11 BMW 1973 R75/5 Measure Cylinders and Install Pistons & New Rings

This page shows how I measured the cylinders and pistons, honed the cylinders and set the end gap for new rings.

I measured the diameter, taper and ovality of the cylinders, the diameter and ring clearance in the pistons (gap between the goove in the piston and the piston ring), and the cylinder-piston clearance.

I used the following tools for this work buying most of them at Amazon.com (price shown in brackets) to add to my tool box.

  • a bore gauge, 2-6 inch range and micrometer with a 0.0005 inch graduation, ($49.95)
  • a 3-4 inch digital micrometer with 0.00005 inch digital readout, ($91.40)
  • a set of feeler gauges in 0.001 increments, (already owned)
  • a cylinder deglazer, honer tool for my drill ($26.28)
  • a piston ring filer ($57.57)

The parts I ordered are first over size rings (+0.5mm) which I got from Hucky’s

11 25 1 256 478 Rings, 1 oversize

Engine Specifications

I used the engine specifications from Duane Ausherman’s web site. There are three cylinder and piston diameter ranges, or size codes, denoted as “A”, “B” and “C”.

Cylinder Specifications

Cylinder Specifications

Piston Specifications

Piston Specifications

This bike has “B” cylinders installed.  You can find the letter stamped on the top of the cylinder at the corner of the base next to the block.

Location of Cylinder Size Mark--"B"--on Top Side, Near Base

Location of Cylinder Size Mark–“B”–on Top Side, Near Base

There are also large “A” and “B” marks on the under side of the cylinders in the center near the base. This is not the size marking.

Cylinder Group Mark "B" on Right Side

Cylinder Group Mark “B” on Right Side

The “B” pistons are stamped “81.975” on the top which is the nominal diameter in millimeters.

Piston Group "B" Size 81.935

Piston Group “B” Size 81.975

Piston Group "B" Size 81.935

Piston Group “B” Size 81.975

Pistons are oval, not round. So the piston diameter is the maximum diameter which is found across the piston skirt perpendicular to the wrist pin holes. This is sometimes called the thrust side of the piston. The side of the piston perpendicular to the wrist pin is the side pushed, or thrust, into the cylinder wall by the connecting rod as it goes around the crank. This is the side of the piston that usually wears more against the cylinder wall due to increased frictional forces.

Here is the digital micrometer I purchased. It can measure from 3 to 4 inches and the digital display can be set to inches (shown) or millimeters. It comes with a reference length to zero the micrometer at 3 inches.

Micrometer

Here is the a micrometer bore gauge I purchased. It can measure openings from 2 to 6 inches and has 0.0005 inch, or 1/2 thousandth, graduations.  It is possible to estimate down to 0.00025 inch which is half way between the smallest graduations.

Bore Guage

Zeroing the Micrometer

The  micrometer comes with a reference standard 3 inch 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.  There was a very small non-zero measurement displayed.

Zeroing Micrometer with 3 inch Reference

Zeroing Micrometer with 3 inch Reference

Note the amount shown here is 0.05 thousands of an inch.  This makes no difference to what I will be measuring, but I zeroed the micrometer anyway pushing the “Set” button until the display showed 3.00000 inch.

Setting the "Zero" on the Micrometer at 3 Inches

Setting the “Zero” on the Micrometer at 3 Inches

Assembling the Bore Gauge

The specifications for cylinder diameter indicate diameter “A” is 3.2283 inches and “B” is 3.2287 inches or 0.4 thousands bigger.  I setup the bore gauge needs to measure in the 3.2 to 3.25 inch range. I have a set of rods to select from, each 0.2 inches different in length and a set of spacers that are 0.01, 0.02 and 0.05 inches thick.

Spacers

Spacers

I use a 3.2 inch rod and 0.05 inch spacer so the maximum length of the rod is 3.25 inches.

3.2 Inch Rod

3.2 Inch Rod

The bore gauge is designed to compress the rod, so I want the initial length to be bigger than the largest diameter I anticipate for the cylinders.

The 0.05 inch spacer slides on the end of the measuring rod.

