Pistons, Rings & Bearings

By where it comes from and how it behaves. Big end knock is a deep, rhythmic sound from low in the block that gets louder under load and often quietens momentarily if that cylinder's injector or plug is disabled. Main bearing rumble is heavier and more general, worst on acceleration. Both usually arrive alongside low hot oil pressure. Piston slap and small end knock sound sharper and are worst when cold. Confirm with a stethoscope and an oil pressure gauge before dismantling anything.

Because clearance is set by selection rather than adjustment. Manufacturing tolerances mean each crankshaft journal and each housing bore falls within a small band, and the factory measures both and fits the shell grade that brings running clearance into specification. Some engines mark the grades on the block and crankshaft with letters, numbers or colours. A rebuild follows the same process: measure, calculate, then order the exact grade for each position, which is why they are listed individually here.

You can, and you will be gambling with an engine. Journals wear, housings distort with age, and a crankshaft that has been reground needs undersize shells rather than standard ones, so fitting a set on the assumption that standard is correct frequently leaves clearance outside specification. Measure the journals with a micrometer, measure the housings with a bore gauge, and check the assembled clearance with plastic gauging material as a cross check. The measuring is the job, and the fitting is the easy part.

It sits on the nose of the crankshaft and absorbs the twisting oscillations that each combustion event sets up along the shaft, protecting the crankshaft, the timing drive and the ancillary belt from resonance. Construction is an inner hub bonded to an outer ring through a rubber layer, and that layer perishes and separates with age and heat. A damper that has begun to separate shows as a visible wobble, a split in the rubber, or a timing mark that has moved relative to the hub.

Almost always. Big end and main bearing bolts are usually torque to yield, tightened to an angle beyond their elastic limit so they hold a very consistent clamp load, and once stretched they cannot repeat that reliably. A rod bolt that fails destroys the engine and often the block with it, so this is not a place to economise. Use new fasteners, follow the specified torque and angle stages exactly, and never lubricate threads unless the procedure calls for it.

Test by when the smoke appears. Valve stem seals typically show as a puff of blue smoke on start up or after idling at a junction, then clear once running, because oil has drained down the guides while the engine sat. Worn rings smoke more consistently under load and are usually accompanied by crankcase pressure and a compression or leakdown reading that is genuinely down. A leakdown test tells you where the air is going and separates the two in about twenty minutes.

Bearing shells and thrust washers sit at the lower end of the range, individually inexpensive though a full set adds up. Connecting rods and torsional dampers are moderate. Crankshafts sit at the top of the category, being heavy, precisely machined and shipped as freight. The live minimum and maximum are shown in the band at the top of this page. Because a rebuild is a long list of small items and one or two large ones, quoting the whole list at once is worth doing.

With a dial indicator against the end of the crankshaft, levering the shaft fully one way and then the other, and reading the total movement against the manufacturer's figure. Excess end float means the thrust washers have worn, which happens quickly on manual vehicles where a driver rests a foot on the clutch pedal. Symptoms include a crankshaft that visibly moves when the clutch is pressed and, in bad cases, a knock at the same moment. Replace the washers whenever the crankshaft is disturbed.

Frequently, and it is a specialist operation. A machine shop measures the journals, grinds them to a standard undersize and polishes them, and you then fit shells of the matching undersize grade. What rules it out is a journal that has been damaged beyond the available undersizes, cracking found on inspection, or a crankshaft that has been hardened in a way that regrinding would remove. Have the shaft crack tested at the same time, since a bearing failure often means it ran hot.

There is nearly always a reason beyond mileage, and finding it decides whether the rebuild lasts. Common causes are oil starvation from a low level, a blocked pickup or a failing pump, contamination from unfiltered debris or an overdue oil change, overheating, or an incorrect clearance from a previous rebuild. Look at the shells themselves, since the wear pattern tells the story: uniform wear points to mileage, scoring points to debris, and localised destruction points to starvation or misalignment.

Only where the number agrees. Stroke, journal diameters, counterweighting, the reluctor pattern for the position sensor and the flywheel mounting all follow the specific variant, and manufacturers change them between engines that share a family name. A crankshaft with a different reluctor pattern will run badly or not at all even when it fits mechanically. Read the number stamped on your own shaft, match it, and use the quote form where the reference brings back nothing.

Yes. Bearing shells are supplied in graded thicknesses and pistons in graded diameters, so each grade is its own reference and most move too slowly to carry a standing price. Send the number together with the grade marking stamped on the crankshaft web or the block deck. Without that marking the reply cannot be precise, and a shell one grade out gives a clearance that either seizes or knocks. Order the shells as a set for the journal you are working on, and add the torsional damper to the enquiry if the engine has high mileage.