Problem: At 150,000kms on a hire car, one bank of this V6 developed very noisey tappets. The tappets are similar to the Mitsubishi and Ford hydraulic lifters fitted in the ends of alloy rockers. Oil flow had been interupted on one bank with some resultant scuffing inside the rocker bores.
Cause: The rockers operate on a pair of rocker shafts with the inlet side anchored by bolts running through them into the head. However, on the exhaust side, the tendancy for the shaft to rotate with the rocker movement is “secured’ by four hollow dowels that locate the assemble onto the top of the head and enable oil transfer to move from the block, through the head and up into the rocker shafts. Transfer to the inlet shaft in via drillings across the rocker posts. The oil flow had been interupted when the small ends of these four dowels progressively failed and permitted the exhaust shaft to rotate thereby cutting off oil flow.
Cure: We have noted that the very loose fit of the small end of these dowels in the rocker shaft actually permits the shaft to oscilate back and forth in the rocker pedestals leading to the ends of the dowels breaking off. We have made heavier section dowels from valve stem material, drilled the oil hole through them and enlarged the small rocker-shaft locating end to achieve a tighter fit in the rocker shaft to reduce the movement.
To make the dowels, use a 7mm valve stem, drilled through with a 3.1mm oil hole. Reduce one end to 5.5 diam, 5.5 long to achieve a spigot that will fit into the rocker shaft. Chamfer this small end 45 degrees by 1mm. Part off at 22mm overall length and chamfer 45 by 1mm. To help extract the dowel at a later date, we tapped a 4 by 0.7 pitch thread into the larger diam end.
Regards
Hedley James
Ph 03 9372 3111
www.engineaction.com