3.2 Inch Rod & 0.05 Inch Washer

3.2 Inch Rod & 0.05 Inch Washer

And a lock nut slides over the rod

Rod, Washer & Nut

Rod, Washer & Nut

to attach the washer and rod to the end of the bore gauge

Measuring Rod Installed on End of Bore Gauge

Measuring Rod Installed on End of Bore Gauge

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 3.228 inches as that’s a little less than the “A” size of 3.2283 inches. I set the micrometer in the rubber jaws of my vice so it’s snug (don’t over tighten the vice) and then set the micrometer to 3.228 inches and locked the anvils with the small black locking handle above the digital readout.

Setting Bore Gauge Reference Diameter on Micrometer

Setting Bore Gauge Reference Diameter on Micrometer

I put the bore gauge in the micrometer getting it centered up and down, left and right between the anvils by rocking it.

Setting Bore Gauge

Setting Bore Gauge in Micrometer

Rocking it a bit shows where the minimum distance is and that’s where I set the outer ring so the zero on the ring matches up with the needle of the dial gauge.  Loosen the locking nut on the outside of the dial (at the 2:00 position in the photo below) and then rotate the outside of the dial to align the zero mark with the needle.  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.

Zeroing Bore Gauge

Zeroing Bore Gauge

Measurement Accuracy

The specifications require measurements that are accurate to less than 0.5 thousandths of an inch. That’s a fine art. I’m not confident I can accurately measure at that distance. Metals expand and contract with temperature. The coefficient of thermal expansion (CTE) tells how much change occurs and is in units of in/in/Deg-F. The longer material is, the more the overall expansion will be per unit change in temperature: a 1 inch length of steel changes less than a 3 inch length for the same temperature difference. Different metals have different CTE. Steel is approximately 0.000007 in/in/Deg-F while aluminum is approximately 0.000012 in/in/Deg-F or about 1.7 times as much expansion per degree of temperature change.

I calculated the change in diameter of a cylinder with a diameter of 3.2273 in. for 10 Deg-F difference is about 0.7 thousandth, while the aluminum piston change in diameter is about 1.2 thousandths.  (I used the circumference (Pi X D) as the length of the cylinder/piston, multiplied by 10 and by the CTE.)  I also did a test of the change in cylinder diameter for 10 Deg-F and got about 0.5 thousandths of an inch difference.

If I am going to get a good estimate of piston-cylinder clearance, I need the cylinders and pistons at the same temperature. I used an infrared temperature gun to ensure everything was at the same temperature, (+/-) a degree or two, before I measured them.

Checking Cylinder and Piston Temperatures

Checking Cylinder and Piston Temperatures

Measuring the Cylinders

One end of the bore gauge with the little wheels has a small piston with a rod on the end that moves in and out.  Tilt the bore gauge so it is at an angle and slide it into the cylinder and then rotate the gauge vertical so it goes across the diameter of the cylinder.

Measuring Cylinder Bore Diameter

Measuring Cylinder Bore Diameter

The trick is to get the measuring rod perpendicular to the bore and vertical in the cylinder. I rock the gauge back and forth watching the dial indicator until it shows the smallest reading. I note how far from the zero mark the needle is. If the needle is below the zero (counterclockwise rotation), the measurement is larger than the zero mark (+) and if it is above the zero (clockwise rotation), the measurement is less than the zero mark (-). The larger ticks are 0.001, or one thousandth of an inch, and the smaller ticks are 0.0005, or one-half thousandth of an inch.  I can estimate as small as 0.00025 inch between two of the small ticks.

Rock Bore Gauge for Minimum Distance

Rock Bore Gauge for Minimum Distance (+3.25 thousandths)

Rock Bore Gauge for Minimum Distance

Rock Bore Gauge for Minimum Distance (+2.5 thousandths)

Rock Bore Gauge for Minimum Distance

Rock Bore Gauge for Minimum Distance (+3.0 thousandths)

I take measurements at the top, middle and bottom of the polished surface of the bore and  at different radials around the circumference. Looking down on the cylinder as if it is a watch, I measured along the 12:00-6:00 o’clock radial and the 2:30-7:30, 3:00-9:00, and 4:30-10:30 radials.

I put a piston ring in the cylinder squaring it up with a piston and then drew a circle with a sharpe pen at the top and bottom of the polished section of the cylinder which is where the rings move up and down the bore.

