Hello I have a Big Cam Cummins III I am in the process of rebuilding this guy to maybe 600 to 700hp I am taking my time with this project I want it done right the first time This engine is in a 85 someone mention that I should have it cut for the LPF liners...Please help any advice is always good...............also can this be cut while the engine is still in the chassis????
Should I have my engine cut for LPF liners???????
Cummins has made many changes to the basic 855 block through the years
Rebuilders continue to come up with variety of proven repair processes.
In the July 1997 issue of Diesel Dialog we discussed the Cummins 855 cid series of engines, specifically, the changes in the engine through the years and how those changes are identified. This month we'll look at some of the repair strategies and updates that Cummins, as well as independent rebuilers, have devised over the 40 years this engine has been in service.
With the understanding that this is a very large subject that cannot be completely covered in short form, we will focus on a few important specifics and provide some of the pertinent information available. As stated in the pervious column, our intent is not to endorse any specific repair, but to report on what is being done with this engine series.
Starting with eh engine block, Cummins has introduced many variations to the basic configurations of the original 855 series block. It has appeared as the Small Cam Non-FFC to the Big Cam Ivs and beyond to the N14. However, many of the repairs have remained consistent through the product line.
Specific repairs are determined by the blocks themselves. As a first step, identify whether the block is a thin deck (3/4" measured from the deck down a water hole) or a thick deck block (1"). Also determine whether the block uses upper press fit liners or the later, lower press fit design, by measuring the counterbore and pilot bore areas and comparing them against specifications for depth and inside diameters.
Completely inspect this area for cracks as the counterbore joint absorbs a great deal of stress during engine operation. Cummins has guidelines for acceptance of cracks, which can be obtained from your local dealer (technical service bulletin #3810450). It is important to also measure the deck height (18" measured from the top of the deck to the centerline of the main housing bore) as this will determine if the block can be resurfaced.
Be sure and not any previous repairs done to the block such as upper or lower repair sleeves, main saddle repairs or camline sleeves. This is very important because previous repairs may become your responsibility after the block has left your shop. There are two basic styles of counterbores in the decks of these blocks - upper press fit (UPF) and lower press fit (LPF). To understand how these blocks seal water it is important to understand the difference between these two types of counterbore joints.
The upper press fit counterbore and line relies on the upper area of the counterbore working in conjunction with the counterbore ledge to support and seal the liner. In 1986, beginning with the BC-IV 444 engine, Cummins introduced the LPF counterbore system. Here the liner is supported by the lower bore of the counterbore joint. This bore is sometimes called the pilot bore.
Water sealing depends on a seal ring or shim which operates between the counterbore ledge of the block and the underside of the liner flange. See Figure 1. This shim also "lubricates" the metal-to-metal contact between the liner flange and the counterbore land.
It is important to understand the two separate purposes for the counterbore joint: the excess of which will inevitably crack the casting; and the obvious importance of sealing the water jacket. This is important to understand as it is very common for people tom think that lower press fit means that the lower liner seal area (where the o-rings seal the water at the bottom of the liner) has an interference or press fit. This is not so.
The lower liner seal area in these blocks is functionally the same, with a crevice seal and two o-rings. Cummins maintains that the lower press fit liner system seals up to three times better than the previous design.
Repairs to the counterbore area must be done carefully to maintain correct dimensions and depths. If this area is machined at any angle other than 90 there will be concentricity problems between the counterbore and the lower liner seal area. This condition will often cause improper sealing of the liner (see Figure 2)
Any tool chatter will also threaten this seal. If the block counterbore dimensions are too tight, bore distortion on the I.D> of the liner will result causing undue stress to the casting which commonly means cracks in the counterbore will appear. Distortion of the I.D. of the liner can be especially devastating with LPF liners as the press fit area of the block is in the same area as the top of the piston ring travel; this can lead to piston ring failure in addition to over stressing the casting.
Figure 1: A lower press fit means there is a .004"-.005 press fit in the pilot bore rather than in the upper counterbore of the block. Lower press fit does not mean there is a press fit in the lower liner seal area of the casting.
It is also important to note that there are two different head bolt designs. The differences in the clamping area in the casting can also affect counterbore distortion. In 1978 a 1" longer head bolt was produced in an effort to lower the clamping forces in the casting away from the counterbore area. These bolts reduce stress in the deck of the block.
In 1982 Cummins offered oversize O.D replacement liners for the upper press fit blocks. This liner had a 0.20" larger O.D. flange and was .010" thicker (deeper). If damage remained after machining this 0.10", counterbore shims were offered in different thicknesses to allow proper liner protrusion after a repair. In 1985 Cummins offered an upper press fit liner with an increased upper press fit liner with an increased contact (press) area by .080" to help make the system more rigid and durable.
