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Pistons with Reduced Compression HeightWhat Does Compression Height Mean?

The compression height is the dimension between the piston head and the centreline of the piston pin (Fig. 1). This is a signifi-cant measurement when over-hauling engines as it ensures that the piston is positioned at the cor-rect distance from the cylinder head at the top dead centre, so optimising the combustion process. MSI have an extensive range of pistons with reduced compression height to facilitate professional engine reconditioning. A table of the fast moving parts is shown on pages 3 and 4. Details are provi-ded in the latest KS Catalogue „Pistons/Cylinders/Kit Sets“.

Compression and Piston Protrusion

Diesel engines have a much higher compression than petrol engines due to their structure. In order to achieve this high compression, the clearance - i.e. the distance bet-ween the cylinder head and the piston top dead centre - should be as small as possible. When design-ing the engines, the desired com-pression is achieved by allowing the piston to protrude a certain distance beyond the face of the engine block in the top dead cen-tre. This is then referred to as pis-ton protrusion (dimension „C“ in

Fig. 2). In spite of this protrusion, the piston does not hit against the cylinder head because the cylinder head gasket also has a specific material thickness and the clear-ance between the piston and the cylinder head is still maintained. Figure 3 shows how the piston protrusion is measured.

Why Pistons with Reduced Compression Height?

Pistons with reduced compression height are always necessary whenever the sealing surface of the cylinder block on diesel engines, especially truck engines, is refinished in the course of over-hauling the cylinder block. This work reduces the distance from the sealing surface to the crank-shaft. The piston height must consequently be reduced by the same amount that was removed by the remachining work on the cylin der block. If a piston with a standard com-pression height is still used despite this work on the surface of the cylinder block, the clear-ance between the piston and the cylinder head is frequently no longer warranted in the top dead centre. As long as the engine is still cold, the clearance is often sufficient, so that when the engine is turned by hand the piston does not necessarily hit against the

Fig. 1

Fig. 2

Fig. 3

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cylinder head. At operating temperature, however, the piston becomes longer due to the natural thermal expansion of the material, thus reducing the clearance and causing the piston to strike

against the cylinder head when the engine is running. The vibra-tions also affect the injection noz-zles. The result is that the nozzles cannot withstand the pressure and the fuel injection is no longer

controlled. Consequence of this is an abnormal combustion with severe damage to the piston and/or fuel overflow, increased piston ring and cylinder wear, and high oil consumption.

Machining the Piston Head to Size

is a method that is still widely used to adapt the compression height of a standard piston to the special requirements of engine reconditioning. At the same time, however, machining the piston head to size alters the geometry and the volume of the combustion bowl and of the valve pockets. This frequently causes problems affecting the free running of compo nents and the compression. New engines that are subjected to stringent exhaust gas ratings for the purpose of observing national and international emission regula-tions react very sensitively to even minor alterations in the shape of the combustion space. Increased pollutant emissions, higher fuel consumption and un satisfactory engine perform-ance are the results. The reason for this is that the shape and injection angle of the injection nozzles are structurally coordina-ted with utmost precision to the geometry of the combustion bowl. The injection nozzle, the piston prechamber and the compression together constitute a combustion system, as it were. By altering this system, the correct function - that has been harmo-nised in the course of endless tests and series of processes - is questionable. Piston damage frequent ly occurs in such a case because, once the piston has

been machined to size, the jet of injected fuel hits against the edge of the combustion bowl that is subjected to higher heat stress.

Pistons with Anodised Piston Heads

are utilised to enhance thermal resistance (Fig. 4). In the course of machining the piston head to size, the anodic layer is partly or completely machined off and the piston no longer has the neces-sary strength and heat resistance to rule out cracking. Pistons with anodised piston heads can be identified in the KS Piston Cata-logue by „Lox“ in Column 4.

Attention:

For the above reasons, we urgently advise

against conducting resurfacing work on the piston. MSI reject all liability, in particular where the liability for defects is concerned, for damage arising as a result of piston modifications.

Fig. 4

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MAN 102 93 721 600 77,30 -0,2 mm