Sharpe Marker to Measure Top & Bottom Cylinder Diameter

Sharpe Marker to Measure Top & Bottom Cylinder Diameter

I put the bore gauge on the top line and then moved down to get the middle and bottom measurements. Then I rotated the bore gauge 45 degrees to the next radial and repeated the measurements.  I took measurement three times and use the average for the estimated diameter. I write down the dial gauge readings and then use a spreadsheet to compute the final diameter based on the 3.228 inches I set as the zero for the dial gauge.

Measuring the Piston Diameter

I used the micrometer to measure the piston diameter. I measured at the widest diameter (perpendicular to the wrist pin hole) taking a number of readings towards the bottom of the piston skirt about a 1/2 inch or so up from the bottom.  I write them down and I’ll use a spreadsheet to average them.

Measuring Piston Diameter on Thrust Side of Skirt

Measuring Piston Diameter on Thrust Side of Skirt

Doing the Math

I set up a spreadsheet to do the math. I wanted to determine a couple of things. For the cylinders, the ovality or maximum difference in diameter around the cylinder, the taper, or maximum difference in diameter from top to bottom and the average diameter to compute the piston ring gap. For the pistons, the average diameter at the widest point.  Here is my spreadsheet.

Cylinder & Piston Measurements

Cylinder & Piston Measurements

At the top I show the specifications for the “A” and “B” class pistons in inches and millimeters and I show the conversion from millimeters to inches (1 millimeter is 0.0394 inches). I recorded the “Zero” distance I used to set the zero mark on the bore gauge.

Next, I show the measurements I took from the dial indicator on the bore gauge. Each row, Top, Mid, Bottom, indicates the vertical location and the 1, 2, 3, 4 columns correspond to that point around the circumference of the cylinder as indicated by the “1”, “2”, “3”, and “4” shown in the diagram between the Left and Right cylinder measurements.

The Ave column is the average diameter around the circumference (sum the measurements divided by the number of measurements).

The Ovality column shows the maximum difference in diameter I measured around the circumference for each vertical location.

The Taper row is the largest difference in diameter from top to bottom for each radial around the circumference.

The Final column adds the Zero distance to the Ave dial gauge reading and is the average diameter at that height in the cylinder.

I show the Max Ovality from the specifications, 0.0004 inches, and compute the difference between the largest ovality (0.00025) and the max and show that as Remaining Ovality (0.00015 for the left and 0.00015 for the right).

The piston diameter measurements are shown in a column with the average value at the bottom next to the Ave Diameter label. I show the average of the cylinder measurements to the left (add the averages for each height and then divide by three).  I compute the difference between the average cylinder diameter and average piston diameter and show that as the Piston Clearance.  The Max Clearance is from the specifications and Remaining is the difference between the max clearance and what I computed for piston to cylinder clearance. (0.0014 inch, Left and 0.0012 inch, Right).

The right side cylinder has more variation in diameter than the left and has more taper than the left. Both tapers are greater than the specified maximum of 0.0004 inches, or 4 ten-thousandths of an inch. That said, the bore gauge measurements I took aren’t 100% accurate.  What I am sure of is that there is taper in the cylinders and the taper is greater for the right than the left. But I don’t trust the actual values I measured are accurate enough to cause any concern.

The engine specifications indicate that boring to 1st oversize adds 0.02 inches to the cylinder diameter. I don’t see anything to indicate the need to bore out the cylinder and add oversize pistons. But there is cylinder wear. So rather than use standard rings and risk them having too large a ring end gap when installed, I bought 1st oversize rings and will grind them to achieve the specified ring end gaps. That will have to do for now.

Cleaning the Pistons

Initially, I used a wire wheel to clean the carbon off the top of the piston. Then, I soaked them in a strong solution (2:5 to 1) of KleenTec 600C water-soluble degreaser I got at NAPA.  Some folks use concentrated Simple Green to soak them in and there may be other solutions sold specifically for removing carbon and varnish from the pistons.

I let them soak for several days and each day I used a tooth brush and stiff nylon brush to remove carbon and varnish on the skirts, the top and inside the piston ring grooves.  All told, the pistons soaked for about 3 days.

Pistons After Soaking and Toothbrushing

Pistons After Soaking and Toothbrushing

Cylinder Honing and Glaze Breaking

The cylinder bores were polished to a mirror shine after 97,500 miles. Very little of the original hone marks, or cross-hatching was left as shown below. The third picture shows the original honing and cross-hatching at the bottom of the cylinder whete the piston does not touch the cylinder.

