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Camshaft Scissor / Preload Gears

The scissor gear on a camshaft is designed to reduce the free play between the camshafts and eliminate any knocking noise by ensuring constant contact is being made between two mating gears.  Basically a spring loaded gear applies constant pressure on the sister gear in order to prevent any rattle or ticking noises occurring.

Fit the service bolt before removing camshafts

The best approach when handling these types of gears is to install a service bolt through the scissor gear BEFORE removing the camshaft.  Most of the time a 15-20mm long 6x1mm bolt can be used, however some applications may vary from this size.  After you refit the camshaft, simply remove the bolt and away you go.

If the camshafts are removed without installing the service bolt, the preload on the gear will be lost and you will have to reset it before re-installing the camshaft.

 

To reset the preload on a scissor gear secure the camshaft in a vice on a section that will not damage the lobes or journals.  You will need to rotate the scissor gear against spring pressure until the service bolthole lines up with the thread.  Normally a large pair of pliers or multi grips will be sufficient to do this.  Holding the gear in place with one hand fit the service bolt all the way through the gear. The bolt only needs to hold the preloaded gear in place and does not need to be any more than finger tight.

Once you have re-fitted the camshafts onto the cylinder head remove the service bolt BEFORE rotating the camshaft, if you try to turn the engine over with the service bolt fitted it will cause significant damage to the engine.

The noise caused by an incorrectly set scissor gear can be similar to a noisy hydraulic lifter or tappet.  After a cylinder head service many people make the mistake of assuming a hydraulic lifter or tappet clearance is causing an excessive ticking or rattle, but often it is the scissor/preload gear that has not been set.

Kia Rio 1.5L 2000 – 2005. Engine- A5D

Head Bolts & Maintenance Issues:

This engine consists of two head bolts at the front of the engine that are shorter than the rest. The two shorter head bolts must be fitted in the corresponding holes at the front of the engine. Fitting the longer bolts in the shorter bolt holes will cause the head bolt to bottom out and the head gasket will not seal correctly. This will result in water and oil mixing, water entering the combustion chamber in cylinder one, and external oil/water leaks around the front of the engine.

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These engines are also notorious for seizing camshafts as a result of poor maintenance and lack of oil changes. If you have an engine with a broken belt, be sure to check under the tappet cover for signs of oil sludge formation. If sludge build up is present, there’s a  good chance one or both of your camshafts have seized in the cylinder head.

Alternately, if your engine is still running ok but you know you’ve been a bit slack with servicing (doing more than 15,000kms without an oil change or using low quality oils), its best to check under the tappet cover for signs of sludge. If you find signs of sludge build up, you should talk to your local mechanic or give us a call as soon as possible to avoid costly engine damage.

NISSAN YD25DDTi

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There are an increasing number of reports of lower timing chain failures in Australian released Nissan YD25DDTi engines. The primary (lower) timing chain and gears are a simplex (single row) design, while t

he secondary (upper) timing chains and gears are a duplex (double row) design. The simplex chains are stretching and ultimately falling, leading to major engine component failure, including valve and piston crown contact and dam

age. It is advisable to replace the simplex timing chains and tensioners before 80,000kms.

Typically the timing chain guides and the vacuum pump timing chain sprocket and shaft are found to be heavily damaged and/or worn as well.