MAN 102 93 722 600 77,10 -0,4 mm

MAN 108 91 075 600 72,70 -0,2 mm D 0824 GF, -GF 01, -GF 02, D 0826 F, -F 01, -F 02, -FO, -

FO 01, -FO 02, -FR, -FR 01, -FR 02, -GF, -GF 01, -GF 02, -

GF 03, -GF 04, -GFA, -GFA 03, -GFA 04, -OH 01, -OH 02,

-OH 03,

MAN 108 93 138 600 72,80 -0,2 mm D 0826 LE 51, -LE 52, -LFL 10, -LUE, -LUE 130, -LUE 160,

-LUE 51, -LUE 52, -TUE 51,

MAN 108 94 413 600 72,80 -0,2 mm

MAN 108 94 414 600 72,60 -0,4 mm

MAN 108 94 415 600 72,40 -0,6 mm

MAN 108 90 049 600 75,20 -0,2 mm D 0826 K, -LF217, -LOH 01, -T, -TF, -TOH 01,

MAN 108 90 050 600 75,00 -0,4 mm

MAN 121 92 988 600 93,60 -0,4 mm D 2156 6 U, -HM 2, -HM 3, -HM 6, -HM 9, -HMN 3, -HMN

6, -HMN 8, -HMN 9, -HNY, -MGN, -MNY, -MY, -MYN,

MAN 125 92 994 600 89,67 -0,2 mm

MAN 125 92 995 600 89,47 -0,4 mm

MAN 125 93 275 600 89,60 -0,2 mm

MAN 125 93 329 600 89,40 -0,4 mm

MAN 125 93 405 600 89,20 -0,6 mm

MAN 125 93 553 600 89,67 -0,2 mm

MAN 125 93 554 600 89,47 -0,4 mm

MAN 128 90 336 600 89,60 -0,2 mm

MAN 128 90 337 600 89,40 -0,4 mm

MAN 128 90 482 600 89,20 -0,6 mm

MAN 128 90 342 600 89,67 -0,2 mm D 2866 E 20, -F, -FOH, -OH, -U, -UE, -UH, -UM,

MAN 128 90 584 600 89,55 -0,2 mm D 2865 LF, -LF 02, -LF 03, -LF 04, -LF 05, -LF 06, -LF 10, -

LF 14, -LF 15, -LOH, -LU 01, -LU 02, -LU 03, -LU 04, -LU

06, -LUH 02,

D 2866 KF 09, -LF 02, -LF 03, -LF 04, -LF 05, -LF 06, -LF

07, -LF 09, -LF 10, -LF 15, -LFG, -LOH 02, -LOH 20, -LOH

21, -LOH 22, -LOH 23, -LOH 24, -LOH 25, -LOH 26, -LOH

30, -LOH 31, -LU 01, -LU 03, -LU 04, -LU 05, -LU 09, -LX, -

LXE 30, -LXFG, -LXOH, -LXU, -LXUH,

Mercedes-Benz 87 94 674 600 44,65 -0,2 mm

Mercedes-Benz 87 94 674 610 44,65 -0,2 mm

Mercedes-Benz 87 94 674 620 44,65 -0,2 mm

Mercedes-Benz 89 94 675 600 44,45 -0,2 mm

Mercedes-Benz 89 94 675 610 44,45 -0,2 mm

Mercedes-Benz 89 94 675 620 44,45 -0,2 mm

D 2565 HM, -M, -MF, -MFR, -MH, -MR, -MUE, -MUH, -

MUL,

D 2566 HM, -M, -ME, -MF, -MFO, -MFR, -MH, -MHO, -

MUE, -MUH, -MUL, -MUM, -MXF,

D 0226 MC, -MCE, -MCFO, -MK, -MKE, -MKF, -MKFA, -

MKFO, -MKH, -ML, -MLE, -MT, -MTE, -MTE 51, -MTXE

51,

D 0824 LF 01, -LFL 01, -LFL 05,

D 0826 LE 10, -LF 01, -LF 02, -LF 04, -LF 05, -LF 06, -LF

07, -LF 08, -LFL, -LFL 01, -LFL 02, -LFL 05, -LFL 06, -LOH

06, -LUH 01, -LUH 02, -LUH 03, -LUH 04, -LUH 05, -LUH

06, -LUH 10,

D 2565 MK, -MKUL, -MT,

D 2566 KUL, -MFT, -MK, -MKE, -MKF, -MKUH, -MKUL, -

MLE, -MLUH, -MLUM, -MT, -MTE, -MTF, -MTH, -MTHO, -

MTU, -MTUE, -MTUH, -MTUM,

D 2565 HM, -M, -ME, -MF, -MFR, -MH, -MR, -MUE, -MUH,

-MUL,

D 2566 M, -ME, -MF, -MFO, -MFR, -MH, -MHO, -MUE, -

MUH, -MUM, -MXF,

D 2866 FZ, -KF, -KFZ, -KOH, -KU, -KUH, -KUL, -LF, -LFZ, -

LU, -LUH, -LUL, -LULK, -LXF, -T, -TOH, -TU, -TUH, -TUH

01,

OM 601.911, -912, -913, OM 602.911, -912, -930, -931, -

938, -939, -990, OM 603.912, -913, -931,

OM 601.900-000, -940, -941, -942, -943, OM 602.900-000, -

940, -941, -942, -946, -947, -948, -991, -994, -995, OM

661.911, OM 662.911,

KH **

*Non-binding extract from our catalogue; for details of the correct use and allocation of components, please refer to the latest KS Catalogue Pistons / Cylinders / Kit Sets

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Mercedes-Benz 97,5 90 532 600 62,50 -0,3 mm