Polished Bore with Minimal Cross Hatch Showing

Polished Bore with Minimal Cross Hatch Showing

Top of Cylinder with Minor Ridge and Polished Bore

Top of Cylinder with Minor Ridge and Polished Bore

Bottom of Bore with Original Cross Hatching

Bottom of Bore with Original Cross Hatching

I attached the hone/deglazing tool to my cordless drill.

Drill with Honing Tool

Drill with Honing Tool

This hone is designed for bores from 2 to 6 inches and has an adjustment ring at the base to add or remove tension on the arms. I adjusted the tension to the minimum since the cylinders are a bit over 3 inches. I put motor oil in the cylinder and on the stones of the hone.

Oil in the Bore Before Honing

Oil in the Bore Before Honing

I squeezed the arms closed and placed the hone in the cylinder to avoid creating vertical scratches.

The cordless drill is variable speed. I want to get a cross-hatch pattern that has 45 degree overlapping swirls. I have to move the hone up and down the inside of the cylinder while the honing stones are spinning to get the cross-hatching, so I use a very slow speed so the hone doesn’t turn around very much as it is moving up and down the bore.

The Hone Iside the Cylinder

The Hone Iside the Cylinder

Here is the cross-hatch pattern I achieved. If you look closely you will see swirl marks that cross each other at 45 degrees and the mirror finish and glaze is roughed up.

New Cross-Hatching In the Cylinder

New Cross-Hatching In the Cylinder

This ensures the new rings are scored by the ridges of the cross-hatching creating a labyrinth-like seal on the outside surface of the ring. This helps the rings seal against the cylinder wall minimizing oil consumption.

Gaping the New Rings

I bought one over size rings (+0.5 mm) to compensate for the wear in the cylinder. This is the package with one set of rings I received from Hucky’s

New Rings, 1 Over Size from Hucky's

New Rings, 1 Over Size from Hucky’s

New Rings in Packaging: Left, Bottom Oil Ring; Right, Top Ring

New Rings in Packaging: Left, Bottom Oil Ring; Right, Top Ring

I grind the end of the rings with the ring grinder to set the gap to the specification. Each ring has its own gap specification:

  • Top                 0.30-0.45 mm
  • Second          0.30-0.45 mm
  • Oil                  0.25-0.40 mm

The rings are marked on one side with “Top” indicating this is the top side of the ring when installed in the piston. If you look closely at the rings, you will see they have a profile and the cross section is not a rectangle.

Top of Ring Stamped on Ring

Top of Ring Stamped on Ring

Top Ring Profile: It is Not Rectangular

Top Ring Profile: It is Not Rectangular

The ring grinder has a circular grinding stone mounted on a base with two pins near the top.

Ring End Grinder Tool

Ring End Grinder Tool

The rings go into the ring grinder with the Top side of the ring facing up and the end opposite the end with the Top marking placed against the grinding stone. The ring is pushed up against the two pins to keep it square to the grinding stone. The crank is turned so the stone rotates from the outside edge of the ring to the inside edge, or counter-clockwise as you face the crank. This is backwards from my normal tendency when turning something so I keep reminding myself which direction to turn the crank as I go.

Positioning the Ring for Grinding

Positioning the Ring for Grinding

Before I started grinding, I test fit the ring into the cylinder. Insert it in the middle with the gap facing up and then push down and rotate the ring into the bore. Since it is one over sized, I can’t rotate it so it is square in the bore as the gap is closed up. But I want to see how much of an incline there is when the gap is closed so I can judge how much material I have taken off and how much more I need to take off. I don’t want to grind too much off the end of the ring or the gap will be too big. That means I buy another set of rings at $65 🙁

Inserting Ring in Cylinder

Inserting Ring in Cylinder

I started out turning the crank counter-clockwise 15 times and then removed the ring to test the fit. Before inserting it, I felt the ends of the cut edge of the ring for a sharp bur and used a Dremel grinding disk by hand to remove it so I wouldn’t scratch the bore when I insert the ring.

Removing Bur with Dremel Grinding Disk

Removing Bur with Dremel Grinding Disk

Each time I test fit the ring, I could see how much more I could tilt the ring into the bore. I kept taking 15 turns of the crank off the end until I could get the ring to just fit into the bore.

