- This bike has 97,500 miles on the engine.
- The cylinders and pistons are original size, not bored out.
- The left cylinder is an "A" group, the right cylinder is a "B" group
- The pistons are "B" group, with 81.975 mm stamped in the top
- I have the cylinders and pistons on the workbench.
- I can see that the bore is smooth with minimal cross-hatch remaining.
- I can see at the top of the cylinders, there is a band of less polished bore.
- I am replacing the rings (I checked original rings and top ring gap about 2x max)
- I bought 1st oversize rings. I will use a ring end grinder to set the end gap to specifications
- I will hone the bores to create new cross-hatch to seat the new rings
I've read about removing the ridge at the top of a cylinder when replacing the rings. What I have read states the ridge is due to less wear of the bore at the very top of the stroke leaving more metal creating a ridge. How much more metal is too much I haven't been able to find out.
I've have also read some statements that ridge reamers can inflict damage on the bore if not well made and properly used. One piece of advice was "never use them", as a ridge that would harm a new set of rings indicates there is so much wear you need to re-bore the cylinder.
1. Ridge reaming advice?
- Skip it.
- Do it
* Good ridge reamer tool recommendation
I've also took some measurements of the cylinder bore and the pistons. I used a bore gauge with dial micrometer (0.0005 inch minimum graduation) and 3-4 inch digital micrometer (0.00005 inch minimum measurement). I measured at the top, middle and bottom of the cylinders and at 4 different directions across the bore at each height. I measured the pistons on the wide side (perpendicular to the wrist pin hole) about 1/2 inch from the bottom of the skirt.
These initial measurements show the cylinder-piston clearance is okay, the ovality is okay, but the taper is just above the maximum of 0.0004 inches on the left cylinder (0.0005) and as much as 0.00025 above in one direction on the right cylinder (0.00075).
Here is a link to my spreadsheet summarizing the initial measurements.
1. Would ridge reaming remove enough metal to bring the taper back to specifications?
- the maximum taper is 4 ten-thousandths of an inch (0.0004), and that's small.
Dumb Question #467
Is the BMW specifications for cylinder-piston gap the difference between the diameter of the cylinder and the diameter of the piston, or the distance from one side of the piston to the cylinder, which would be the (Dia. of Cylinder - Dia. of Piston)/ 2.
Any one have recommendations for who I would send the cylinders to for honing, or boring out, preferable in the Metro-Denver, Colorado area?
Thanks for the collective wisdom.