readout.
I put the bore gauge in the micrometer getting it centered up and down, left and right between the anvils by rocking it.
Rocking it a bit shows where the minimum distance is and that’s where I set the outer ring so the zero mark on the face matches up with the needle of the dial gauge. This is fiddly and takes some patience. When the zero is aligned with the needle, I tighten the ring locking screw to hold the zero mark in place.
Marking Cylinders For Measurements
I want to measure how out of round the cylinders are, called “ovality” and how out of parallel the sides are, called “taper”. The specifications for ovality have two values, one at 20 mm (0.8 inches) from the top of the cylinder and the second at 115 mm (4.5 inches) from the top. I can use measurements at these two heights to compute the taper as well.
I insert a piston ring and square it up with the piston inside the cylinder bore. I push the ring down and check how far it is with Vernier calipers until I get it about 20 mm from the top. Then I use a Sharpie pen and draw a circle around the bore at that height.
I repeat this for the 115 mm depth.
I’m going to use four radial measurements to get a good estimate of the ovality. I mark the number of each measurement on the top and bottom of the cylinders so I record them consistently in a spreadsheet.
Using A Spreadsheet To Record Cylinder Information & Measurements
I use a spreadsheet to record specifications and measurements for the two cylinders. I enter the details about the bike, engine and specifications for the cylinder from Duane Ausherman’s document as shown below (You can click the image to enlarge it).
I record the following information:
- Make/Model: 1983 BMW R80ST
- Engine: R80
- Cylinder Class: B
- Piston Class: B
- Nominal Bore: 84.8 mm (3.3386 inch)
- “B” Cylinder Diameter Range: 84.815-84.805 mm (3.3392-3.3388 inch)
- “B” Piston Diameter: 84.775 mm (3.3376 inch)
- Max Ovality-0.8″ (20 mm): 0.005 mm (0.0002 inch)
- Max Ovality-4.5: (115 mm): 0.010 mm (0.0004 inch)
- Max Taper: 0.020 mm (0.0008 inch)
- Standard Piston Clearance: 0.040-0.030 mm (0.0016-0.0012 inch)
- Max Piston Clearance: 0.080 mm (0.0031 inch)
I record the bore gauge calibration: 84.811 mm (3.3390 inch) and the zero reading of the micrometer: 76.20 mm (3.0000 inch).
I also show a diagram with the numbered locations of the radials I use to measure the diameter of the cylinder.
Making The Measurements
I insert the calibrated cylinder bore dial gauge assemble into the cylinder so the two wheels are against the wall and the nose is against the other side of the wall and move the assemble down so the nose is on the line at 20 mm.
I rock the bore gauge slowly up and down and watch the dial indicator until I see the minimum reading. If the needle is above the zero, it’s a positive reading and if it’s below it’s a negative reading.
The value I read on the dial indicator is the deviation from the calibrated length so to get the actual diameter I add the dial indicator value to the calibration value of 3.3390 inches.
Left Cylinder Measurements
I record my measurements in the spreadsheet with a section for each cylinder. Below is the section for the left cylinder (You can click the image to enlarge it).
The “Position” columns labeled 1-4 show the measurements I made at each of the four radials. I make three sets of measurements at the 0.8 inch (20 mm) and the 4.5 inch (115 mm) depths. Then I compute the average of the four radial measurements in the “Ave” column.
The “Ovality” column subtracts the smallest measurement from the largest. The “Final” columns show the average diameter of the cylinder using the “Ave” measurement added to the calibration value of 3.3390 inches (84.811 mm).
The row labeled “Taper” is computed by taking the average of the three measurements at the 0.8 inch (20 mm) height and the average of the three measurements at the 4.5 inch (115 mm) height. Then I subtract the 4.5 inch average from the 0.8 inch average. The value in the “Ave” column is the average of the four taper column values.
For the “Max Ovality-0.8″” value I average the three ovality measurements at 0.8 inch and compare them to the specification. They are equal to the 0.0002 inch specification.
For the “Max Ovality-4.5″”, value I average the three ovality measurements at 4.5 inch and compare them to the specification. They are 0.0000 inches so well below the specification of 0.0008 inches.
For the “Piston Clearance” I use the piston diameter, 3.3376 inch (84.775 mm), and subtract the “Final” cylinder diameter, 3.3391 inch (84.8136 mm), from it. The clearance is 0.0015 inch (0.039 mm). This is within the “standard” piston clearance and is well below the maximum clearance value of 0.0031 inch (0.080 mm).
The left cylinder is in good condition.
Right Cylinder Measurements
I do the same for the right cylinder and the table below shows the results (You can click the image to enlarge it).
This is the summary of measurements for the right cylinder.
- Max Ovality 0.8″: 0.00015-0.00020 Inch
- Max Ovality 4.5″: 0.00005 Inch
- Average Taper: 0.0005 Inch
- Final Diameter: 3.3395 Inch
- Piston Clearance: 0.0019 Inch (0.048 mm)
The ovality is within specifications and so is the taper. The piston clearance is just above the maximum “standard” clearance of 0.0016 inch, but is comfortably below the maximum clearance of 0.0031 inch (0.080 mm). The average cylinder diameter, 3.3395 inch, is greater than the maximum for the B-class range, 3.3392 inch. But, since the ovality, taper and clearance are within specifications, the right cylinder is fine as well.
Honing The Cylinders With Brush Research Flex-Hone®
After 64,000 miles, the walls of the cylinder are glazed and this prevents the new piston rings from embedding tightly against the cylinder walls to create an oil tight seal. I use the Brush Research Flex-Hone® that is a ball hone to remove the glaze. The hone I use has 240 grit aluminum-oxide abrasive and is 3-1/2 inch diameter. I also use the Brush Research honing oil.
What I am actually doing is removing the glaze on the cylinder walls. The term honing is often used for this operation and also for the removal of material from the cylinder walls to get them to an exact diameter. Flat hones are typically used to remove material from the cylinder walls to get them to the proper dimension. That is why I’m using a ball hone as all I want to do is remove the glaze and not damage the thin Nikasil plating.
I use my variable speed portable drill. It has two speed ranges: range “1” is a maximum of 600 RPM and range “2” is a maximum of 1500 RPM. I use the lower speed range setting as Brush Research recommends using 500-800 RPM for the ball hone.
Since the nickel silicon carbide plating on the cylinders walls is very thin, I hone the cylinders only for about 15 seconds at two strokes per second to remove the glaze but not damage the plating. This is a short video from Brush Research that explains how to hone cylinders.
VIDEO: Learn How To Use The Flex-Hone In Minutes
Here is the before and after picture of the cylinder walls. The black circle in the before picture is from using a Sharpie to mark where I measured the cylinder diameter as explained above. The cross-hatch is still visible in the cylinders indicating they have not worn very much.