Then I reduced the turns to 10 and test fit again using an inverted piston to push the ring square into the bore lining up with a groove in the piston at the top of the cylinder.

Squaring Ring in Bore with Inverted Piston

Squaring Ring in Bore with Inverted Piston

As the gap opened up, I used my feeler gauges, to measure the gap. My gauge set is in 0.001 inch but shows the mm as well. I used the 0.012 and 0.013 inch feelers (0.305 and 0.330mm).

Feeler Gauges: Just Right and Too Big

Feeler Gauges: Just Right and Too Big

The goal is to just fit the 0.012 in. gauge in the gap but not be able to fit the 0.013 in. gauge.

As soon as the gap opened up with the ring square in the bore, I reduced the turns to 2 turns until the gap was just a bit smaller than the 0.012 in. gauge. Then I only did 1 turn until the gauge could just enter the inside of the gap. At than point I turned the crank only 1/2 turn and finally about 1/4 turn until the gauge just slipped in and pressed against the cylinder.

Measuring Ring End Gap: Perfect Size

Measuring Ring End Gap: Perfect Size

I couldn’t get the 0.013 inch feeler to go in. So, good to go for the top ring.

I used the inverted piston to push the top ring down further into the bore and remeasured to be sure the 0.012 inch still fit but the 0.013 inch wouldn’t. Then I left the top ring in the cylinder and started to work on the second ring and then the oil ring. In total, it took about an hour and a half to gap the rings for one cylinder. I was in no hurry.

I keep the rings in the cylinder they are ground to fit with the top ring at the bottom and the oil ring at the top. That way I won’t get them mixed up and when I’m ready to put them on the piston. And, they are in the order I need to put them on the piston, bottom ring (oil) first and top ring last.

Gaped Rings Stored in Cylinder

Gaped Rings Stored in Cylinder

22 thoughts on “11 BMW 1973 R75/5 Measure Cylinders and Install Pistons & New Rings

  1. You must have done some research to decide on the tools you needed. This is inspiration for me! I just picked up my restored & repainted headlight bucket. I am hoping your website is still up when I begin to work on my own R75/5 project (1972 red toaster). I will be following your progress with the 1973 R75/5 much more closely come winter… the winter after that I shall be retired and will have some time to get started.

    Regards,

    Ken Cruise

    • Hi Ken,

      I looked at the specifications Duane published and realized I had to be at 1/2 thousandth to be useful. The measuring tools were not premium priced IMHO. They would not be a good choice for a machinist with daily use, but I think they are adequate for a “hobbiest” such as I.

      I have no plans to retire my site and have been creating web pages as I go so folks like you can take advantage of what I learn.

      Best of success with the R75/5 project this winter. I’m hopeful I may get this project done by late fall depending on how the painting goes when I do it.

      Best.
      Brook.

  2. I’ve enjoyed reading your posts, Brook. What did you do to clean out the metal dust after the hone? Did you experience any cylinder smoking before you tore down the bike?

    Thanks,
    Josh

    • Hi Josh,

      Thanks for dropping by.

      I oiled the cylinder and hone so the metal filings were trapped in the oil. I cleaned them with blue shop towels and then rinsed them in the parts washer. You could use soap and water with towels as well.

      This bike has been parked for six years. If memory serves, it wasn’t using a lot of oil.

      Best.
      Brook.

  3. Pingback: 1973 BMW R75/5 Rebuild: Measuring Cylinders, Pistons and Installing New Rings | Motorcycles & Other Musings

  4. Having carefully measured everything as you described and building a spreadsheet as you did, I am ecstatic at the results. I plan on honing the bores before assembly but wonder how much, if any, the bores will change from my current measurements.

    • Hi Picinisco,

      I found no real measurable difference after honing. The measurement error was on par with the roughness of the honing.

      I’m glad this approach worked for you.

      Best.
      Brook

  5. Nice writeup. I am redoing a 90/6. I did not think about setting the dial bore indicator and then using it to find dimensions over or under the setting. I just made a spreadsheet as you suggest. Ain’t the internet great?! Thanks.

