Measuring, Honing and Silicon-Lapping Cylinder Bores
Matching Pistons to Crankcase
Using the engine identification code found on the cylinder block, use the following tables for piston to cylinder matching, refer to Identification. Application and ID
Matching Pistons To Crankcase With Numerical Identification
119.960/970/972/974:
119.971/975/981/985:
Matching Pistons To Crankcase With Letter Identification
119.980/982/970/972/974:
119.971/975/981/985:
Piston Clearance
Piston Clearance
When new 0.004 - 0.026 mm
Wear limit 0.080 mm
Maximum wear limit of cylinder bores in
direction of travel or transverse direction
at top and bottom reversal point of
top piston ring 0.100 mm
Machining Tolerances
Permissible variation of:
Cylinder Shape:
When New 0.013 mm
Wear Limit 0.050 mm
Rectangularity related to cylinder height 0.050 mm
Averaged peak-to-valley height (Rz):
After polishing 0.001 mm
After silicon-lapping 0.001 - 0.003 mm
Permissible wave depth (Wt)
After silicone-lapping 50 % of peak-to-valley height
Chamfer of cylinder bores see step 14 below*
* Step 14 of "Honing and Silicon-Lapping" procedure, within this article.
NOTE: See piston, matching and dimensions for further tolerances and assignments. Specifications
NOTE:
- Use only the equipment and materials listed above or their exact equivalent for light alloy cylinder bore refinishing.
- The light alloy cylinder bores are very sensitive to damage, scratching and also dirt and must therefore be handled with great care.
- The cylinder bores should be adapted when honing to the sizes of the available repair size pistons with the Group No. 0, 1 or 2.
- The specified piston clearance must be observed.
Measuring
Use a measuring instrument with measuring point pressure relief for measuring the cylinder bores as the cylinder barrel will otherwise be scored by the measuring point tip and the measuring point tip of the instrument will wear prematurely.
Set the self-centering internal measuring instrument to the cylinder dia. before measuring. Measure the cylinder bores at a room temperature of 22 - 24°C.
Following silicon-lapping (final state) it is only possible to measure the amount by which the aluminum surfaces stand back between the silicon crystals (0.5 - 1.5 micro meters) indirectly by means of the averaged peak-to-valley height (Rz).
The specified exposed depth of the silicon crystals results from the limited-time control during silicon-lapping and from the silicon paste used.
Honing and Silicon-Lapping
Where severe scoring and severe cylinder wear exists (greater than 0.100 mm) etc., the cylinder bores can be honed to the stated repair sizes. The silicon crystals must be undamaged and be flattened on the cylinder surface following the honing operations.
After the honing operations (prehoning, final honing and polishing) it is essential to perform "silicon-lapping" based on the Sunnen method so that the silicon crystals are exposed.
CAUTION:
- If The "SILICON-LAPPING" Operation Is Not Performed, This Will Inevitably Result In Piston Seizure.
- The following operations can only be performed with a honing machine with honing oil filter and oil cooler.
- The silicon particles must be filtered out of the honing oil.
- The honing oil must be cooled to a constant temperature of 20°C to avoid excessive heating of the crankcase.
- Only the specified honing oil may be used to achieve good honing quality.
1. Prepare honing machine.
- Perform all the honing and lapping operations without the direction guide shoes.
- The honing stone runout (arrow) size x:
Engine 119.96: 14 mm
Engine 119.97: 13.6 mm
2. Cut off projection of bronze bars at bottom of main guide shoe (arrow) as only approx. 14 mm undercut for the honing stone runout is available at the cylinders.
3. The specified stone sets of 89 mm length must be shortened to 70 mm.
- Only the honing stone, not the holder, should be cut off at the top with a hacksaw.
- Size a = 70 mm
4. Tighten crankshaft bearing cap bolts and nuts to 50 Nm in the order of the lightening diagram.
5. Fix disassembled and cleaned crankcase with crankshaft bearing caps installed and tightened in the honing machine with the square steel bar (1).
6 Set stroke setting (cylinder length) on the stroke scale (arrow) according to the "prehoning" table shown below.
7. Set stone projection (arrow) with the gauge according to the table shown below.
8. Set feed (arrow) according to table.
9. Set strokes/min. and rpm according to table shown below.
Prehoning Table:
10. Prehone all the cylinder bores at full honing oil feed up to approx. 0.08 mm before final size otherwise the silicon crystals will be torn out or destroyed by the cutting pressure.
NOTE:
- A deviation of approx. + 0.02 mm is obtained when measuring immediately after prehoning as a result of healing.
- The heating of the crankcase is dependent among other things on the outside temperature.
11. Insert stone set for final honing and cut to length as described in step 3 above.
12. Set honing machine according to the "Precision honing" table.
Precision Honing:
13. Precision-hone all cylinder bores at full honing oil feed up to approx. 0.02 mm before final size.
NOTE:
- A deviation of approx. + 0.01 mm is obtained when measuring directly after precision-honing as a result of heating.
- Heating of the crankcase is dependent among other things on the outside temperature.
14. Chamfer cylinder bores according to the sketch prior to "polishing".
15. For chamfering, use a suitable hand milling cutter with an angle as shown in the illustrations above.
16. Insert stone set for polishing.
- New polishing stones should be corrected in the tightest cylinder bore before starting machining.
Polishing Table:
17. Set honing machine according to "polishing" table.
18. Polish all cylinder bores at full honing oil feed up to end size.
19. Allow crankcase to cool.
20. Measure cylinder bores.
- When performing this step, allow for the required cylinder dia. (Group No.) for the existing pistons and the specified piston clearance. Application and ID
- Re-polish once again if necessary.
21. Clean cylinder walls with filtered honing oil so that all silicon particles are removed and do not cause any scratches during the subsequent silicon-lapping operation.
22. Press felt inserts into the holders and the latter into the stone holders.
- After pressing in the felt inserts, remove all material deposits on the holder.
23. Perform setting of cylinder diameter with the setting gauge.
24. Soak felt inserts with filtered honing oil and insert into the honing head.
25. Shut off honing oil feed.
Silicon Lapping:
26. Set honing machine according to "Silicon Lapping" table.
27. Thoroughly stir AN-30 silicon paste and then rub into the dry cylinder walls fully.
28. Likewise apply silicon paste to felt inserts.
29. Introduce honing head with felt inserts into cylinder bore.
30. With honing machine running, slowly turn the feed wheel to the right until the indicating instrument has reached 30 %.
31. Set feed scale to 18 graduations.
- The honing machine switches off after approx. 70 seconds.
- The cylinder surface then has a mat appearance.
- No honing angle is visible.
- The peak-to-valley height is 0.001 - 0.003 mm.
32. Thoroughly clean all silicon paste residues from cylinder bores with filtered honing oil and a suitable soft-bristle brush and dry cylinder bores.
NOTE: Used silicon paste must not be re-used!