After the introduction of the LPF liner all engines relied on a .020" liner shim placed under the flange of the liner. In 1987 Cummins introduced an oversize liner for the lower press fit system, called the 20/40 liner. This liner featured a .020" larger O.D. in the flange area. The pilot bore areas remained the same as other lower press fit liner - .040" larger than the earlier upper press fit liner.
This liner is often a first choice repair for a block that has not had any upper repair sleeves installed. It effectively allows the use of the lower press fit liners in upper press fit blocks. Two different upper counterbore repair sleeves are available, one at .600" and one at .750" thick. The first is used upper press fit blocks, the second is for lower press fit blocks and can be used to retrofit older style, thick-deck blocks for use with upper press fit liners. The .750" thick repair sleeves cannot be used in thin deck blocks as there is not enough material to support a sleeve this thick.
Block repairs, such as installation of lower liner seal inserts, align boring, saddle repairs and cam-line repairs are, for the most part, the same as any other wet sleeve diesel block.
There are a couple of other changes to the blocks that should be mentioned at this point. Early blocks have a smaller cam bore and require a special cam bearing which is available through bother OEM and aftermarket sources. One repair option is to alignbore these cam bores for the later cam bearings, the benefit of which is that the thicker bearing, are more durable, especially when used in higher horsepower applications.
Also for blocks which have minimum deck height and cannot be surface and counterbored, or blocks which cannot be surface for other reasons, the aftermarket has responded with an "extreme-service" gasket. This composite gasket made of a graphite base is designed to seal these blocks when fitting freshly resurfaced heads. Response from the field has been positive, considering what we are trying to accomplish.
As for the rotating assemblies, there are two basic crankshaft styles - the "tapered-nose" crank and the "stub-nosed' crank. The earlier tapered-nose cranks are common in lower horsepower, small cam engines. Cummins recommends that some of these cranks not be ground and other castings be confined to low horse-power naturally aspirated applications.
Table 1 is a list of the casting numbers of the cranks which are not suggested to be ground. This information can be found in Cummins service letter #104-6826, published in 1966, which was the first of a series of these service letters concerning regrinding of these cranks.
These crankshafts had a problem with breaking in the radius areas after regrinding. They were weak for basically, two reasons depending on the casting number. The first reason was that some of the early cranks were "non-fillet.hardened" and must be confined to naturally aspirated, low horsepower applications.
Figure 2: Repairs to the counterbore area must be done carefully or improper sealing of the liner will result The second reason was that with the early attempts at "fillet-hardening" there was a problem controlling depth, making the fillets especially susceptible to grinder burn, which can cause a stress riser in the fillet area, resulting in breaking. Cummins released specific instructions for the grinding of these crankshafts, which had to do with special grooved wheels, turning the grinder at a different rpm, as well as a special cooling equipment. However, for the most part, many rebuilders seem to feel that they are doing the customer a favor be selling a replacement crankshaft with a stronger characteristics, rather than grinding the weaker casting numbers and risking failure.
One interesting note taken from these three service letters was the final test procedure which involved using hydrochloric acid and acetone to test for invisible grinder burn in the radius area. I have spoken with rebuilders who do this as a standard procedure on crankshafts that they known are going to be used in very high horse-power applications and put under extreme loads.
The Small Cam engine and the Big Cam engine use different connecting rods. The Big Cam rods use two dowels to eliminate cap shift. Small Cam rods use a through-type bolt. Due to the common 12"center-to-center dimension, rode width and pin size, Big Cam rods can be physically used in Small Cam engine. Some rebuilders prefer to retrofit to the later style rod. The Big Cam rods have a weight code stamped into the side of the rod and should match as a set. Also, the proper rod bearings must be used if the Big Cam rods are used.
As for piston and cam combinations, there are many, many different options available depending on the CPL (Control Parts List) number. A discussion of this, however, is best let for another column.
There are basically three types of Cummins heads. The original casting has smaller pushrod cavities which will not work with the Big Cam engines. The second type is the head introduced with the Big Cam blocks. The housing of the upper rocker assemblies on the first two types of heads, are the same, but a hollow set screw must be used with compression brakes (Jakes).
The third type of head is referred to as the NTA-88 or formula heads. These heads are different and also use different upper rocker assemblies. Also, be very careful retrofitting different turbo and Jake combinations. With some of these combinations the boost rise specific turbos (such as the Holset T46) is so fast that there is a risk of piston-to-valve contact. There is a kit available through aftermarket sources to update the early Jakobs 30s to the later style that will respond to the faster boost rise.
Rebuilders are continuing to develop new fixes and finding ways to make this versatile engine even better. There is a good volume of these engines in the field, and their design is straightforward.
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