      • Hi again. Just thinking about cylinder grades (a, b, or c). Like you i have two cylinders with a raised “A” and “B” but both mine have a hammer-stamped “C” in a bottom corner. So i am thinking that these are “c” grade cylinders and the raised A and B are identifying left and right cylinders. My “A” cylinder has a casting number on the fins of 731-2-1182 and my “B” cyl is 731-1-1182. Could not confirm this via google. What do you think?

        • Hi Kent,

          Thanks for stopping by and your note. I think you are correct about where the bore size code is located, which is at the corner of the cylinder base on the top side of the cylinder. I believe the pictures I show of the size codes is incorrect and is the larger letter cast into the middle of the bottom side of the cylinder. Haynes shows a picture of the size code where you found it on your cylinders.

          I’ll update those pictures when I get the chance to take some new photos.

          Thanks for pointing out this error. I appreciate it.

          Best.
          Brook.

  6. Hi. I have a general question about your rebuilds. I noticed you apply blue Hylomar to the bottom gasket when assembling the cylinders but didn’t see you apply any compound to the head gasket. when I stripped my R80 down I found no gasket compound whatsoever? Did you apply compound to both sides of the cylinder gasket and the same for the heads?

    • Hi Steve,

      Thanks for reading about this project.

      I don’t use any sealant on the head gaskets or the valve cover gaskets. I believe the valve cover gaskets have a heat activated adhesive that makes them stick to the head side, not the cover side.

      I used Hylomar on both sides of the base cylinder gasket. You can see the procedure I used to assemble the top end in this write-up.
      11 BMW R75/5 Install Engine Top End

      –>Note, most of the airhead Guru’s no longer recommend Hylomar for base gaskets. For example see this article from Bob Fleischer (aka Snowbum).
      http://bmwmotorcycletech.info/chemicalsetc.htm

      Skip to the “Sealants” section for his view on sealants and what to use where,
      Skip to the “Cylinder base sealants” section for specific recommendations.
      Skip to the “General sealants notes” which lists what NOT to use a sealant on and includes the head gasket.

      I hope this helpful to you.

      Best.
      Brook.

  7. Hi Brook,have 1976 R100S,it says in the Haynes manual that the cylinder barrels are identical(i have Kolben schmidt markings on mine),at the moment i have yet to find out which barrel went on which side,how important is this?,and what would be the consequences of fitting them incorrrectly? Ian

    • Hi Ian,

      Well, you have a 50% chance of putting them back on the side they came from. 🙂

      You don’t say if you are replacing rings and/or honing the cylinders. If so, the rings will wear into the new cross-hatch and seal fine to whatever cylinder they go into. Other than that (which in the grand scheme of things is a minor thing), I’d not worry about it.

      I put masking tape on the top studs (the one the nuts at the 12:00 and 6:00 positions go on) of mine before I slide them off so as I inspect them, should I find anything, I can correlate it one side or the other.

      Best of success on your project.

      Best.
      Brook.

  8. Thanks for the advice Brook,will have another go at being Sherlock Holmes,and failing that i have some souvenir dice from Las Vegas. : )

    • Hi Steve,

      Although I didn’t do that measurement, it’s worth taking it. The ring grooves do need to be clean of all carbon and deposits and they can become enlarged.

      Hope this helps.

      Best.
      Brook

  9. I believe that the cast-and-raised A and B on the jugs refer to the engine-side as this is common to all airheads. Another poster above asked about it as well. The size-range for the A, B, C is stamped on the base, in a place that can be read with the jugs installed.

    Very nicely done, thank you. I have earmarked your postings for consultation as I tear into my own!

    • Hi Bob,

      Indeed, my photo was of the wrong marking. Thank you for clarifying this.

      I’m glad this material is helpful.

      Best.
      Brook.

  10. Brilliant tutorial. But I’m curious about your bore gauge. I’ve never seen one with those little wheels. I’m more accustomed to using a Fowler dial gauge. Would this not achieve the same effect?

    • Hi Matt,

      I’m pleased this was helpful to you.

      The wheels are an aid for aligning the gauge so it is centered in the bore. You rock the gauge up and down looking for the minimum measurement which occurs when the gauge is exactly horizontal in the bore. So, the wheels eliminate one worrying about one of the two planes you have to get the gauge centered in.

      In theory, this tool is more precise than a snap gauge and measurement transfers to a micrometer.

      Best.
      Brook.

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