Mercedes-Benz 97,5 90 532 610 62,50 -0,3 mm

Mercedes-Benz 97,5 90 533 600 62,20 -0,6 mm

Mercedes-Benz 97,5 91 551 610 62,50 -0,3 mm OM 356.901, -902, -903, -904, -905, -906, -907, -908, -909,

-910, -911, -912, -914, -915, -916, OM 364.900-004, -918, -

919, -920, -921, OM 366.900-004, -911, -915,

Mercedes-Benz 97,5 93 964 600 62,50 -0,3 mm

Mercedes-Benz 97,5 93 964 610 62,50 -0,3 mm

Mercedes-Benz 125 94 656 600 87,15 -0,3 mm OM 401.901-504, -970, -972, -973, -974, -975, -976, -977, -

978, -979, -980, -981, -984, -985, -986, -987, -988, -989, -

990, -991, -993, -994,

OM 402.901/-504, -970, -971, -972, -973, -974, -975, -976, -

977, -978, -980, -981, -982, -983, -984, -985, -986, -987, -

988, -989, -990, -991, -992, -995, -996,

Mercedes-Benz 128 91 597 600 89,70 -0,3 mm OM 447.900/-000, -901/-200, -905, -906, -907, -908, -910, -

911, -912, OM 475.907-505, -907-506, -950, -951, -952, -

953, -954, -982, OM 485.980,

Mercedes-Benz 128 91 622 600 89,55 -0,3 mm OM 427.902/-400, -500, -903/-600, -905, -950, -951, -952,

OM 429.951, -952, -953, -955,

OM 447.902, -902-400, -902-500, -902-520, -902-521, -

903/-600, -903-705, -950, -951, -952, -953,

Mercedes-Benz 128 91 630 600 81,05 -0,3 mm OM 441.901, -901/-400, -901-500, -901-505, -901-520, -

901-530, -950, -951, -953, -955, -960, -961, -962, -981, -

982, -983, -985,

OM 442.901/-400, -901-500, -901-501, -901-502, -941, -

942, -943, -944, -945, -946, -950, -951, -952, -953, -954, -

955, -957, -958, -959, -960, -961, -962, -963, -964, -965, -

966, -967, -968, -969-502, -969-503, -971, -972, -974, -975,

-976, -980, -982, -983, -985, -989, -991, -992, -993, -994,

OM 443.901/-400, -901-500, -940, -980,

OM 444.901/-400, -901-500,

Mercedes-Benz 128 94 516 600 81,05 -0,3 mm OM 441.901/-400, -901-500, -901-505, -901-520, -950, -

951, -953, -955, -960, -961, -962, -981, -982, -983, -985,

OM 442.901/-400, -901-404, -901-500, -901-501, -901-502,

-901-505, -901-506, -901-520, -941, -942, -943, -944, -945,

-946, -950, -951, -952, -953, -954, -955, -957, -959, -960, -

961, -962, -963, -964, -965, -966, -968, -971, -972, -974, -

975, -976, -980, -982, -983, -985, -989, -991, -992, -993, -

994, OM 443.901/-400, -901-500, -940, -980, OM 444.901/-

400, -500,

Mercedes-Benz 128 94 655 600 81,15 -0,3 mm OM 440.940, -941, -942, -945, -978, -979, OM 441.990,

Mercedes-Benz 130 91 621 600 81,05 -0,3 mm OM 440.909, -910, -911, OM 441.900/-000, -905, -906, -

907, -908, -909, -911, -912, -913, -914, -915, -916, -917, -

918, -919, -920, -921, -923, -924, -925, -926, -927,

OM 442.900/-000, -905, -906, -907, -908, -909, -910, -911, -

914, -915, -916, -917, -918, -919, -920, -921, -922, -923, -

924, -925, -926, -927, -928, -929, -930, -932, -933, -934, -

935, -939, OM 443.900/-000, -905,

OM 304.900-001, -010, -401, -410, -016, -017, -018, -413, -

414, -415, -416, -417, -510, OM 356.940, -984, -985, OM

364.901/-400, -500, -950, -951, -952, -954, -980, -981, -

984, -901/-400, OM 366.901/-500, -540, -541, OM 366.944,

-945, -946, -947, -948, -949, -951, -952, -953, -954, -955, -

9 9 8 960 962 963 96 966 96 968 969

OM 356.913, OM 364.900/-000, -900-040, -906, -907, -908,

-909, -911, -912, -913, OM 366.900/-000, -905, -906, -907, -

908, -909, -910, -912, -913, -914, -917, -918, -919, -930, -

931, -932, -933, -935, -936, -939, OM 370.905, -952, OM

372.906, -907, -930,

KH **

*Non-binding extract from our catalogue; for details of the correct use and allocation of components, please refer to the latest KS Catalogue Pistons / Cylinders / Kit Sets