Avoid lugging the engine. A damaged cylinder may need reconditioning or replating. The same goes for when you find your intake boot and airbox was not sealed properly. Rebuilding the bottom end of your engine, whether two or four-stroke, can be a fun and rewarding job. Additionally, a considerable amount of money can be saved by taking on the work yourself versus tasking a shop to perform the work. Rebuilding a motorcycle engine can be very easy whether you have a single cylinder 2-stroke or a multi-cylinder 4-stroke engine.
This preparation must include having the workshop and motorcycle thoroughly clean especially the external engine components. The top signs that a 4 stroke dirt bike needs to be rebuilt are: Hard to start. Low power. Noisy engine. Due to the unevenness of load distribution that can result from pounding the bearings out with a hammer, we caution against this as a primary form of removal without the aid of heat.
At this point, it is my recommendation that all components that originated inside the crankcases be thoroughly cleaned. Clean components will ensure easy and accurate inspections. On four-stroke engines that have oil passages running through the crankcases, cylinder, and cylinder head, it is imperative that these are cleaned and blown out.
This is especially true if the engine suffered a major failure where oil contamination was a resulting issue. Similarly, on two-strokes, the passages that lead to the crank bearings should be cleaned. Component inspections should be conducted to assess the condition of the gearbox, crankcase bearing bores, and crankshaft. The crankshaft should either be rebuilt or replaced depending on the severity of wear and desires of the builder. Should you determine you'd like to rebuild your crank, ProX offers OEM quality connecting rod kits that can be used for a crankshaft rebuild.
Unless you are experienced in this field and have the tools, crankshaft rebuilds should be trusted in the hands of a reputable shop. Read about the advtantages of ProX connecting rods here. Regardless of whether a new or rebuilt crankshaft is utilized, the trueness of the crankshaft must be checked. This can be farmed out to a competent shop, machinist, or if properly equipped, performed in-house.
While it should normally be expected that new or rebuilt cranks are within runout specifications, the trueness of the crankshaft is imperative to long-term durability. Similar to removal, heat can also aid in bearing installation. The same heating recommendations apply, and once at temperature, the majority of the bearings should fall to the bottom of their bores without any input.
The caveat to this is the bearing dropping into the bore cock-eyed. When this happens, a punch and hammer should be used be help square the bearing to its bore. Be sure to tap on the outer race of the bearing. To ensure the bearings are at the bottom of their bores, they should be tapped to confirm they are fully seated.
Be sure to load the bearings through their outer races when pressing them in place. Seals can be tapped into place with a seal driver or socket and hammer. Alternatively, a press can be used. The important checks to perform are to ensure the seals have been installed squarely in their respective bores, and at any prescribed depths outlined in the service manual.
Any seals installed cock-eyed will wear out prematurely. To prepare for the new bearing and seal installation, complete crankshaft bearing and seal kits are available through ProX. Ordering in kits creates less hassle and is a budget-friendly step in your rebuild. Click here to check out ProX bearings and seals. Crankshafts that utilize an interference fit with their mating crank bearings can be installed two ways.
Shrinking the crankshaft in place using a combination of heating and cooling of components works well. Alternatively, utilizing a crankshaft puller is another great way to install the crankshaft. Pounding or pressing the crankshaft into place should never be considered because the trueness of the crankshaft can be affected.
To shrink the crankshaft in place the crankshaft should be set in a freezer for about an hour, and the inner race of the crank bearing should be carefully heated with a torch. The cautionary point here is to use care when heating if the crank seals have already been installed. Inner race temperature can be gauged by applying a drop of water to the surface of the race. If the water droplet sizzles, the inner race is plenty hot.
At this point, the cooled crankshaft can be dropped through the heated bearing. Once seated, work to button up the crankcase assembly should progress quickly, and the remaining inner race can be heated and the crankcase installed. Using a crankshaft puller is an equally acceptable method and is incredibly straightforward.
The puller is threaded and seats against the crank bearing or crankcase and attaches to the end of the crank via a nut or retaining ring. Rebuilding the bottom end of your engine, whether two or four-stroke, can be a fun and rewarding job.
Additionally, a considerable amount of money can be saved by taking on the work yourself versus tasking a shop to perform the work. A 4 stroke that is difficult to start may be a sign that it needs to be rebuilt. This could be caused by valves that are worn or out of adjustment. It could also be a worn top-end that is low on compression. Start the engine and allow it to run at idle speed until it is thoroughly warmed up, then stop the engine and allow it to cool completely.
Start the engine and allow it to warm up thoroughly. Then ride the motorcycle for ten minutes using no more than half throttle. Avoid lugging the engine. A damaged cylinder may need reconditioning or replating. The same goes for when you find your intake boot and airbox was not sealed properly.
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